[Topics in Applied Physics] Spin Dynamics in Confined Magnetic Structures I Volume 83 ||...

Stroboscopic Microscopy

of Magnetic Dynamics

Mark R. Freeman and Wayne K. Hiebert

Department of Physics, University of AlbertaEdmonton, Alberta, T6G 2J1, Canadafreeman,[email protected]

Abstract. The enhanced capabilities of contemporary pulsed light sources have ledto the reflourishing, in recent years, of ultrafast imaging of micromagnetic dynamics.Concurrently, interest in the subject has been intensified by other factors, such asthe emergence of intrinsic magnetic response times as a potential limitation tothe ultimate bandwidth of magnetic data storage and by increasingly powerfulcomputer models of magnetic dynamics which call for experimental comparisons.This review contains a discussion of the experimental details behind ultrafast time-resolved magneto-optic imaging, sandwiched between a brief historical overviewand a presentation of some recent results, and accompanied by an outline of somefuture prospects.

1 Historical Overview

The ongoing development of ultrafast laser technologies has made strobo-scopic imaging of fast dynamics in microscopic structures very convenient.The stroboscopic, or “pump-probe” method as it is traditionally named bythe optics community, has been grafted onto many different varieties of mi-croscopy, including electron beam, scanning probe (force and tunneling), and,of course, optical (both conventional and near-field) [1,2,3,4]. Some applica-tions of ultrafast optical microscopy are more fully developed than ultrafastscanning probe microscopies, many of which are still not too far beyond the“proof-of-principle” stage. This is largely because the development time foran ultrafast optical microscope is much shorter than that for combinationsof ultrafast lasers with other imaging methods.

Magnetic structures in particular have provided a major test bed for de-velopments in ultrafast optical microscopy. The magneto-optic activity offerromagnetic materials ideally suits them for this kind of experimental anal-ysis. With characteristic relaxation times and oscillation periods ranging intothe low picosecond range, and with domain wall widths and spin-wave wave-lengths in the nanometer range, spatiotemporal investigation of these mate-rials poses a difficult challenge for any type of microscopy. Ferroelectrics areanother class with similar characteristics [5].

As is often the case, the territory we explore now and find so fertile turnsout to have been well surveyed by our predecessors, using the best tools of

B. Hillebrands, K. Ounadjela (Eds.): Spin Dynamics in Confined Magnetic Structures I,Topics Appl. Phys. 83, 93–129 (2002)c© Springer-Verlag Berlin Heidelberg 2002

94 Mark R. Freeman and Wayne K. Hiebert

their day. Most of the current experimental activity in the area of fast mag-netic dynamics was foreshadowed by work conducted during the late 1950 sand early 1960 s. This was a time of much groundbreaking magnetics research,fueled initially by the relatively new availability of tools for microwave spec-troscopy and by the early success of hard disk memories, and sustained by theincorporation of other advances such as laser technology. Nonlinear magneto-optics is perhaps the most notably different new technique available today,but even this example has roots which trace back to the birth of nonlinearoptics in the early 1960 s [6].

The overriding problem of interest throughout this time has been magne-tization reversal in thin film structures [7,8]. To draw some parallels to workstill ongoing today, consider the study of timescales for switching magneticbits in which the changes in magnetization are detected either by inductiveor magneto-optic methods. Switching speeds may be recorded directly bymeasuring the voltage in inductive pickups with fast oscilloscopes [9,10], andvectors are measured with different pickup coil geometries [11]. In an exampleof related work, switching times were estimated through the application ofpulses of varying duration, followed each time by postinspection to determineif the element actually switched [12]. The most creative “reincarnation” ofthis type of experiment in the 1990 s was the collaboration between ETHZurich, IBM, and the Stanford Linear Accelerator Center, firing picosecondbunches of relativistic electrons through magnetic films and inspecting themagnetization reversal patterns after the fact [13,14] (see also [15] for a the-oretical grounding and [16] for simulation of this experiment). The foregoingtypes of pulsed experiments have been the basis of a large proportion of theadvances in understanding magnetization reversal [17], but they neverthelesscarry with them the sense that one is missing the complete picture.

Reversal processes in general are highly complex, and it has also long beenrecognized that the only way for experiments to address the problem withoutcompromise is to achieve a high degree of simultaneous spatial and temporalresolution in tracking the magnetization changes. Such efforts in the 1960 sculminated in the “nanosecond Kerr magneto-optic camera” of Kryder andHumphrey [18]. Reference [18] also nicely summarizes the varied approachesdifferent investigators had pursued until that time. Before widespread avail-ability of the laser, the variety included a bright, strobed light source madeby passing sunlight through a slit in a rapidly spinning disk (an experimentperformed in California) [19].

The Kryder and Humphrey system had (by today’s standard) only modestspatial and temporal resolution — about 10µm and 10 ns, respectively. Thesenumbers were achieved in single exposures, however, which was an enormousachievement. For the subsequent period of about 15 years, qualitatively newexperimental advances in time-resolved magnetic imaging were sparse. Duringthis time, the predominant concentration of high-speed magnetics was inthe frequency domain, and experiments largely used nonimaging microwave

Stroboscopic Microscopy of Magnetic Dynamics 95

methods. Interest in time-domain techniques was rekindled partly as a resultof the advancement of short pulse techniques to the femtosecond range, wherethey tread in a regime of equivalent frequencies inaccessible to microwavemethods. (In another interesting bit of historical foreshadowing, frequency-domain optical detection of ferromagnetic resonance was achieved by Hanlonand Dillon, again in the 1960 s [20]. The experiment was not time-resolvedbut still exploited the high bandwidth of optics.)

Revival of interest in high-speed imaging methods began in the mid-1980 s.This was motivated by the scale reduction of features in semiconducting andmagnetic technologies (as indicated, for example, by the almost simultane-ous appearance of stroboscopic scanning electron microscopy probes for inte-grated circuits and for recording heads [21,22]), and also by the atmosphereof the “microscopical renaissance” which has swept through scientific andtechnical communities since the debut of the scanning tunneling microscopein the early 1980 s.

2 Experimental Details

Time-domain magneto-optical measurements offer two possible advantagesrelative to more conventional microwave measurements. The first is the veryhigh bandwidth derived from the ultrafast laser pulses and the ability todeliver this bandwidth optically without expensive or awkward microwaveelectronics or plumbing. The second is the spatial resolution one obtainsfrom optical detection. Ferromagnetic dynamics tends by default to be spa-tially inhomogeneous. Optical detection does not suffer loss of signal-to-noiseas rapidly as inductive detection when it is focused on smaller areas (andindeed no loss of signal-to-noise if the laser power can be left unchanged.)The bandwidth advantage is moderated in practice by the fact that it isstill simpler to precisely control and vary the excitation parameters (rise andfall times, pulse widths) electronically than it is optically. Under ambientconditions in particular — in air at room temperature — it is common touse a combination of lasers and microwave sources. For vacuum or low tem-perature environments, the convenience of performing all of the “high speedcommunications” with the sample through an optical window is a great ad-vantage.

Four basic components are required to perform stroboscopic magneticimaging: the pulsed optical source, a synchronous means for magnetic exci-tation of the sample, a microscope, and some mechanism for polarization-resolved image capture. Various individual components and their assemblyinto complete measurement systems form the subject of this section. Thebasic measurement scheme can be described with reference to Fig. 1 [23]. Inthe figure, transient quasi-metallic photoconductivily induced by the pumppulse [24] at the end of the coplanar transmission line structure launchesa current, which in turn drives the sample away from equilibrium with its as-


Fig. 1. Sample geometry for a pump-probe, time-resolved nonequilibrium magneti-zation measurement using a coplanar transmission line ultrafast transient magneticfield generator

sociated transient-applied magnetic field. This excitation is repeated periodi-cally, allowing a sufficient interval between each pulse for the sample to returnto equilibrium (with the exception of cases of resonant amplification, wherethe period between pulses in the train is less than the relaxation time [25]).The sample magnetization is measured stroboscopically via magneto-opticinteraction with the probe beam, a train of optical pulses synchronous withthe excitation train. The relative arrival time of the two beams is varied us-ing an optical delay line. The time resolution is limited in principle only bythe duration of the optical pulses. Performing the experiment in an imagingmode captures the spatial information.

