Giant Magneto Impedamce

Impacts of amorphous and crystalline cobalt ferrite layers on the giant magneto-impedance response of a soft ferromagnetic amorphous ribbonD. Mukherjee, J. Devkota, A. Ruiz, M. Hordagoda, R. Hyde, S. Witanachchi, P. Mukherjee, H. Srikanth, and M.H. Phan Citation: Journal of Applied Physics 116, 123912 (2014); doi: 10.1063/1.4896583 View online: http://dx.doi.org/10.1063/1.4896583 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/116/12?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Tailoring magnetic and microwave absorption properties of glass-coated soft ferromagnetic amorphousmicrowires for microwave energy sensing J. Appl. Phys. 115, 17A525 (2014); 10.1063/1.4868329 Giant magneto-impedance effect in amorphous ferromagnetic wire with a weak helical anisotropy: Theory andexperiment J. Appl. Phys. 113, 243902 (2013); 10.1063/1.4812278 Stress-induced magnetic hysteresis in amorphous microwires probed by microwave giant magnetoimpedancemeasurements J. Appl. Phys. 113, 17A326 (2013); 10.1063/1.4798278 Enhanced giant magneto-impedance effect in soft ferromagnetic amorphous ribbons with pulsed laser depositionof cobalt ferrite J. Appl. Phys. 113, 17A323 (2013); 10.1063/1.4795802 Permeability and giant magnetoimpedance in Co 69 Fe 4.5 X 1.5 Si 10 B 15 ( X=Cr , Mn, Ni) amorphous ribbons J. Appl. Phys. 89, 7218 (2001); 10.1063/1.1359226

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Impacts of amorphous and crystalline cobalt ferrite layers on the giantmagneto-impedance response of a soft ferromagnetic amorphous ribbon

D. Mukherjee, J. Devkota, A. Ruiz, M. Hordagoda, R. Hyde, S. Witanachchi, P. Mukherjee,H. Srikanth, and M. H. Phana)

Center for Integrated Functional Materials, Department of Physics, University of South Florida, Tampa,Florida 33620, USA

(Received 5 May 2014; accepted 16 September 2014; published online 26 September 2014)

A systematic study of the effect of depositing CoFe2O4 (CFO) films of various thicknesses

(d 0600 nm) on the giant magneto-impedance (GMI) response of a soft ferromagnetic amor-phous ribbon Co65Fe4Ni2Si15B14 has been performed. The CFO films were grown on the amor-

phous ribbons by the pulsed laser deposition technique. X-ray diffraction and transmission electron

microscopy revealed a structural variation of the CFO film from amorphous to polycrystalline as

the thickness of the CFO film exceeded a critical value of 300 nm. Atomic force microscopy evi-

denced the increase in surface roughness of the CFO film as the thickness of the CFO film was

increased. These changes in the crystallinity and morphology of the CFO film were found to have a

distinct impact on the GMI response of the ribbon. Relative to the bare ribbon, coating of amor-

phous CFO films significantly enhanced the GMI response of the ribbon, while polycrystalline

CFO films decreased it considerably. The maximum GMI response was achieved near the onset of

the structural transition of the CFO film. These findings are of practical importance in developing

high-sensitivity magnetic sensors. VC 2014 AIP Publishing LLC.[http://dx.doi.org/10.1063/1.4896583]

I. INTRODUCTION

The giant magneto-impedance (GMI) effect of a soft fer-

romagnetic material is of growing research interest for its

potential applications to the fabrication of highly sensitive

and cryogen-free magnetic sensors.13 GMI is a large change

in the ac impedance of a ferromagnetic conductor subject to

an external dc magnetic field.4 The effect has been mostly

observed in rectangular and cylindrical geometries, such as

ribbons,46 films,2,7 and micro-wires2,8 of different soft ferro-

magnetic materials. Among them, Co-based amorphous rib-

bons with nearly vanishing magnetostriction (k 0) havebeen reported to exhibit GMI effects with a high degree of

field sensitivity and are therefore promising candidate mate-

rials for making advanced magnetic sensors.911 For a given

ribbon of thickness 2t, the complex impedance ZR jX(where R and X are resistance and reactance, respectively; jis the imaginary unit) is given by ZR jX

