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Broadband strip‑line ferromagnetic resonancespectroscopy of soft magnetic CoFeTaZrpatterned thin films

Jin, Tian Li; Nongjai, R.; Asokan, K.; Ghosh, A.; Aparnadevi, M.; Suri, P.; Piramanayagam, S.N.; Gupta, Surbhi; Kumar, Durgesh

2018

Gupta, S., Kumar, D., Jin, T. L., Nongjai, R., Asokan, K., Ghosh, A., et al. (2018). Broadbandstrip‑line ferromagnetic resonance spectroscopy of soft magnetic CoFeTaZr patterned thinfilms. AIP Advances, 8(5), 056125‑.

https://hdl.handle.net/10356/86295

https://doi.org/10.1063/1.5007943

© 2018 The Author(s) (published by American Institute of Physics). All article content,except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY)license (http://creativecommons.org/licenses/by/4.0/).

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AIP ADVANCES 8, 056125 (2018)

Broadband strip-line ferromagnetic resonancespectroscopy of soft magnetic CoFeTaZrpatterned thin films

S. Gupta,1 D. Kumar,1 T. L. Jin,1 R. Nongjai,2 K. Asokan,2 A. Ghosh,3M. Aparnadevi,4 P. Suri,4 and S. N. Piramanayagam1,a1School of Physical and Mathematical Sciences, Nanyang Technological University,21 Nanyang link, 637371, Singapore2Materials Science Division, Inter-University Accelerator Centre, New Delhi 11006, India3Data Storage Institute, A∗STAR (Agency for Science, Technology and Research),2 Fusionopolis Way, 08-01 Innovis, 138634, Singapore4Heraeus Materials, 1 Tuas South Street 3, Singapore 569059

(Presented 7 November 2017; received 4 October 2017; accepted 8 November 2017;published online 18 January 2018)

In this paper, magnetic and magnetization dynamic properties of compositionally pat-terned Co46Fe40Ta9Zr5 thin films are investigated. A combination of self-assembly andion-implantation was employed to locally alter the composition of Co46Fe40Ta9Zr5

thin film in a periodic manner. 20 keV O+ and 60 keV N+ ions were implantedat different doses in order to modify the magnetization dynamic properties of thesamples in a controlled fashion. Magnetic hysteresis loop measurements revealed sig-nificant changes in the coercivity for higher influences of 5 × 1016 ions per cm2.In particular, N+ implantation was observed to induce two phase formation withhigh and low coercivities. Broadband strip-line ferromagnetic resonance spectroscopyover wide range of frequency (8 – 20 GHz) was used to study the magnetizationdynamics as a function of ion-beam dosage. With higher fluences, damping constantshowed a continuous increase from 0.0103 to 0.0430. Such control of magnetic prop-erties at nano-scale using this method is believed to be useful for spintronics andmicrowave device applications. © 2018 Author(s). All article content, except whereotherwise noted, is licensed under a Creative Commons Attribution (CC BY) license(http://creativecommons.org/licenses/by/4.0/). https://doi.org/10.1063/1.5007943

I. INTRODUCTION

Investigation of soft amorphous magnetic films is a topic of interest for magnetic recording, spinelectronics, thin film transformers, magnetic inductors and microwave communication. In the field ofmagnetic recording, the soft magnetic films are useful in the writing heads and in the perpendicularmagnetic media for the purpose of writing operation.1,2 In the field of microwave communication,they are ideal candidates for high frequency harmonic noise suppression due to the ability of theirmagnetization to precess at a high frequency (GHz range).3,4 In particular, CoZr based system hasbeen investigated extensively for microwave communication in the past.3–6 In such a system, theaddition of elements such as Nb, Ta etc. is carried out to reduce the magnetostriction to nearly zeroand to stabilize the amorphous phase of the CoZr-based magnetic material.1 In particular Y. Endoet al.4 and G. Hao et al.5 studied the effect of composition variation and electrical pulse annealingrespectively, on high frequency magnetic properties. Interestingly investigations on CoZr based sys-tems, thus far, have mainly been focused on continuous films. Since the magnetization permeabilitylargely depends on the ferromagnetic resonance (FMR) frequency, lamination as well as compos-ites of CoZr and oxides has also been investigated as alternative to increase the FMR frequency.7–9

aCorresponding author: prem@ntu.edu.sg

2158-3226/2018/8(5)/056125/6 8, 056125-1 © Author(s) 2018

056125-2 Gupta et al. AIP Advances 8, 056125 (2018)

In the case of laminations, the magnetic and non-magnetic layers are stacked one over the other toimprove the high-frequency performance of the stack. Recently T. N. Kołtunowicz et al.9 discussed theFMR of “FeCoZr core – Al2O3 oxide shell” nanoparticles and observed the composition dependentbehavior.