2.1 Pulsed Optical Source

Today, the light source is almost always a short-pulse laser of some kind. Thecurrent default is the present commercial standard, the Kerr lens mode-lockedtitanium-doped sapphire laser. These lasers offer more stable beam intensityand consequent improved measurement signal-to-noise ratio than their mode-locked dye laser predecessors. With the routine availability of pulse widths aslow as 30 fs or less, these lasers are also very attractive for the new imagingmethod of nonlinear magneto-optics [26,27,28,29,30]. Mode-locked argon ionlasers have been used in the past and are adequate for linear Kerr effectimaging if picosecond time resolution is not required [31]. The Hillebrandsgroup is now obtaining beautiful results using a 10-ps pulsed semiconductordiode laser. External cavity mode-locked semiconductor lasers will soon be


used in these applications (if they are not already), and related work usingmode-locked fiber lasers has already been reported [32].

2.2 Transient Magnetic Excitation

The recent era of ultrafast pump-probe magnetization dynamics studies be-gan with techniques of direct optical pumping. In the classic experiments ofAwschalom and co-workers [33], optically induced nonequilibrium magnetiza-tion was created in dilute magnetic semiconductor materials over time-scalesof a few picoseconds, using pulses from a mode-locked organic dye laser. Forcircularly polarized excitation from a p-like valence band to an s-like conduc-tion band, with pump photon energy not too far above the semiconductorbandgap (such that the spin orbit split-off component of the valence bandis not coupled), 50% initial polarization of the optically generated electronsand holes is obtained via the angular momentum-conserving selection rules.Faraday effect measurements made with the weaker probe beam can be usedto monitor the magnetization change as a function of pump-probe delay. Theobserved relaxation behavior generally consists of several steps. Spin relax-ation of the transient hole population is typically very rapid, on timescalesranging from subpicosecond to a few picoseconds. Across the tens to hundredsof picosecond time interval, the conduction electron spin polarization also de-cays, partly through spin-flip scattering with magnetic ions. The decline ofthe charge carrier population through recombination may be monitored in-dependently via photoluminescence. Indirectly induced ionic magnetizationoften remains as the last memory of optical excitation and may be unam-biguously identified as such due to the separation of timescales. The favor-able disposition of the magnetic semiconductor materials toward both opticalexcitation and detection of magnetization has been used to great advantageby the Awschalom group in particular. Effects of quantum confinement, re-duced dimensionality, and more recently, spin transport have been exploredin samples ranging from II–VI heterostructures to bulk GaAs [25,34].

In metallic ferromagnets, on the other hand, direct pulsed optical exci-tation is used primarily for transient heating of the electronic system. Thisreduces the magnetization on very short (picosecond or femtosecond) time-scales and has been exploited in combination with both magneto-optical andpolarized photoemission detection in measurements of spin–lattice relaxationin ferromagnets (as discussed in contribution by Zhang) It has not yet provenpossible to substantially change the level of electronic polarization by directoptical pumping in metals (the number density of electrons is simply toohigh, unlike the semiconductor case where the carrier population itself is cre-ated by the excitation). The Nurmikko group has taken an exciting steptoward this goal. From measurements on an exchange-coupled FM/AF bi-layer, they reported convincing evidence of suppression of the magnetizationof the top layer resulting from transient destruction of exchange coupling atthe interface due to back-side laser excitation [35]


Perhaps the only “universal” (sample material independent) means ofdriving a magnetic system out of equilibrium on picosecond timescales is touse the laser pump pulses to trigger a change in the magnetic field appliedto the sample. One of the most flexible geometries for driving a transientmagnetic field using a pump laser pulse is the coplanar transmission line on asemiconductor substrate (Fig. 1). Closely related to the microwave strip-linesin use for decades for high-speed excitation of magnetic structures, this ar-rangment is very convenient for delivering high bandwidth to the sample viathe pump pulse. With the coplanar geometry, one can obtain rise times intothe subpicosecond regime [36]. The geometry integrates the idea of a photo-conductive (Auston) switch [24] with a transmission line so that the electricalpulse (actually pulses, one in each direction) can be launched at any pointalong the line accessible to the laser. Using femtosecond laser excitation, thetransient magnetic field rise time at the sample is typically limited by dis-persion of the pulse during propagation from the switch point (although ithas been demonstrated that subpicosecond rise times may propagate for mil-limeter distances on superconducting transmission lines [36]). Rise times ofa few picoseconds or less at the sample are attainable very easily.

At the photoconductive switch itself, the pulse rise time is limited by car-rier transport in the semiconductor or by switch capacitance if the structureis large. The decay time of the pulse is determined by the electron–hole re-combination rate in the semiconductor or by the sweep out of carriers fromthe illuminated region, if the latter proceeds more quickly than recombina-tion. For impulse excitation, the semiconductor can be intentionally damagedto decrease the carrier lifetime (ion implantation or low temperature growthare the traditional means). Reduced efficiency comes with the increase inspeed, so one must trade off amplitude to obtain shorter pulse widths. Directgap semiconductors are normally used for higher efficiency in generating pho-tocurrent (GaAs and InP are well suited to Ti:sapphire excitation), but thesetend to have subnanosecond carrier lifetimes at most. The original Austonswitches were based on silicon, and Auston showed how fairly square pulsesin the nanosecond regime could be generated, with the trailing pulse edgecontrolled either by carrier sweep out or by shorting the line to ground witha second optical pulse. New means of pulse control continue to be reportedwithin the photoconductive context [37].

The coplanar line geometry itself also offers some tunability of pulses.Terminating the line in an open circuit and using the open end as the opticalexcitation point, as illustrated in Fig. 1, doubles the amplitude of the tran-sient field at the sample by superposition of the electrical pulse reflected fromthe end and that initially launched toward the sample. Moving the pump fo-cus position along the line affects the pulse shape at the sample, and this cansometimes be used to advantage [38].

Magnetization reversal of micrometer-scale structures on nanosecondtime-scales is normally driven by current pulses from commercial avalanche-


transistor-based electronic pulsers propagating through lithographic striplines. The arrangement is then very similar to setups from decades earlier [18],reconstituted in more miniature form and with faster laser sources. Withthese pulsers again, there is a trade-off between amplitude and rise time. Inour lab, with pulsers from Picosecond Pulse Labs and Directed Energy Inc.,the available combinations are 200mA (into 50 Ω), 45-ps rise time; 1A, 250-ps rise time; and 18A, 5-ns rise time – corresponding to a nearly constantmaximum voltage slew rate of about 200mV/ps across this entire range. Aswith earlier systems, the key to exploiting the speed of these drivers is to payclose attention to the jitter between the laser pulses and the electrical pulses.

Careful characterization of the pulses used in experiments is also required,particularly as input for comparison computer models. It is possible to mea-sure the current waveforms optically in a very high bandwidth (more than50GHz), essentially noninvasive manner, which also provides an “absolute”time reference (identical time origin and scale) for comparison with the time-resolved measurements of the magnetic structures under investigation [39].The optical current probe looks at the parametric response to the fring-ing fields of the current of either a fast relaxing paramagnetic sensor or ofa dc magnetic field biased ferromagnetic sensor (see Sect. 3.1 for more dis-cussion). The amplitude of the sensor response can be calibrated in knownlow-frequency fields (with known low-frequency currents), so the method pro-vides the amplitude of the magnetic transient, as well as its temporal profile.Custom inductive probes have also been developed by several groups, and aconvenient 2-GHz current probe is available commercially (Tektronix CT-6).

2.3 Microscope and Polarization Imaging

The most convenient and economical microscope for use with pulsed lasersources is a stripped-down bench-top design containing only essential opticalcomponents. At the heart of the microscope is an infinity-corrected micro-scope objective, mounted very stably with respect to the sample on a piezo-driven flexure stage (we have had good success with the Elliot Scientific designmarketed by Thor Labs in North America.) A higher end approach is to usea nice metallurgical microscope (such as the Zeiss Axiomat, favored by theIBM groups of Bernie Argyle and Jurgen Heidmann). Strain-free objectiveshave a large advantage in limiting the “depolarizing cross” effect on outputlight and are particularly useful for imaging where the absolute polarizationneeds to be well known.

The effective spatial resolution in imaging is determined by the maximumspatial frequency at which the signal of interest can be resolved above thenoise background of the instrument [40]. In magneto-optical imaging, thisis determined by the combination of the focusing acuity and the sensitiv-ity to polarization or intensity changes. This creates a strong incentive toconcentrate on methods of improving focus which do not involve a large sac-rifice in optical efficiency or “photon budget” (and hence in signal-to-noise).