Z Rdcjktcothjkt; (1)

where Rdc is the dc resistance and k (1 j)/dm is related tothe skin depth dm

ffiffiffiffiffiffiffiq

plTf

qof the ribbon. Here, q and lT are

the resistivity and transverse permeability of the ribbon,

respectively, and f is the frequency of an ac current. At low

frequencies (f kHz) where the skin effect is weak, the pri-mary contribution to lT and hence to the magneto-impedance (MI) comes from inductive voltage. However, at

moderate frequencies (f 1 MHz), the skin effect is strongand both R and X contribute to the MI.8,11 At higher frequen-cies, the ac current is confined to the sheath of the ribbons

surface so that the skin effect is very strong. In this fre-

quency range, dc resistance dominates the GMI and the sur-

face of the ribbon is very sensitive to its electric and

magnetic environment.3 A small modification of physical

properties on or near the surface of the ribbon could thus

lead to a large alteration in its GMI response.

A controlled engineering of the surface of a soft ferro-

magnetic ribbon has proved useful in enhancing the GMI

effect and its field sensitivity (g).1217 For example,Taysioglu et al.,12,13 Peksoz et al.,14 Laurita et al.,16 andChaturvedi et al.17 reported the enhanced GMI response ofan amorphous Co-based ribbon when coated with copper and

zinc oxide, diamagnetic organic thin film, cobalt, and carbon

nanotubes, respectively. Recently, we have achieved a large

enhancement in the GMI response of a Co-based amorphous

ribbon coated with a 50 nm thick CoFe2O4 (CFO) film using

the pulsed laser deposition (PLD) technique.18 The improved

effect was attributed to the reduction of the surface rough-

ness of the ribbon and the closure of magnetic flux paths due

to the CFO layer. On the other hand, it has been noted that

the crystallinity and magnetic properties of a CFO film

grown on an amorphous substrate (e.g., SiO2/Si) varied sig-

nificantly with film thickness.1921 The structure of the CFO

film varied from amorphous to polycrystalline as the film

thickness exceeded a critical value.21 Since GMI is a

surface-related phenomenon, this structural transformation

of a CFO film grown on a Co-based amorphous ribbon upon

variation in CFO film thickness is expected to have distinct

impacts on the GMI response of the ribbon.

To elucidate the influence of the coating layer crystallin-

ity on magneto-impedance, we have performed a thorough

study of the structure, magnetic properties, and GMI

effect of a commercial Co65Fe4Ni2Si15B14 amorphous

a)Author to whom correspondence should be addressed. Electronic mail:

[email protected]

0021-8979/2014/116(12)/123912/8/$30.00 VC 2014 AIP Publishing LLC116, 123912-1

JOURNAL OF APPLIED PHYSICS 116, 123912 (2014)


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ribbon coated with CFO films of various thicknesses

(d 0600 nm) by PLD. A careful analysis revealed that thestructure of the CFO film changed from amorphous to poly-

crystalline as the film thickness exceeded 300 nm. This crys-

tallization of the CFO film was shown to coincide with a

change in the trend of maximum GMI ratio with film thick-

ness. The CFO coating enhanced the GMI response of the

ribbon for d 300 nm while reducing it for d> 300 nm.Since both coating and annealing effects occurred simultane-

ously during PLD, our study demonstrates the usefulness of

an in-situ annealing and coating method based on PLD forimproving the GMI response of soft ferromagnetic amor-

phous ribbons for high-performance sensor applications.