As an alternative to lamination, we have attempted to investigate the high frequency performanceby local patterning in the lateral direction. In contrast to a continuous Co-based amorphous film withlaminations, the composition that is modulated periodically in the lateral direction is expected toperform differently. Simulation studies on patterned soft magnetic underlayer has been proposed toimprove recording performance of magnetic media.10 However, there are not many experimentalresults based on nanometer scale patterning.11 For example, Ikeda et al.11 explored high frequencyproperties by designing micro-patterned CoNbZr films of different feature size for band stop filterapplication. In order to understand the effect of patterning on a nano-scale, by inducing periodic com-position modulation, we employed a combination of self-assembly and high-energy ion-implantation.Self-assembly based on di-block copolymers can be effectively used to achieve periodic patterns ofvarying pitch over a magnetic layer.12 Ion-implantation of non-magnetic species (such as O and N)was carried out through the self-assembled mask patterns, in order to modulate the composition ofpresent Co-based amorphous films periodically. Such patterned Co-based amorphous films offer aninteresting system to study for high frequency applications. In addition, we have chosen a CoFeTaZrsystem instead of commonly studied CoZr, with a purpose of achieving higher magnetization (dueto the presence of CoFe), relative softness (i.e., low coercitive field Hc) and their low magnetizationprecessional damping. The understanding based on hysteresis loop and FMR investigations on suchcompositionally patterned CoFeTaZr films are reported in this paper.

II. EXPERIMENTAL DETAILS

Soft CoFeTaZr magnetic thin films were deposited on silicon substrates by DC magnetron sput-tering using a 2-inch Co46Fe40Ta9Zr5 alloy target. A high pressure deposition (50 mTorr) was usedto achieve a granular CoFeTaZr film, instead of a continuous film.12 Surface morphology of filmswere scanned by Atomic Force Microscopy. Figure 1(a) shows the microstructure of as-depositedpristine films in scanned area of 1 µm × 1 µm. The AFM shows the granular nature of CoFeTaZr filmas intended, where cluster size is found to be ranging from 10 to 60 nm. Self-assembly technique ofdi-block copolymer i.e. polystyrene (PS)-polydimethylsiloxane (PDMS) was employed to fabricatethe patterned mask of oxidized PDMS/SiO2 spherical dots on CoFeTaZr magnetic thin films as shownin Figure 1(b). For this purpose, commercially available PS-PDMS (88:22 weight fractions) di-blockcopolymer of molecular weight 54 kg/mol was mixed in Toluene and spin coated over magnetic thinfilms. Then solvent annealing was performed to enable the complete micro-phase separation betweenPS and PDMS.12 Subsequently the di-block copolymer film was reactive ion etched to remove thePS matrix as well as oxidize the PDMS nanostructure. AFM micrograph in Fig. 1(b) shows thehexagonally coordinated sphere morphology (average feature size ∼ 35 nm ± 2 nm) in PDMS mask.For implantation, 20 keV of O+ and 60 keV of N+ were chosen to incorporate the maximum fluence

FIG. 1. AFM micrograph of (a) as-deposited CoFeTaZr thin film and (b) Pattern mask of oxidized PDMS/SiO2 sphericaldots on CoFeTaZr magnetic thin film and (c) Ion profile obtained from TRIM on implanting 20 keV O+ and 60 keV N+ inCoFeTaZr thin film.

056125-3 Gupta et al. AIP Advances 8, 056125 (2018)

at about half the film thickness and to minimize the ion-beam mixing at the CoFeTaZr/Si interface.The range of implanted ions in the different layers was verified using the Transport of Ions in Mat-ter (TRIM) program from the SRIM software.13 The implantation profile obtained from TRIM for20 keV of O+ and 60 keV of N+ are shown in figures 1(c). It should be noted that maximum implan-tation fluence is centered either before or half the film thickness. However, slight spread of ions canbe seen in substrate in addition to the targeted layer in case of N+ implantation. The implantationwas performed at two fluences, i.e. 1 × 1016 and 5 × 1016 ions per cm2 through oxidized mask ofPDMS/SiO2 spherical dots.