The solid immersion lens is a particularly attractive alternative, naturallyextending the progression of air and liquid immersion microscopy [41]. Inaddition, most groups employing femtosecond titanium:sapphire lasers forimaging also use frequency doubling of the beam to obtain higher spatialresolution. This trend will undoubtedly continue from the blue into the ul-traviolet, for example, using higher harmonics or parametric amplification offemtosecond pulses. At some point, however, a crossover to near-field tech-niques becomes essential if one hopes to extend ultrafast optical imaging tothe nanometer scale [5].

In studies of ferromagnetic dynamics, one must follow the behavior of theentire magnetization vector. Fortunately, linear magneto-optic measurementsare inherently vectorial in nature: the probe light couples to the component ofmagnetization in its direction of propagation. With high numerical apertureillumination of the sample, the three orthogonal components of magnetizationare sampled nearly equally and can be elegantly separated from one anotherby using quadrant photodetectors [42,43], as discussed below. This approachis adopted from static Kerr imaging and works equally well in stroboscopictime-resolved measurements where we integrate the response across manypulses and do not require any high-speed differential electronics. One canalso obtain linear combinations of the in-plane and out-of-plane componentsof the magnetization by masking off halves of the input or output beam (atthe expense of some spatial resolution). Whichever approach one adopts, allthree components should be extracted at each pixel during an individualimage scan to avoid possible misregistration from positional drift betweensuccessively rastered images.

A dual quadrant detection system is schematically depicted in the over-all experimental layout (Fig. 2). When only the sum signal from each set ofquadrants is used, the system reduces to a polar Kerr detector. The Thom-son polarizing beam splitter is set at 45 to the incident polarization so thatequal intensities are sent through each arm. Then, differential detection ofone quadrant sum from the other takes advantage of common-mode rejectionwhile doubling the signal [44] (45 is also the angle most sensitive to smallpolarization changes). In split-signal mode (one-half of the quadrant minusthe other half), the polar Kerr effect is subtracted by symmetry. The longi-tudinal Kerr effect remains because the sense of rotation is opposite for thek-vector parallel or antiparallel to the in-plane magnetization, and the twodetector halves see correspondingly opposite intensity shifts (via the Thom-son). Subtracting the split signal of one quadrant from the other retains thecommon-mode rejection inherent in the simple polar detection system. Sinceall data are collected from the quadrants simultaneously, the three decoupledcomponents of magnetization at the surface are captured at the same time.

The use of this dual-quadrant scheme presumes a highly symmetricalbeam profile so that each quadrant receives the same “quarter” of the beam.This necessitates spatial filtering for lasers with poor transverse-mode pro-


Fig. 2. Layout for experiments wherein the transient magnetic excitation is drivenby a transistorized current pulser, synchronized to the mode-locked laser (opticaland electronics schematic)

files, such as the cavity-dumped dye lasers. A single-mode-fiber spatial filtergave better results than a pinhole filter when a dye source was used in ourexperiments.

Time-resolved vector magnetometry is now also being performed using thesecond-harmonic generation (SHG) magneto-optic Kerr effect (MOKE) [45].Both the SHG efficiency and the SHG polarization rotation (and ellipticity)are monitored simultaneously, the former by simply photon counting and thelatter by locking to photoelastic modulation of the same photons, to givetransverse and longitudinal SH-MOKE effects, respectively.

2.4 System Operation

A schematic diagram for the entire system is shown in Fig. 2, including theoptical and electronic layouts. Details of the particular parts of the systemare discussed below. The electronic pulses are synchronously triggered fromthe laser pulses and gated at a lock-in frequency to encode phase sensitiveinformation directly on the magnetic state of the sample. This particulargeometry creates a transient external magnetic field H(t) that is strongerthan the static external biasing field Hdc provided by the permanent magnet.

For synchronization, a small part of the laser beam typically is split offand directed to a fast photodiode (e.g., ThorLabs DET210) which triggers the


current pulser via a variable electronic delay generator (Stanford ResearchSystems DG535.) Older, actively mode-locked lasers have an rf synchroniza-tion output which may also be used; some passively mode-locked lasers with-out such a master clock replicate this output by using a built-in photodiode.With the pulsed semiconductor diode laser, one obtains the simplicity of trig-gering the probe pulse in the same manner as the electrical current excitation.Kerr lens passively mode-locked Ti:sapphire lasers may be phase locked toan external oscillator through feedback control on the laser cavity length.

The maximum trigger rate of the SRS electronic delay generator is 1MHz.Therefore pulse picking or cavity dumping of the modelocked laser pulse trainis required to reduce the pulse repetition frequency to this level. When us-ing electronic delay and electronic pulsers, the minimum propagation delaythrough all of the electronics is of the order of 100 ns and requires an addi-tional measure to achieve temporal synchronization with the optical probepulse. The probe pulse can be delayed an equivalent amount (e.g., by propa-gation through a length of optical fiber), or additional electronic delay maybe inserted until the current pulse is actually synchronized with the laserpulse immediately following that by which it was triggered. (The pulse-to-pulse jitter in the mode-locked laser pulse train is less than a few picosecondsand is not a limiting factor.)

Individual sources of jitter can be progressively eliminated from the sys-tem, according to the time resolution required. The electronic delay stage isvery convenient, particularly when delay ranges of 10 ns or more are needed,but adds jitter of 50 ps or more (increasing with total delay). Alternatively,with an optical delay line to control the timing and a fast photodiode to trig-ger the pulser, the jitter remains very low (as small as 1 ps for the PicosecondPulse Labs products, for example.) In this case, it is recommended to bypassas much of the internal delay in the pulser as possible (it can be reduced toless than 20 ns) and to delay the probe pulse sufficiently for synchronization.

Finally, trigger jitter is fully absent when one uses photoconductiveswitches to generate the current transients. Because the limiting repetitionrate is often determined by the delay and/or pulse generator, another advan-tage of using an optical delay line for timing control and a photoconductiveswitch for pulsed excitation is that pulse repetition frequencies may be in-creased to levels limited by the relaxation times of the samples. This maxi-mum duty cycle will optimize the signal-to-noise ratio. Some high repetitionrate electronic devices exist commercially or have been custom manufactured,including pulse generators and electronic delay generators capable of highertrigger rates but smaller maximum delays (Jan Schaapman, at the Universityof Alberta, has made one based on the Analog Devices AD9500BQ chip.)

One can often extract a majority of the information of interest very effi-ciently in raster scanned-mode. In many cases of spatially nonuniform dynam-ics (when “spot” measurements of the time-dependent magnetization aloneare not informative enough), there is nevertheless enough symmetry in the


problem that most of the spatial structure may be captured by raster scan-ning along just one or two particular cutlines across the sample. Most of theinformation can then be distilled into two-dimensional images mapping themagnetization as a function of time delay and of position along these lines.

If a large number of images scanned in two spatial dimensions is required,the data acquisition can sometimes be accelerated by an “adaptive” steppingprocedure. For example, the pixel dwell time can be a variable determined ateach location after inspection of a parameter such as the reflected intensityor the magnetization (time averaged for a short interval). If the parametercheck shows that the location is of interest, the magnetization can then beaveraged for a longer period or saved in a time series of points for analysislater (e.g., averaging and noise analysis.)

One weakness of our time-resolved scanning Kerr effect microscopes isthat they measure changes in magnetization, instead of the magnetizationdirectly. Therefore, one must add information about the initial state of mag-netization, to extract the actual time-dependent magnetization. Because theinitial magnetization will not always be devoid of spatial features and poten-tially might even change during the course of a repetitive measurement [46],it must be borne in mind that this is a potential hazard.

The temptation to measure changes in magnetization is well known; itallows one to remove polarization-dependent artifacts that do not originatewith the magnetization of the sample. For example, in wide-field Kerr imag-ing, a reference image of a magnetically saturated sample (containing nodomain walls) is often subtracted from images acquired in other fields, to ob-tain a crisper representation of the domain configurations [47,48]. In raster-scanned time-resolved experiments, we have opted most frequently to mod-ulate the magnetic excitation, subtracting the state in which no excitationis present from that with excitation. The excitation is easily modulated onand off at kilohertz frequencies (above the 1/f knee of the system noise) byinterrupting of the train of trigger pulses. An optical chopper in the pumpbeam accomplishes this when a photoconductive switch is used to drive themagnetic excitation. Chopping the split-off trigger beam has the same effectwhen an electronic trigger originating in a fast photodiode is used. For anactively mode-locked or other (including pulsed diode) laser synchronized toan external source, used in combination with an electronic current pulser,the train of trigger pulses is easily “chopped” using an rf switch controlledby a TTL square wave.