II. EXPERIMENT

A. Growth of CFO films on amorphous magneticribbons

Co65Fe4Ni2Si15B14 ribbons with a thickness of 15 lm(Metglas

VR2714 A) were prepared by a rapid quenching

method. Cobalt ferrite (CFO) coatings with nominal thick-

nesses of 50 nm, 100 nm, 200 nm, 300 nm, 400 nm, and

600 nm were deposited on the ribbon pieces of width 2 mm

via PLD. The CFO films were deposited at 450 C and theincrease in the film thickness (50 nm 600 nm) corresponded

to that of the annealing time (8.33100 min). A compressed

powder target of CFO was ablated using an excimer (KrF)

laser of wavelength 248 nm and operating at 10 Hz while

providing an energy density of 2 J/cm2 at the target surface.

The distance between the heater element or substrate holder

and the target was 4 cm. Under these conditions, the growth

rate was measured to be around 0.1 A/pulse. In a typical dep-

osition process, first two ribbons were attached to the heater

element with one of the ribbons covered completely with a

shadow mask to provide a control sample. Once a base pres-

sure of 1.0 106 Torr was reached, the heater element wasraised to 450 C. It was critical to attain high vacuum in thedeposition chamber before heating the ribbons, in order to

prevent their surface oxidation. If oxidized, the ribbons

became brittle and crumpled on touching. After the deposi-

tion, the chamber was kept under vacuum as the heater block

was very slowly cooled to room temperature (2 h). To under-

stand the growth mechanism of CFO film on an amorphous

substrate, the CFO coatings were also deposited on amor-

phous SiO2/Si (100) substrates (dimensions: 1 cm 1 cm 0.5 mm) under the same conditions as those on the ribbons.

B. Sample characterization

The interfacial microstructure in the PLD-deposited

CFO/SiO2/Si layered structures was analyzed using a high-

resolution transmission electron microscope (HRTEM) (FEI

Tecnai F 20 S-Twin TEM). The sample for cross-sectional

HRTEM analyses was prepared by milling a 5 lm 10 lmrectangular strip with 100 nm thickness from the sample sur-

face using a focused ion beam (FIB) (JEOL 4500 FIB/SEM)

and Pt welding it to a Cu TEM grid. The phase formation and

crystallographic orientation of the films on the ribbons were

analyzed using X-ray diffraction (XRD) with Cu Ka radiation

(Bruker D8 Focus Diffractometer with Lynx Eye position

sensitive detector). Peak shifts due to sample misalignment

were accounted for while performing the XRD scans. The

surface morphologies were observed using an atomic force

microscope (AFM) (Digital Instruments III). The chemical

composition analysis was performed using energy dispersive

spectroscopy (EDS) (Oxford Instruments, INCA x-sight). The

magnetic properties of uncoated and CFO-coated ribbons

were measured by a vibrating sample magnetometer (VSM).

C. Magneto-impedance measurements

MI measurements of all uncoated and CFO-coated rib-

bon samples were performed by the four-point measurement

technique using an HP4192A analyzer with a constant ac

current of 5 mA along the ribbon length of 5 mm over a fre-

quency range of 0.113 MHz and in the presence of axial dc

fields up to 6120 Oe. The details of the measurement systemcan be found elsewhere.22 The GMI ratio (DZ/Z) and its fieldsensitivity (g) in the samples were calculated as according to

DZZ% ZH ZHmax

ZHmax 100; (2)

g 2 DZ

Z

max

DH; (3)

where Z(H) is the impedance of the samples in the presence ofan axial dc field H and Hmax (120 Oe) is the magnetic fieldthat saturates the Z. DH is the full width at half maximum(FWHM) of a field dependent GMI curve at a given frequency.