In-plane magnetic hysteresis loops were traced using vibrating sample magnetometer. Ferromag-netic resonance (FMR) in field sweep mode was recorded using a vector network analyzer, connectedto co-planar waveguide (CPW) using 50 ohm coaxial cables and high-frequency connectors. Thesamples were mounted in flip-chip geometry such that external static magnetic field is applied in thein-plane direction.

III. RESULTS AND DISCUSSION

Figures 2(a) and 2(b) show the hysteresis loops measured along the in-plane direction beforeand after the 20 keV O+ and 60 keV N+ implantation respectively. As shown in the inset, pris-tine CoFeTaZr sample showed isotropic soft magnetic behavior with high saturation magnetizationMs = 1021 emu/cc and a coercive field (Hc) of 15 Oe. This value of Hc is smaller in comparison tothat in crystalline CoFe thin films, and is good for high frequency application.3,15 It may be seen fromfigure 2(a) that a low dosage of 1× 1016 O+ ions/cm2 did not induce significant changes in magneticbehavior as a marginal increase in remnant magnetization (Mr) is observed. However, dosage of5× 1016 O+ ions/cm2 led to visible increment in Mr as well as Hc attributed to compositionallymodified regions, acting as pinning centers and thereby increasing the coercivity.14

Surprisingly, 60 keV N+ implantation led to contrasting observations (shown in fig. 2(b)). In caseof small fluence of 1× 1016 N+ ions/cm2, Hc first decreased to 9 Oe while Mr showed a significantupsurge. At a higher fluence of 5× 1016 N+ ions/cm2, an evident enhancement in Hc with the highestvalue of 80 Oe is observed with a drop in Mr. Similar trend has also been observed for Ne, Xe implantedFeCo thin films in literature.13 Also, the squareness factor (Mr/Ms) is found to be improved with highimplantation dosage irrespective of the ion species.

Magnetization dynamics in the sample was induced by a microwave Oersted (dynamic) magneticfield hrf, which is generated by a microwave current (0 dBm, 8 - 20 GHz) flowing through the signalline of the co-planar waveguide and is maintained to be perpendicular to external magnetic field, asshown in inset of Figure 3. Figure 3(a) shows the typical FMR spectra for pristine CoFeTaZr thin film,recorded for different applied frequencies (8 GHz to 20 GHz). The external magnetic field was sweptup to 4000 Oe. Here scattering parameter S21 corresponds to ratio of absorbed microwave power atport 2 to the incident fixed microwave power of 0 dBm at port 1. The characteristic spectrum showeda complex line shape signal for all the applied frequency ranges where continuous decrease in the

FIG. 2. M-H Hysteresis loop CoFeTaZr thin films implanted with different fluences of (a) 20 keV O+ and (b) 60 keV N+

respectively. Inset shows the isotropic behavior of pristine sample.

056125-4 Gupta et al. AIP Advances 8, 056125 (2018)

FIG. 3. (a) Field sweep FMR spectra of pristine CoFeTaZr thin film, (b) Resonance field (open circle) as obtained fromexperiments and a curve fit by Kittel formula (solid line) and (c) Linear fit of field linewidth (∆H) versus applied frequency.Inset shows the sample location around the CPW in flip chip geometry.

absorption peak intensity is attributed to high frequency power losses. The obtained FMR signals as afunction of applied magnetic field were fitted with sum of derivative of symmetric and antisymmetricLorentzian functions as given in equation 1 to estimate the linewidth (∆H) and resonance field (Hres).16

S12 (H)=∑

n

(L

(∆H)2

(∆H)2 + (H − Hres)2+ D

∆H(H − Hres)

(∆H)2 + (H − Hres)2

)(1)

where L and D is the respective amplitude of symmetric and antisymmetric Lorentzian functions.The submission over the symmetric and antisymmetric component is performed with “n” defin-ing the number of ferromagnetic elements in the system. The different values of resonance field(Hres) and field line-widths (∆H) thus obtained are plotted in Figure 3(b) and 3(c) respectively.In Figure 3(b), the plot of applied frequency (ω) versus Hres is further fitted by Kittel formula,17

ω= γµO√

Hres(Hres + 4πMeff) to determine the effective magnetization (4πMeff) to be 1.25 T. Hereγ = gµB/h is the gyromagnetic ratio, g is the Lande’s factor, µB is the Bohr magnetron and h is thePlanck constant.