The advantage of this approach is that the optical component of the signalarises entirely from changes in the magnetization of the sample. The litho-graphic conductors used to generate the transient fields at the sample aretoo small for their fringing fields to cause detectable Faraday rotations in thefocusing optics. Depolarization effects such as the collimating cross of themicroscope objective also have no signal at the locking frequency. There area couple of disadvantages to chopping the magnetic excitation in the above


manner, however. One disadvantage is that the high-frequency componentsof the excitation pulses can radiatively couple to the detector side electronics,giving rise to a background “pickup” on the signal (also modulated and hencedetected by the lock-in.) Careful wiring layout and grounding can minimizethis effect, but it tends to be very sensitive to small changes in the configu-ration. Fortunately the pickup can be characterized and subtracted througha measurement with the optical probe beam blocked. Complete electromag-netic shielding of the sample housing should eliminate the effect, but we havenot adopted such a measure because of other constraints it would place onthe operation of the microscope.

The other drawback of chopping the excitation pulse train is subtler andis associated with the avalanche-transistor pulsers. The propagation delaytime through such a pulser has a small dependence on the repetition rate,explained by the manufacturers as stemming from a duty cycle dependenceon the junction temperature in the transistor and its subsequent effect on theonset of the avalanche. The repetition rate of the pulser is modulated whenthe trigger pulse train is chopped. The net jitter induced by toggling the gen-erator between near-megahertz and near-zero repetition rates on millisecondtimescales can be more than 100 ps, dominating the rise time of the fastestpulsers.

For avalanche-transistor pulsers, then, the modulation scheme should al-low the pulser to operate at a constant repetition rate. Microwave switches,that can handle the high amplitude and bandwidth characteristics of our exci-tation pulses and operate at kilohertz rates for long periods, are not availablecommercially. This is why the most obvious solution, namely, to gate the exci-tation between the pulser and the sample, is not an option. However, it is alsopossible to access a reference point corresponding to the absence of excitationby modulating the relative delay between the optical and electronic pulses. Inthe scheme illustrated in Fig. 3, one sees the optical probe pulse and sampleexcitation response shifted out of temporal alignment every second half cycleof the square wave modulation waveform. It is almost as straightforward toimplement this idea as it is to gate the trigger pulses. We use a microwaveswitch to route the trigger pulse alternately through two different lengths ofdelay cable, tying the two arms together with a pulse combiner prior to thetrigger input to the avalanche pulser. In systems locked to a master clock,the same result is accomplished by phase modulation of the excitation [49].The delay-time or phase-modulation approach has the additional advantagethat the unwanted background pickup that troubled the earlier setup is com-pletely absent; there is no modulation of the amplitude of any componentparasitically coupled to the detectors.

If this delay-time approach has one disadvantage, it is its reliance ontriggering from the next pulse in the optical train. This is not from the pulse-to-pulse jitter (which, as mentioned, should be only a few picoseconds) butbecause the system is less flexible to change. If one wants to use a different


Fig. 3. Timing diagram for delay-time modulation of the magnetic signal, one ofthe schemes used with lock-in detection to isolate changes in magnetization withhigh signal-to-noise. In this case, the trigger signal to the current pulse generatoris toggled between two different lengths of delay cable, one resulting in a magneticresponse which overlaps in time with the arrival of the probe pulse at the sample,and the other phased so that the probe senses only the equilibrium state betweenpump pulses

rep rate, for example, one would have to change the delay cable length toaccommodate the new temporal distance between pulses in the train. Oneother minor concern is the number of current pulses that overlap the opticalprobe pulse compared to the number that do not during a single modulation.Close inspection of the figure reveals that there will be one extra currentpulse that does not overlap than does (i.e., for 401 pulses during a singlemodulation cycle, 200 will be “on”, and 201 will be “off”). For small reprates or fast modulation, this could become a considerable difference.

Sample stability is a prime concern. Unlike feedback-scanned probes,a method is not so simple for feedback to keep a very stable position forthe long scans that are inherently needed for a raster-scanned technique.Feedback piezo-driven translation stages are an option. One could also con-sider a built-in auto-focusing algorithm that would periodically correct drift.Thermal drift is probably the main concern in this. Another possible solutionwould be feedback temperature control on the flexure stage.

2.5 New Opportunities in Optical Imaging

One of the most promising current directions in which to move, within thearena of time-resolved optical imaging, is toward single-shot capture of mag-netic dynamics. Many factors can contribute to variations in the dynamics ofreversal from pulse to pulse, details that would fall within the spatiotemporalresolution of the experiment. As a consequence, some information is lost when


the measured response is averaged across many pulses. This final limitationhad already been overcome by Kryder and Humphrey through their pioneer-ing use of a Q-switched ruby laser (pulse energy > 50 mJ!) as the cameraflash for high-speed single-shot imaging [18]. This distinguishing feature ofthe KH system still has not been replicated today.

Single-shot imaging naturally also requires full-field image capture. Thetrade-offs between raster-scanned and full-field imaging involve many ad-ditional factors. A considerable inefficiency of the raster-scanned mode isexposed by the very small fraction of the available optical power actuallyused, at least when the source is a mode-locked laser. With our mode-lockedTi:sapphire, an average power of the order of milliwatts remains after pulsepicking (at a sub-MHz repetition rate). This power is usually reduced by a fac-tor of the order of 100 before being brought to a sharp focus on a sample,to avoid permanent damage to the surface. Substrate selection and samplepreparation are also important variables in the equation. The Silva groupuses films grown on sapphire substrates for second-harmonic measurements,reporting that they can withstand higher flow from the laser. Not surpris-ingly, Kryder and Humphrey encountered related difficulties with their pulsedruby laser, speculating that damage originated at the interface between themagnetic film and the blooming layer used to enhanced longitudinal Kerrcontrast. In optical studies at the air-bearing surface of recording devices,empirical evidence suggests that it is advantageous to remove the carbonpassivation layer.

A move to full-field image capture would allow using all of the probelight without damaging the sample. Unfortunately, there is not as eleganta solution to the problem of acquiring full-field longitudinal Kerr imagesas the quadrant detectors provide in the raster-scanned mode. Recordingintensity changes through nearly crossed polarizers is a possibility, but thisreintroduces some inefficiency in using the optical power (requiring an evenbrighter laser source again) and increases again the attendant risk of sampledamage. Nonlinear Kerr measurements may represent a superior solution.

Higher optical pulse energies are easily available today through cavity-dumping [32] or, more dramatically, regenerative amplification [50] of mode-locked titanium:sapphire lasers. An increase in the amplitude of opticallydriven transient magnetic field pulses will be an attendant benefit to stemfrom the use of these sources. This will open the way to investigations ofdynamics in harder magnetic materials, of faster switching, and of large-tipping angle FMR. Older, laser-based methods of generating intense, shortmagnetic field pulses (e.g., 1 ns, 60 T driven by massive CO2 laser pulses [51])should be revisited using these contemporary, small-scale laboratory sources.Significantly higher currents from semiconductor photoswitches can also beexpected.

An excellent compromise between full-field, single-shot and stroboscopic,raster-scanned imaging would be “single-shot, raster-scanned” (one pulse per


pixel) signal acquisition. This would capture both the reproducible responseand the frame-by-frame variations through a series of shots at each position,and the information could easily be built up into images as well. As thestructures under investigation evolve in size toward the limiting resolution ofthe microscope, the meaningful size of a full-field image shrinks toward onepixel in any case, but the utility of single-exposure capture remains.

3 Discussion of Representative Results

In this section, we survey recent experiments which exploit both the temporal-and spatial-resolving capabilities of ultrafast laser probes of magnetic mate-rials. We do not focus on the extensive work using pump-probe methods tomeasure spin dynamics timescales where spatial resolution is not an essentialfactor, as discussed in detail in contribution by Zhang. Similarly, nonlinearmagneto-optics is a growth industry exploiting the very high peak intensityin ultrashort pulses and has broader applications to static imaging and tononimaged time-resolved studies, both of which we will not consider further.These topics, of course, can overlap significantly with ultrafast microscopy. Inmany cases, tight focusing is required to couple well to a small structure, tocreate higher intensities to enhance nonlinear response, or to drive a systemfar enough from equilibrium to detect the response to an optical excitation.In other cases such as transient field measurements on electrically conductingmaterials, additional requirements for high-speed response (such as picosec-ond eddy current decay) can be satisfied only in microscopic geometries.