III. RESULTS AND DISCUSSION

In order to understand the growth mechanism of CFO

films on Co65Fe4Ni2Si15B14 amorphous ribbons, we first

examined the growth of the CFO layer on the widely used

amorphous substrate SiO2/Si (100) under identical condi-

tions, the results of which are shown in Fig. 1. In contrast to

the thin and readily deformed amorphous ribbons, the

0.5 mm silicon-based substrates provided the necessary me-

chanical strength for the ion-beam milling process required

for the HRTEM sample preparation. Consequently, micro-

structural analysis was performed on CFO/SiO2/Si, which

can be assumed to exhibit a similar morphology to the CFO

deposited on the Co-based ribbons subject to the same

growth parameters. Unlike the deposition of CFO on single-

crystal substrates, such as Si (100),19 Al2O3 (0001),19 SrTiO3

(100),23 and MgO (100),23 using PLD, for which the crystal

lattice of the substrate surface assisted the nucleation of pol-

ycrystalline CFO nano-islands during the initial growth

phase,20 the use of an amorphous substrate prevents their for-

mation. This is evident in the cross-sectional HRTEM

images shown in Figs. 1(a) and 1(b) for the 50 nm thick CFO

film grown on amorphous SiO2/Si. From this figure, it can be

seen that although the CFO layer makes a sharp interface

with the substrate, the presence of a thin (3 4 nm) layer of

SiO2 (observed in all images as a result of surface oxidation

of single crystal Si (100) substrates) prevented a direct inter-

face between CFO and the underlying Si (100) surface,

123912-2 Mukherjee et al. J. Appl. Phys. 116, 123912 (2014)


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eliminating a preferred direction for the formation of CFO.

As a result, the initial CFO layers did not exhibit any long-

range polycrystalline structure and were mostly amorphous

in nature. However, as more and more flux of ablated species

arrived at the heated substrate, the adequate substrate tem-

perature possibly resulted in the crystallization of the subse-

quent layers. In Figs. 1(a) and 1(b), a well-defined crystal

structure with continuous sharp lattice fringe spacings is

observed far from the substrate, consistent with the face-

centered cubic CFO crystal structure. The crystal planes of

CFO as marked in this figure were identified from the lattice

spacings. Also within each grain boundary (marked by white

dashed lines in Figs. 1(a) and 1(b)), the grains exhibited

single-crystalline nature with a size distribution of

1030 nm, suggesting that the CFO coatings were composed

of single-crystal nano-grains.24 The overall polycrystalline

nature of the CFO coatings is evidenced from the selected

area electron diffraction (SAED) pattern in Fig. 1(c) obtained

near the interface. EDS spectra collected from different

zones of the amorphous CFO layer (Fig. 1(d)) revealed a

stoichiometric composition with the atomic % Co:Fe ratio of

1:2 (within an error limit of 0.01 atomic %). Similar charac-

teristics within the CFO layer grown on amorphous

Co65Fe4Ni2Si15B14 ribbons are inferred from the above

analysis.

Figure 2 shows the XRD patterns of CFO-coated mag-

netic ribbons for various thicknesses of the CFO layer start-

ing from 0 nm (uncoated) up to 600 nm (denoted as

CFO-0 nm, CFO-50 nm, CFO-200 nm, CFO-300 nm, CFO-

400 nm, and CFO-600 nm, respectively). For CFO-coated

ribbons with d 0200 nm, the XRD patterns showed a typi-cal amorphous behavior with a broad hump peak at low

angles from 20 to 30. No crystalline peaks were detectedin the scans within the resolution limit of the XRD.

However, staring from CFO-300 nm, one can observe the

appearance of CFO peaks along with the amorphous back-

ground of the ribbon. We define dc 300 nm as a criticalthickness at and beyond which the CFO film is of detectable

FIG. 1. (a) and (b) Cross-sectional

HRTEM images captured at different

locations along the interface of 50 nm

thick CFO coating deposited on amor-

phous SiO2/Si (100) substrates under

the same conditions as CFO coated rib-

bons using PLD; (c) typical SAED pat-

tern obtained near the interface of the

CFO coating on SiO2/Si substrate; (d)

representative EDS spectrum obtained

from the amorphous CFO layer show-

ing stoichiometric composition within

error limit of 0.01 atomic percent.