The other fundamental quantity that characterizes high-frequency response of a ferromagneticmaterial is the damping, which basically tells about intrinsic relaxation mechanism of the precessionalmotion of magnetization vector/dynamics. The dimensionless Gilbert damping coefficient (α) inaddition to extrinsic magnetic inhomogeneity (∆Ho) was determined from ∆H, according to theequation µO (∆H - ∆Ho) = αω/γ.15 Here linear increase in ∆H with applied frequency as seen infigure 3(c) indicates the Gilbert-type damping in CoFeTaZr thin films with damping coefficient of0.0103 which is found to be in agreement with the damping constant of CoFeZr.3 In addition, zero-frequency contribution, i.e. the intercept∆Ho corresponding to the inhomogeneous broadness is foundto be 2.89 mT. Similar measurements were also performed for 20 keV O+ implanted samples.

No significant change could be observed for low dosage of 1× 1016 ions/cm2. Figure 4(a) showsthe FMR spectrum of sample with 5× 1016 O+ ions/cm2 implantation for selected frequencies.

FIG. 4. Typical FMR spectra of CoFeTaZr thin film implanted with higher 5 × 1016 ions/cm2 fluence of (a) 20 keV O+ and(b) 60 keV N+ ion. Inset shows the M-H loop of N+ implanted film.

056125-5 Gupta et al. AIP Advances 8, 056125 (2018)

TABLE I. The magnetic and high frequency parameter of pristine and ion-implanted samples.

Species and Dose Hc (Oe) Lande g factor 4πMeff (T) α ∆Ho (mT)

Unimplanted 15 2.18 1.25 0.0103 2.891 × 1016 O+ ions/cm2 13 2.18 1.30 0.0112 1.175 × 1016 O+ ions/cm2 40 2.18 1.29 0.0137 2.411 × 1016 N+ ions/cm2 9 2.18 1.18 0.0184 0.735 × 1016 N+ ions/cm2 80 2.18 1.35 0.0430 1.10

Broadness in all microwave absorption spectra is quite evident in comparison to pristine sample.In contrast, two resonance peaks were observed in FMR spectrum of 5 × 1016 N+ ions/cm2 implantedCoFeTaZr samples. We attribute this to the possibility of two magnetic phases in the films after implan-tation (Figure 4(b)). This is further supported by the observation of two reversals in the M-H loops of5× 1016 N+ ions/cm2 implanted CoFeTaZr thin film samples (shown in inset). In case of N+ implantedsample, the fitting of complex FMR signal has been performed using equation 1 and keeping n = 2owing to double peak spectra.

Typical magnetic and high frequency properties of pristine as well as implanted samples extractedfrom the M-H hysteresis and FMR measurements are tabulated in Table I. There is clear increase inHc for higher fluences of implantation while damping showed the steadily increase with implantationfluence. It may be noted that N+ implantation has induced larger impact on the magnetic propertiesin comparison to O+ implantation. Owing to higher energy of the 60 keV N+ implanted ions, weproposed the significant change is may be due to enhanced scattering and modification in the spin-orbit interaction. As far as Gilbert damping is concerned, we observed monotonic increase in thedamping with increase in ion-dose, found to be matching with literature.18 It is important to mentionthat higher damping is also desirable for increasing the switching rate to ensure coherent reversal inmagnetic elements in magnetic storage devices.18 However, the estimated homogeneous linewidthcontribution is observed to be scattered and requires further understanding. The results from thisstudy indicate that the proposed method of localized patterning is an alternative way to tailor themagnetization dynamics of soft magnetic films.

IV. CONCLUSION

In conclusion, static as well as dynamic magnetic properties of patterned CoFeTaZr filmsdeposited by DC magnetron sputtering on silicon substrates were investigated using VSM and FMR.We have shown, by means of 20 keV O+ and 60 keV N+ implantation, that soft magnetic properties ofCoFeTaZr thin film can be tuned over a large range. Implanted O+ ions increased the coercivity as wellas the remnant magnetization. Samples with a higher fluence of 5 × 1016 N+ ions/cm2 implantationshowed two magnetic phases, resulting in a significant enhancement in the coercivity and dampingconstant as well. This combined methodology of self-assembly and ion-implantation for local controlof damping and static magnetic properties is expected to be useful in the field of spintronics as wellas microwave communication.19

ACKNOWLEDGMENTS

The authors gratefully acknowledge MOE AcRF Tier1 grant RG163/15 and NRF-IIP grant((NRF2015-IIP003-001) for the partial financial support. We duly acknowledge the low energy ionbeam facility (LEIBF) at Inter University Accelerator Centre (IUAC), New Delhi.1 J. Shi, Y. Yang, H. K. Tan, S. N. Piramanayagam, C. B. Lim, H. L. Seet, S. L. Ho, and J. F. Hu, Phys. Stat. Sol. PSS-RRL

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