3.1 Relaxation, Resonance, and Small Angle Excitation

Figure 4 shows an example of spin relaxation measured on a thin film of EuS,using the experimental geometry illustrated in Fig. 1 with the transmissionline fabricated on an InP substrate [23]. The photoconductively generatedcurrent transient is effectively a step function in this case, because the carrierrecombination lifetime in the InP is long relative to the spin lifetime beingmeasured. The timescales are in fact very well separated in this example. Therise time of the transient field at the sample is no more than a few picoseconds,as dictated by the laser pulse width and by dispersion of the rising edgeof the current transient through propagation along the transmission line.The rising edge of the magnetic response seen in the time-resolved Faradayrotational signal is dictated by the longitudinal spin relaxation time of theEuS, determined 120 ps. The slow decay of the signal is dictated by the920-ps carrier recombination time of the InP photoconductive switch, underthe 10V applied bias voltage. This example also nicely illustrates portabilityto low temperatures and/or vacuum environments. The entire structure wasin cold helium gas at 11K during the measurement.

The decay time constant of the current was determined by an autocor-relation measurement using the “sense” line, with the probe light refocused


between the central and sense conductors to create a transient photoconduc-tive tap into the current waveform. (It is assumed that the time constant forthe sense contact is the same as that in the current-launching switch, whichis accurate at low enough bias that carrier sweep out plays a negligible role inthe decay of the current.) In the recent experiments of Hicken and Wu [52],a surface-mounted resistor has been added in series with the transmission linestructure for convenient monitoring of the decay time with a fast oscilloscope.The rise time of the current was not measured directly in the work of Fig. 4.An upper limit of a few picoseconds was estimated from measurements onother samples having faster spin relaxation times.

The center-to-center distance between the conductors was 100µm for thepulse field generator used in the measurements reported in Fig. 4. The max-imum transient magnetic field strengths we obtained from such structuresare in the 10 kA/m range, corresponding to peak pulsed current amplitudesless than 1A. The thin film samples were on separate substrates, diced intosmall pieces of characteristic linear dimension 0.5mm and placed facedownon the pulsing structure. At these relatively large sizes, the sample materialsare restricted to poor conductors or to very thin films, so that the magneticresponse is not limited by the eddy current screening. For a good conduc-tor having resistivity in the µΩcm range and a film thickness of the order of100 nm, disk-shaped samples must be restricted to diameters of less than a fewmicrometers so that eddy current decay times are less than a few picosec-onds. Efforts to scale down the geometry to the smallest possible dimensions

Fig. 4. Sample relaxation data from a spin relaxation measurement on a thin filmof EuS [23]


offer the advantage of stronger transient fields. This extends the range ofthe technique to measuring spin relaxation times in materials having weakermagneto-optic response [53].

Ferromagnetic resonance data can also be seen in the time domain [54].One instance is for a YIG substrate using a geometry similar to Fig. 1 butwith the transmission line formed into a tight, one-loop coil to maximize thetransient magnetic field amplitude [39]. The actual photoconductive current isgenerated on a separate semiconducting substrate with interdigitated fingersand connected to the transmission line with indium bonds. This experimentwas done as an example of the use of YIG as a fringing field sensor forcharacterizing of fast currents (cf, Sect. 2.2). The resonant oscillations for this(large size) insulating ferromagnet should follow very closely the analyticaldescription (known as the Kittel equation) for an infinite layer ferromagneticmaterial, as long as there is enough static external field to sweep out thedomain structure and dominate anisotropy. The Kittel formula is as follows:

ω = γ√|H|(|H |+ 4πMs)

with γ the gyromagnetic ratio, H the applied field, and Ms the saturationmagnetization of the sample. Clearly, the oscillation frequency will be in-creased for increasing applied field. For an applied external field of 100mT,the quickly rising current pulse contains a strong Fourier component at theresonant frequency and excites large oscillations in the YIG. In an exter-nal field of 1.5T, the Kittel frequency is essentially above the bandwidthcontained in the rising edge of the current pulse, and little or no energy istransferred to the precessional mode. The tip of the magnetization vectorinside the material, instead, follows the field direction “parametrically”, thatis, the spins are sufficiently stiffened to follow the “slow” change of the mag-netic field quasi-statically. The result is a precise temporal mapping of thecurrent pulse in the line. As long as a strong enough field can be applied, thebandwidth of this sensor can, in principle, be increased indefinitely (thougha strong enough field would shrink the tipping angle of the magnetizationvector below the experimental detection limit). The oscillations even givea built-in measure of the bandwidth of the sensor device, which is about50GHz in this case.

An interesting pulsed ferromagnetic resonance experiment was done byBauer et al. in which tailoring of the magnetic field pulse was used to sup-press the resonant oscillations [55]. A similar experiment was done by Craw-ford et al. using thin film Permalloy [38]. A thin film BIG sample on topof a microstrip transmission line was used in the stroboscopic time-resolvedKerr experiment because of its low damping and low-frequency resonance.The sample was placed in a static magnetic field of 4Oe along the transmis-sion line axis. A variable length, 2 ns rise and 2 ns fall time, current pulsedown the line created a transient magnetic field of 0.6Oe in the transversedirection (perpendicular to the static field). This field excited the BIG magne-tization into resonance with a maximum of 9 tilt angle of the magnetization


vector from the equilibrium direction. The component along the transversedirection was detected by the longitudinal Kerr effect.

The results are shown in Fig. 5. The magnetization ringing upon termi-nation of the pulse is alternately enhanced or suppressed, depending on thefield pulse length (note that, although ringing can be tailored after termi-nation, the behavior is not affected during application of the pulse). Thiscan be understood upon inspection of the magnetization vector position asa function of time. The vector will oscillate between two positions, one farfrom and the other close to the static equilibrium position. If the pulse isterminated when the vector is close to the static position, pulse terminationsimply leaves the vector aligned along the net field with no energy for furtherprecession. Alternatively, if the field pulse is viewed in the frequency domain,the alternating enhancement and suppression correspond to the times whenthe (resonant) Fourier components of the rising and falling pulse edges arein phase and 180 out of phase, respectively.

Figure 6 brings us back into a microscopy regime. The figure shows a snap-shot at one instant of the spatial magnetization response of an 8µmPermalloy(80/20 NiFe) disk during transient magnetic field pulse induced ferromag-netic resonance excitation [56,57]. This example clearly shows the need formicroscopy at this time-resolved level because the response has incrediblyrich spatial structure. The 100- nm thick disk (with eddy current decay times

0 10 20 30 40 50 60

pulse

2.0 ns3.2 ns4.4 ns6.4 ns8.6 ns

10.8 ns12.2 ns13.8 ns15.2 ns17.8 ns20.0 ns

M(a

rb.units)

x

Time (ns)

H = 4 Oe

9 deg

static

max

Fig. 5. Temporal evolution of the x component of the magnetization Mx measured(at the center of the sample) during and after field pulse excitation for differentpulse durations Tpulse, as indicated. The beginning of the pulse launch is at t = 0ns. All measurements are performed in the center of the sample


Fig. 6. Snapshot during ferromagnetic resonance of an 8m Permalloy disk afterdouble excitation: experiment and simulation [56]

in the ps regime) is immersed in a 250-Oe external field (in the horizontaldirection) in this case and subjected to a transient magnetic field induced bya one-loop coil/indium/photoconductive switch system. The transient fieldhas a peak height of 120Oe with a fast rise and exponential decay temporalshape but with a reflection (at the indium-coil interface) as fast and almost aslarge as the initial rise. Because of this, we can consider that there is a “dou-ble excitation” acting on the sample (only the rise of initial and rise of thereflection contain enough bandwidth to transfer energy to the precessionalmodes which is what is important here). The snapshot is taken on the secondresonant peak (from the point of view of the center of the sample) after thedouble excitation. The rich structure is due to this extra energy pumped intothe system before it has time to relax.

The system (spatially) can be modeled but must be modeled numerically(analytical modeling would have to assume uniform demagnetizing energyand, a priori, uniform magnetization behavior). The temporal shape of thetransient field pulse (sensed at high biasing field using the Permalloy itselfas the sensor) is interpolated to create the tipping pulse field used in thesimulation and allows assigning of the same (arbitrary) time zero.