FIG. 2. XRD patterns of CFO coated amorphous ribbons grown for various

thicknesses of the CFO layer staring from 0 nm (uncoated), 50 nm, 200 nm,

300 nm, 400 nm, and 600 nm, denoted as CFO-0 nm, CFO-50 nm, CFO-100 nm,

CFO-200 nm, CFO-300 nm, CFO-400 nm, and CFO-600 nm, respectively.



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polycrystalline phase. The peaks that appeared match with

the CFO face-centered cubic lattice with a space group Fd-

3 m (227). The XRD peak intensities increased as the thick-

ness of the CFO layer was increased. This implied that after

a critical thickness (dc 300 nm), crystallization of CFOoccurred on the amorphous ribbons. On the other hand, XRD

scans for the just annealed ribbons (with no CFO coating)

exhibited an amorphous like behavior irrespective of the

annealing time. These results indicated that while the ribbons

remained amorphous in nature, the structure of the CFO film

transformed from amorphous to polycrystalline with an

increase in the film thickness from 0 to 600 nm.

Figure 3 shows the AFM 3-D images of the (a) uncoated

ribbon, (b) CFO-50 nm coated ribbon, (c) CFO-300 nm coated

ribbon, and (d) CFO-600 nm coated ribbon with the z-heights

of 25 nm, 50 nm, 50 nm, and 250 nm, respectively. The scan

areas for all samples were 5 lm 5 lm. From Figs. 3(a) and3(b), it is evident that there is an increase in the surface rough-

ness with a 50 nm thick CFO layer on the ribbon as compared

with the surface of the uncoated ribbon. As the CFO layer

thickness is increased, the increase in the surface roughness is

observed in Fig. 2. Due to the large surface features for CFO-

600 nm, a z-height of 250 nm was required for imaging. We

had earlier reported a similar trend in surface roughness for

CFO thin films grown on Si substrates using PLD.19

In order to visualize the enhanced surface roughness

with thicker CFO layers, representative AFM images

acquired under similar magnifications are compared in Fig.

4. The comparison of these images clearly suggests that the

thicker the CFO film, the rougher the surface. The root mean

square surface roughness (Rrms) values increased from

5.2 nm for CFO-50 nm (Fig. 4(a)) to 10.4 nm for CFO-

600 nm (Fig. 4(b)). Such structural and morphological varia-

tions are shown below to have significant impacts on the

magnetic properties and GMI response of the ribbon.

To assess the effect of CFO coating on the magnetic

properties of the ribbon, the magnetic hysteresis M(H) loops

of the uncoated and CFO-coated ribbons were recorded at

300 K. Figure 5 shows the normalized M(H) curves of these

samples. It can be seen that the magnetic hysteresis of all the

samples was minimal and the magnetization reached satura-

tion at small applied fields, indicating their soft

FIG. 3. AFM 3-D images of (a)

uncoated ribbon, (b) 50 nm thick CFO

coated ribbon, (c) 300 nm thick CFO

coated ribbon, and (d) 600 nm thick

CFO coated ribbon, respectively. The

scan areas are 5 lm 5 lm. The z-heights are (a) 25 nm, (b) 50 nm, (c)

50 nm, and (d) 250 nm, respectively.

FIG. 4. AFM 3-D images of (a) 50 nm thick CFO coated ribbon and (b)

600 nm thick CFO coated ribbon, respectively, shown on the same scan area

of 2 lm 2 lm and z-height of 100 nm.



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ferromagnetic characteristics. Relative to their uncoated

counterparts, the CFO-coated samples exhibited slightly

larger susceptibility (see Fig. 5(a), for example, for the

uncoated and CFO-300 nm coated ribbons). As the thickness

of the CFO film was increased from 50 nm to 600 nm, the

shape of the M(H) curves remained almost unchanged (Fig.