The time-domain simulation is based on the Landau–Lifshitz–Gilbertequation, taking into account Zeeman, exchange, and demagnetizing energyterms. The magnetostatic field was calculated using fast Fourier transform(FFT) methods. LLG equations were integrated using a fourth-order Runge–Kutta method with a variable stepper. Exchange is calculated only betweennearest neighbors and on boundaries the inner nearest neighbor cell is repli-cated in place of the missing outer neighbor cell.

Shown on the left in the figure is the snapshot of experimental data forthe out-of-plane (polar) component of magnetization at t = 1316 ps. The grayscale is normalized to give maxima and minima of the data close to whiteand black, respectively, and is much smaller than the absolute scale frompositive saturation to negative saturation. On the right is the snapshot ofthe simulation data for the out-of-plane magnetization at t = 1322 ps and is


similarly normalized to give near white and black response for max and min(though some of the high spatial frequency pixels near the left and right edgesare clipped to white and black). The 2-D excitation profile is reproduced ex-tremely well by the simulation. In [57], the authors had conjectured that thenonuniform response was due to unsaturated initial conditions prior to ex-citation. The argument was that demagnetizing energy would have causedflaring of the spin direction through the thickness of the sample (100 nm is10–20 exchange lengths), especially at the left and right edges, giving nucle-ation conditions for the excitation. The demagnetizing energy is definitelythe crucial factor in the nonuniformity shown; however, the picture of flaringthrough the thickness is most likely not the most important element. Morelikely, as evidenced by excellent comparison with a 2-D simulation (that hasuniform magnetization through the thickness), flaring of the magnetizationin the plane, due to free poles at the left and right edges, is the main causeof a nonuniform profile.

This can be understood very well upon inspection of the figure. The staticfield (in the horizontal direction) holds the sample in an unsaturated stateprior to excitation; spins everywhere but in the vicinity of the left and rightedges align with the static external field. As the out-of-plane magnetic fieldexcitation pours energy into the system, the “unsaturated” spins offer thequickest avenue of relaxation and lead the (k = 0) resonant oscillation. Thisset of images, occurring on the second peak after double excitation, has al-lowed time for the strongest Mz deviation to “propagate” toward the middleof the structure (shown as two white peaks). Because the sample would havebeen in a fairly nonuniform state when the second pulse added new energyto the system, a richly structured picture develops. It is not hard to imaginethat, during this second pulse, a considerable manifold for high k-vector spinrelaxation exists, especially at the edges, and, indeed, the simulation showshigh spatial frequencies of the Mz response in this vicinity. Even the experi-mental data at the edges can easily be imagined as a blurring of this high-kresponse due to finite spatial bandwidth of the measurement.

Though there may be some avenue for high-k relaxation, as has been men-tioned, the gray scale has been greatly exaggerated in these images (with Mz

less than a few percent of Msat), so the actual angles of oscillation are small.This may be why the simulation can track the experiment so well (see, forinstance, [58] for discussion of small angle versus large angle motions in thephenomenological theoretical footing).

Hicken and Wu have also done work on FMR in a metallic ferromag-net (Fe) [52] using a Fig. 1 style apparatus. Their focused probe beam is offnormal incidence by about 20, and they accounted for a combined longitu-dinal and polar Kerr effect in their signal by including both components intheir analytical calculation of the Landau–Lifshitz–Gilbert equation (assum-ing Kittel-like response). Performing a one-dimensional spatial scan across


the transmission line (but reporting only two spot locations), they show rea-sonable agreement of the oscillation amplitude and phase with calculation.

3.2 Dynamic Reversal and Large-Angle Excitation

We can move fully to the large-angle regime by considering dynamic magne-tization reversal experiments. Silva and co-workers have recently been doingconsiderable work in this area. Most of their reported results are for excitationof a Permalloy (80/20 NiFe) bar on a triaxial microstrip line. The bar (withlong and anisotropic axes parallel to the transmission line) is placed on topof the center conductor strip of the same width. Step and impulse excitationsare then introduced in the plane in the hard (transverse) direction.

For initial work, the net transverse magnetization response of the wholesample is determined by an inductive sampling technique [59,60]. The chang-ing transverse magnetization gives rise to a changing flux that encircles thecenter conductor line and creates an electric field by Faraday’s law. Rota-tional times as short as 200ps are observed. They achieve good agreementwith a (single α) numerical simulation that takes demagnetizating fields intoaccount (discretized only in the transverse dimension) and show that thedemagnetizing conditions are important (when the simulation magnetizationis constrained to be uniform, the comparison becomes poor). They furtherexplore the nature of damping in Permalloy by comparing the inductive mea-surements with intense numerical fitting of the Landau–Lifshitz equations.The response for step excitations needs to be characterized by an anomaloustransient damping and the introduction of two separate α. It is suggestedthat this effect and a higher order precessional mode seen in the data areconnected to the large-angle rotations.

Time-resolved microscopy is brought to bear with second-harmonic mag-neto optic Kerr effect (SHMOKE) measurements [61,10]. The frequencies ofunderdamped precessional response from the inductive measurement and theSHMOKE measurement were slightly different. This was attributed to subtledifferences in the sample bulk and surface properties. This time, inductivedata fit well with a single damping parameter, whereas optical data requireda “two-regime” (two α) fit with Landau–Lifshitz.

Work was also done on high coercivity films [62,63], though without actualtime resolution. Thermally assisted switching was investigated in CoCr10Ta4

from 180 magnetization reversal due to nanosecond field pulses. A crossoverfrom exponential to logarithmic decay behavior occurs as pulse lengths exceed10 ns. This is attributed to nonequilibrium magnetization-driven relaxation(dynamic reversal) for short pulse lengths crossing over to metastable equi-librium and thermally assisted relaxation for longer pulse lengths.

Recent results from Silva et al. boast 5µm spatial resolution SHMOKEwith vector-resolved magnetometry [64]. A 1 cm × 1 cm × 50 nm Permalloysample is placed on top of their triaxial excitation line. They report thatthey find complicated intermediate states (or metastable states) that are


accessible only through dynamic excitations. For example, for an easy axis(Hk = 320A/m) external field of 80A/m, (transverse axis) pulse field of1.04 kA/m (200 ps rise-time), and zero hard-axis bias field, they see the angleof magnetization stabilize at 90 to the original direction for many nanosec-onds.

Koch and co-workers also investigated magnetization reversal in micron-sized structures. They measured sample response as a whole and modeledthe spatial excitation [65]. Hillebrands et al. numerically modeled Stoner-like magnetic particles subjected to short magnetic field pulses of varyingstrength, direction, pulse length, and shape [66]. Long pulses (compared tothe precession time) yield switching behavior still governed by the magneticdamping term, but for short pulses, switching is dominated by the details ofthe magnetic precession that allow the ability to control switching character-istics by pulse tailoring. Experimental work on this topic using time-resolvedMOKE showed that the precessional ringing could be turned on and off,depending on the pulse length [55,16].

Representative work from our group in the large-angle regime [67,68,69]is shown in Fig. 7 in an 180 dynamic magnetization reversal experimentwith a stitched-in SEM image of a sample on a transmission line. The samplestructure (slightly darker than the line) is 15 nm thick (order of exchange

Fig. 7. Geometry of a reversal experiment for microstructure excitation


length), 80/20 Permalloy, with an easy axis along the horizontal. The insetshows a close-up micrograph of the structure, a “nominal” rectangle withactual dimensions of 11µm× 3.5µm. The layout is similar to that in Fig. 3.The current pulse travels up the 40-micron wide transmission line creatingan 160-Oe transient magnetic field H(t) in the plane of the sample. For thisgeometry, there is a 4-Oe/ µm gradient of field out of the plane as well.A permanent magnet provides a variable strength (40Oe up to 160Oe) dcmagnetic field Hdc in the opposite direction. This configuration makes −Hdc

(to the left) the net external field for the initial state. During the 10 ns ofpulse on, the net external field becomes Hnet = 160−Hdc (to the right). Forexample, for Hdc = −100Oe, Hnet = +60Oe during the pulse (this is thesituation for Fig. 8). The pulse rise is 0.5 ns, and the fall is about 1.0 ns.

Fig. 8. Experimental measurement and numerical simulation of 180 dynamic mag-netization reversal in a Permalloy microstructure [70]. The 10 panels correspond to10 different times over the course of a turn on/turn off dynamic reversal process.The x component of magnetization (Mx) is in the switching direction. My is forthe transverse direction, and Mz is for the out-of-plane direction


Some results on the structure from Fig. 7 are shown in Fig. 8, includ-ing comparison to simulation. The experimental data include all three com-ponents of magnetization at the surface with approximately 0.7µm resolu-tion from a 0.75NA air objective. The 2-D discretized numerical simulationtakes care to mimic the sample conditions as closely as possible; the actualsample shape is incorporated by inspection of the SEM micrograph and thez-gradient of the tipping pulse H(t) is included as well. The data are normal-ized to full magnetization in that black corresponds to Mi/Ms = −1, whiteto Mi/Ms = +1 (where i is for x, y, and z, for the Mx, My, and Mz images,respectively) and gray is zero. The ten sets of images are representative ofthe reversal process, though the exact time step is not always the same forthe experiment and simulation.

The reversal process agrees very well between the experiment and simu-lation, which might be considered surprising in light of the large angle andstrongly driven reversal. The first thing to note is that the reversal startsfrom the ends and propagates toward the middle. This is consistent with thenotion that there are small end domains to start the nucleation process. Alsoevident, however, is a stripe-like development along the length (as evidencedmost clearly by the zigzag My response) which is consistent with the no-tion of spin-wave excitation in the x direction. The simulation sees higherorder waves (and higher order structure in general) because of the better“resolution”. The experimental images appear like a “blurred” version of thesimulation.

The small bite in the lower right of the structure is obviously playinga role as a demagnetization energy “takeoff” point (for both experiment andsimulation) as seen in time steps 4 and 5. It should be mentioned, however,that the simulation was also performed with a perfect edged 10× 2 rectangleand showed a stripe-like development very similar to the reversal (albeit moresymmetrical than images shown here) and of approximately the same spatialfrequency. Also of note is the fact that the reversal (which really occurs overroughly 2 ns) is locking into a y configuration that does not travel. After“meeting in the middle”, the switched areas push out to the top and bottomedges to complete the reversal. Looking at steps 7 through 10, we see thatthe back reversal is more abrupt and more complicated than the front. Theabruptness can be explained because the net external field driving the switchis larger (100 instead of 60Oe). Both effects probably have to do with the lackof saturation just prior to “pulse off” (step 7), leaving a highly structuredenergy landscape for back nucleation. It is not hard to believe that one wouldsee extensive energy transfer to high k-modes in the spin manifold with suchan initial condition, and in fact, the simulation looks more “gray” as thereversal proceeds than for the front. This optical blurring illusion of the eyeis closely related to what is happening in the experiment.


3.3 Magnetic Device Characterizationand Nonrepetitive Processes

Stochastic behavior is sometimes observed during the course of stroboscopicimaging. For example, very infrequent (mHz rate) random swapping of thenucleation point between different corners was seen in early time-resolved im-ages of magnetization reversal in a rectangular Permalloy element [69]. Ran-dom behavior of a subtler nature or behavior on a wider range of time-scales,becomes observable as the signal averaging time decreases through reductionsof laser and detector noise (we have not yet reached optical shot noise limitedsensitivity.) An example is shown in Fig. 9 from recent measurements of fluxreversal in a magnetic recording device [71]. All of the information representedin the image is acquired during a single raster scan, and each frame shows thesame 6× 6µm area. Panel (a) is a reflected intensity optical image, in whichthe magnetic pole tips “P1” and “P2” appear bright. Panel (b) shows theperpendicular magnetization at one instant during a reversal sequence, cap-tured by a time-resolved polar Kerr effect measurement. Because this deviceexhibited significant random magnetic noise at certain drive currents, a timerecord of 400 separately sampled measurements of the average magnetizationwas collected for each pixel in the image (using a 10-ms lock-in time con-

Fig. 9. Noise spectroscopy: information about nonrepetitive processes [71]


stant) and Fourier transformed to produce a noise spectrum. A contour plotof the integrated noise amplitude between 0.25 and 6.25Hz is superimposedupon the Kerr image in Fig. 9c. What is striking here is that these carefulobservations of noise reveal additional spatial structure not found directly inthe stroboscopic Kerr image. The noise structure can be seen more clearlyin Fig. 9d, where the integrated noise amplitude has been rendered with alinear gray scale, and the contour lines trace constant Kerr amplitude at 4%of saturation to delineate the pole tips. The structure is suggestive of ran-dom switching between nearly degenerate domain configurations in the P2pole tip during the course of the stroboscopic reversal measurement. Theseresults represent a small step in the direction toward the single exposure-perpixel imaging goal discussed in Sect. 2.5.

Combining time-resolved magnetic imaging measurements with magneto-optical current probing yields a nice characterization tool for magnetoelec-tronic devices. Figure 10 is taken from a case where the noninvasive high-speed characterization of the current was particularly useful, due to a designin which the final stage current amplifier was integrated onto the cantileverfrom which the head was suspended (to achieve a higher signal bandwidthfor the device.) The figure shows both the current waveform and the inducedmagnetization response in an example where two successive transitions (bits)were separated by 10 ns. The short interval between the current and magneti-zation pulse edges is the gyromagnetic delay of the device, and the overshootson the current are intended to force a faster flux rise-time. Writing speeds upto 500Mbit/s were demonstrated with this technique, and the simultaneouscurrent interrogation made it possible to show that this limit was imposedby the speed of the driver, not by the magnetic response of the head itself.

Heidmann et al. also use time-resolved magnetic microscopy in charac-terizing thin film magnetic recording devices [72]. As well as measuring thetime-resolved flux response (polar mode) at the gap in different geometries,

Fig. 10. Analysis of current on suspension recording head performance


time-resolved flux propagation in the yoke is observed (with longitudinal Kerrmicroscopy) and found that it is a mixture of wall displacement and mag-netization rotation. The combination of static wide-field Kerr images andconsideration of micromagnetic structure, along with time-resolved informa-tion on the yoke and air bearing surface, are used to try and understandthe nonlinear behavior of the flux reversal. A ripple domain wall structure inthe FeN yoke is cited as a key factor in degraded performance. The authorsgo on [73] to investigate nonlinear transition shifts in high-frequency mag-netic heads caused by transient flux effects associated with high data rates(as opposed to nonlinear effects caused by areal density). Previous opticaltime-resolved recording head work also includes [74,75,76,77,78].

4 Summary and Prospects

Roughly speaking, the ultrafast magneto-optic microscopes in operation to-day can replicate the Kryder–Humphrey camera with more than 10000 timesfaster time resolution, less than 1 ps, and with almost 100 times finer spatialresolution, approaching 100 nm (keeping in mind, of course, the very impor-tant distinction that this is not yet being accomplished in single-exposureimaging). The factors limiting these parameters are the speed of the tran-sient magnetic field sources and the spatial resolution of the optical systems.In terms of further improvements that may be possible in a system of thekind we have described in Sect. 2, at these values one is reaching a point ofdiminishing returns. On the spatial resolution side, large improvements in theefficiency of near-field optical imaging schemes are required before they cancompete with the magneto-optic signal-to-noise ratio one obtains in far-fieldKerr imaging, although as new tip geometries continue to be explored, thesituation is still very hopeful.

4.1 Solid Immersion Lens and Confocal Microscopy

To illustrate the promise of the solid immersion lens for higher spatial res-olution in magneto-optic imaging, in Fig. 11 we show a polar Kerr imageobtained with a truncated-sphere solid immersion lens or super-SIL [79] andusing another magnetic recording device as a resolution test specimen [80].The SIL was polished from a LASF9 fiber coupling sphere (Melles Griot)of refractive index n = 1.89, and the light at 633nm was nearly optimallycoupled in by a focusing objective of numerical aperture 0.55, only slightlygreater than 1/n. In the figure, a three-dimensional rendering of the Kerrsignal is shown, centered on the gap between the pole tips, with underlyingcontours of constant Kerr amplitude. The actual out-of-plane magnetizationof the device peaks very sharply (in opposite directions) right at the edges ofthe gap on either side, which is almost ideal for these tests – indeed if onlyone peak were present, the spatial resolution would be given directly from


its width. Panel 11a is the result obtained with the T-SIL on its own, andin Panel 11b the resolution has been further augmented by confocal filtering.The final spatial resolution is 220 nm (Rayleigh criterion), corresponding toa resolution of λ/2.9, relative to the wavelength. This is particularly excit-ing, given the room for further improvement through a combination of shorterwavelengths and higher refractive index SILs. The solid immersion lens alsoallows for full-field imaging (trading off resolution against field of view, ofcourse), and so stands as an excellent prospect for single-shot measurements,even in a superresolution mode.