5(b)) while the change in coercive field (HC) with the CFOfilm thickness was noticeable (Fig. 5(c)). As one can see in

Fig. 5(c), the HC had the smallest value for the CFO-50 nmsample and it increased with increasing the thickness of the

CFO film. A considerable increase in HC observed for CFO-

coated ribbons with d 300 nm was likely associated withthe transition from the amorphous to polycrystalline struc-

ture of the CFO film.21

In order to understand the impact of CFO coating on the

GMI response of the ribbon, magnetic field and frequency

dependences of GMI ratio (DZ/Z) of the uncoated and CFO-coated ribbon samples were studied systematically. Figure 6

shows, for example, the magnetic field and frequency de-

pendence of DZ/Z for the uncoated ribbon and CFO-coatedribbons with film thicknesses of 50 nm, 200 nm, and 400 nm.

It can be observed that low frequency MI curves of the sam-

ples exhibited a single-peak behavior, whereas high fre-

quency MI curves possessed a double-peak behavior. The

observed single- and double-peak behaviors are associated

with the longitudinal and transverse alignment of magnetic

anisotropy with reference to the external field direction.1,25

It is also noted in Fig. 6 that for all samples investigated,

with increasing frequency, the maximum MI ratio (i.e., [DZ/Z]max) first increased, reached a maximum at a particular fre-

quency f0 (often defined as a characteristic frequency), andthen decreased for higher frequencies (f> f0). This trend canbe interpreted by considering the relative contributions of

domain wall motion and magnetization rotation to the trans-

verse permeability and hence to the MI.2 At very low fre-

quencies f< 1 MHz (t< dm), [DZ/Z]max was relatively smalldue to the dominant contribution of the induced magneto-

inductive voltage to the measured magnetoimpedance. In the

range 1 MHz f f0 MHz (t dm), the skin effect was dom-inant, hence a higher [DZ/Z]max was observed. Above f0,[DZ/Z]max decreased with increasing frequency. This isbecause, in this frequency region, the domain wall displace-

ments were strongly damped owing to eddy currents, thus

contributing less to the transverse permeability and hence

less [DZ/Z]max. A similar explanation can also be applied tothe frequency dependence of g shown later in Fig. 8.

It has been reported that heat treatment26,27 and laser

annealing28 of a soft ferromagnetic amorphous ribbon could

have a significant impact on the MI response of the ribbon.

The MI effect and g are highly sensitive to the annealingtemperature and time. As we noted above, the PLD tech-

nique allowed a simultaneous variation in the thickness of

the CFO film and the annealing time. Since the CFO films of

different thicknesses were grown on the amorphous ribbons

at a fixed temperature of 450 C with the same rate of0.1 nm/s, thicker CFO films took longer time to grow. In

addition to the effect of CFO coating, the annealing effect

(e.g., annealed at 450 C for various times) should alsoimpact the MI response of the CFO-coated ribbon samples.

Therefore, to isolate the effects of CFO coating, the MI

effect of each coated ribbon was compared with its corre-

sponding control samplethe ribbon annealed simultane-

ously but completely masked during deposition. Figure 7

shows the frequency dependence of [DZ/Z]max for selectedsamples of uncoated (control) and CFO-coated ribbons. As

one can see in this figure, the [DZ/Z]max reached peak valuesof 33.3%, 47.7%, and 20.7% for 50 nm, 300 nm, and 600 nm

thick CFO-coated ribbons, respectively, as compared with

17.3%, 48.6%, and 28.9% for their corresponding controls.

These observations are of particular interest. In the case of

FIG. 5. Magnetic hysteresis loops of uncoated and CFO-coated ribbons.



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the control samples, it appears that with increasing the

annealing time, [DZ/Z]max first increased (Fig. 7(a)), reacheda maximum at a certain time (Fig. 7(b)), and then decreased

for longer annealing time (Fig. 7(c)). Since the control sam-

ples were annealed at 450 C, well below their crystallizationtemperature of 550 C (as provided by MetglasV

R

), heat treat-

ment could not affect their crystallinity. However, annealing

in vacuum could cause relaxation of domain structure to first

increase lT and hence [DZ/Z]max up to its maximum valuefor short annealing time and then negate the effects for fur-

ther annealing. A similar trend has been reported previously

for annealed Co-based28 and Fe-based27 amorphous ribbons.