4.2 Alternative Time-Resolved Magnetic Microscopies

With scanning probe microscopes, the rastered nature of image acquisitionitself restricts time-resolved experiments to studies of repetitive phenomena.However, the lack of the possibility of single-exposure imaging (in the absenceof very improbable, or at least very distant, developments such as dense ar-rays of parallel probes!) is balanced by the promise of much finer spatialresolution in ultrafast measurements. Ultrafast time-resolved scanning probemicroscopy projects began in earnest in the early 1990 s [81]. One of thepresent authors started an ultrafast STM effort in response to the obviousshortcomings of optical probes for spatially resolving nonequilibrium dynam-ics in superconductors [82].

Of the list of alternative magnetic microscopies offering superior limit-ing spatial resolution, all could potentially incorporate ultrafast time reso-lution. The techniques based on X rays, magnetic circular dichroism [83],and photoemission electron microscopy [84,85] are poised to make majorleaps forward with the concurrent development of ultrafast stroboscopic x raytechniques [86]. Among electron-based methods, stroboscopic electron beamtomography is by far the most developed [87]. The challenges faced by this ap-proach at high resolution arise from the reduction of sensitivity as a function

Fig. 11. T-SIL and T-SIL confocal microscope images of a magnetic recording headresolution test specimen


of decreasing interaction distance with the beam and from specimen prepara-tion. Ballistic electron magnetic microscopy [88] and spin-polarized STM [89]both offer slight promise for stroboscopic implementations, although eachadded feature (magnetic and temporal resolution) typically reduces the sig-nal current by a factor in the range 100 to 1000 and the image acquisitiontimes would be extraordinarily long. High-speed force microscopy has alsoemerged as another complementary tool in such investigations [90]. Oldermethods, including miniature Hall probes, magnetoresistive sensors, and in-ductive microloops [91], also continue to be very competitive possibilities.

Irrespective of the spatial resolution question, the physics potential ofstroboscopic magneto-optical imaging has been exploited very little to date.The key aspect here is the suitability of the approach for in-situ measure-ments in ultrahigh vacuum and at low temperatures. Efforts to address topicsincluding the dynamics of phase transitions and quantum magnetism are nowdeveloping in some laboratories.

Acknowledgements

We are indebted to Greg Ballentine for performing the micromagnetic simu-lations shown in this chapter and for help with the preparation of some of thefigures. We thank Abdul Elezzabi, Andrzej Stankiewicz, Geoff Steeves, andJames Stotz for their contributions at earlier stages. This work is supportedby the Natural Sciences and Engineering Research Council of Canada, withadditional assistance for the device work from the National Storage IndustryConsortium.

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Index

alternative magnetic microscopies, 120avalanche pulser, 104avalanche-transistor pulsers, 104

ballistic electron magnetic microscopy,121

CoCr10Ta4, 113confocal filtering, 120coplanar transmission line, 95, 96, 98current pulse generator, 105current pulses, 98current transient, 107

damping in Permalloy, 113damping parameter, 113demagnetization, 113, 116demagnetizing energy, 111, 112dilute magnetic semiconductor, 97domain wall width, 93dynamic excitations, 114dynamic magnetization reversal,113–115

dynamic reversal, 113

electronic delay generators, 102EuS, 107, 108excitation pulse, 104

ferromagnetic dynamics, 95, 100ferromagnetic resonance (FMR), 95,109–112

ferromagnetic sensor, 99flux reversal, 119fringing field sensor, 109

high-speed imaging, 95high-speed magnetics, 94

inductive data, 113inductive measurement, 113inductive method, 94inductive sampling, 113InP, 107

Kerr, 96, 100, 106, 112, 117–119Kerr imaging, 103, 118

Landau–Lifshitz equation, 113Landau–Lifshitz–Gilbert equation, 111,112

large-angle excitation, 113large-angle motion, 112large-angle rotations, 113

magnetic circular dichroism, 120magnetic damping, 114magnetic dynamics, 93, 94, 105magnetic excitation, 95, 103magnetic field pulses, 106magnetic microscopy, 118magnetic noise, 117magnetic precession, 114magnetic recording device, 119magnetic semiconductor, 97magnetization dynamics, 97magnetization response, 113magnetization reversal, 94, 98, 113,114, 117

magneto-optic activity, 93magneto-optic interaction, 96magneto-optic method, 94magneto-optic signal-to-noise ratio, 119magneto-optical imaging, 93, 99, 119,121

magneto-optical Kerr effect, 95, 97, 101magneto-optical measurement, 100

128 Index

magneto-optical response, 109magnetoelectronic devices, 118micromagnetic dynamics, 93microscopy, 110microwave, 94, 95microwave spectroscopy, 94microwave strip-line, 98

near-field optical imaging schemes, 119NiFe, 110, 113noise spectroscopy, 117noise spectrum, 118non-uniform response, 112nonequilibrium dynamics in supercon-ductors, 120

nonequilibrium magnetization, 97, 113nonlinear magneto-optics, 94, 96, 107nonuniform, 112nonuniformity, 112nucleation, 112, 116numerical fitting, 113numerical simulations, 113, 115, 116

optical imaging, 105

paramagnetic sensor, 99Permalloy, 110, 111, 113, 115, 117photoconductive current, 109photoconductive switch, 98, 102, 103,107, 111

photoconductivily, 95, 107photoemission electron microscopy, 120precession ringing, 114precession time, 114precessional mode, 109, 111, 113precessional response, 113pulsed excitation, 102pulsed laser sources, 99pulsed optical source, 95, 96pump-probe, 93, 96, 107

quadrant, 106quadrant detection system, 100quadrant photodetectors, 100quadrant sum, 100

recording head, 118relaxation time, 93, 96, 102resonance, 112

reversal, 105reversal process, 116

scanning electron microscopy, 95scanning probe microscopes, 120scanning tunneling microscope, 95second-harmonic generation, 101second-harmonic magneto optic Kerreffect, 113

sensor, 109SIL, 119, 120single-shot capture, 105single-shot imaging, 106small angle excitation, 107small angle motion , 112solid immersion lens, 100, 119, 120spatially inhomogeneous, 95spatially nonuniform dynamics, 102spatiotemporal, 93, 105spin dynamics, 107spin manifold, 116spin polarization, 97spin relaxation, 97, 107–109, 112spin–lattice relaxation, 97spin-flip scattering, 97spin-polarized STM, 121spin-wave, 93spin-wave excitation, 116spins, 112stochastic, 117stroboscopic, 93, 95, 96, 118, 121stroboscopic electron beam tomography,120

stroboscopic imaging, 93, 106, 117stroboscopic magnetic imaging, 95stroboscopic reversal measurement, 118switching, 94, 106, 114switching behavior, 114switching speeds, 94switching times, 94synchronization, 101–103

T-SIL, 120thin film magnetic recording devices,118

time domain, 109time-domain techniques, 95time-resolved, 93, 95, 96, 103, 105, 107,110, 114, 117–119

Index 129

time-resolved magnetic imaging, 94,118

time-resolved microscopy, 113

time-resolved recording head, 119

transient damping, 113

transient external magnetic field, 101

transient field, 98, 107, 109

transient field pulse, 111

transient magnetic excitation, 97, 101

transient magnetic field, 98, 108–111,115, 119

transient photoconductive tap, 108

transient-applied magnetic field, 96

transistor-based electronic pulsers, 99

transistorized current pulser, 101

transmission line, 98, 107–109, 113, 114

triaxial excitation line, 113

triaxial microstrip line, 113

ultrafast imaging, 93ultrafast laser, 93, 95, 107ultrafast magneto-optic microscope,119

ultrafast measurements, 120ultrafast microscopy, 107ultrafast optical imaging, 100ultrafast optical microscopy, 93ultrafast pump-probe, 97ultrafast STM, 120ultrafast stroboscopic x ray, 120ultrafast time resolution, 120ultrafast time-resolved scanning probemicroscopy, 120

ultrafast transient magnetic field, 96

X rays, 120

YIG, 109

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