It is also interesting to note that relative to their control coun-

terparts, the coating of a 50 nm CFO film (Fig. 7(a)) showed

the greatest increase in the MI response among the CFO-

coated ribbons. The positive effect of the CFO coating on

MI response became weaker as the CFO thickness was

increased from 50 nm to 300 nm, and was actually negative

for thicker CFO thicknesses (d> 300 nm). These results canbe explained by considering the magnetic flux closure and

surface irregularities due to the deposition of a thin CFO

layer.18 Since the CFO film was amorphous in the CFO-

50 nm/ribbon bilayer (as seen in the XRD), the presence of

the CFO film could allow the magnetic field to penetrate

through the ribbon better than in its control (uncoated) coun-

terpart, thus enhancing lT and [DZ/Z]max. In the case of theCFO-300 nm/ribbon bilayer, however, the control was

observed to have the maximum lT and the CFO film waspartially crystallized, the flux closure was not sufficient to

further increase lT and hence [DZ/Z]max. The CFO-600 nm/ribbon bilayer showed presence of polycrystalline CFO

phases that resulted in the formation of rigid and random

magnetic domains (as evidenced by the increase in Hc in Fig.6(c)). This possibly resulted in the decrease in lT and conse-quently the [DZ/Z]max.

To better illustrate the effect of CFO coating, we plotted

in Fig. 8 the frequency dependence of [DZ/Z]max (Fig. 8(a))and g (Fig. 8(b)) for various CFO-coated ribbon samples.The highest (47.7%) and lowest (20.7%) values of [DZ/Z]maxwere observed at f 0.9 MHz and 2 MHz for CFO/ribbonbilayers with the CFO thicknesses of 300 nm and 600 nm,

respectively. [DZ/Z]max values 33.3%, 42.8%, and 25.3%

FIG. 6. Field and frequency dependences of the GMI ratio for (a) an uncoated annealed ribbon and coated ribbons with CFO layer thicknesses of (b) 50 nm, (c)

200 nm, and (d) 400 nm.



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were obtained at f 1.5 MHz for the bilayers with the CFOthickness of 50 nm, 200 nm, and 400 nm, respectively. The

values of g calculated for 50 nm, 100 nm, 200 nm, 300 nm,400 nm, and 600 nm CFO-ribbon bilayers at 1 MHz were

determined to be 0.84, 0.98, 2.54, 2.31, 1.08, and 0.85%/Oe,

respectively. It can be seen that although the highest value of

[DZ/Z]max was achieved for the CFO-300 nm sample, theCFO-200 nm sample showed the largest g among the CFO-coated samples. The reason for this was that the crystalliza-

tion of CFO in CFO-coated ribbons with d 300 nm broad-ened the FWHM of GMI curves and consequently reduced g.Since the CFO-300 nm ribbon and its control had similar val-

ues of [DZ/Z]max in the frequency range of 0.113 MHz (Fig.7(b)), the highest peak of [DZ/Z]max observed in Fig. 8(a) forthis sample could be attributed to the annealing effect, rather

than the CFO coating. We recall that the surface roughness

of the CFO film increased gradually with increasing the film

thickness, and the crystal structure of the CFO film changed

from amorphous to polycrystalline at dc 300 nm. In con-nection with the GMI analyses, it appears that the largest

GMI ratio and field sensitivity of CFO-coated ribbon sam-

ples were achieved near the onset of the structural phase

transition of the CFO film. A similar trend was also reported

by Coisson et al.,29 but for the structural phase transition of aCo-based ribbon itself. We note that we used the commercial

Co65Fe4Ni2Si15B14 amorphous ribbons to fabricate the CFO/

Co65Fe4Ni2Si15B14 bilayer structures for demonstrating our

concept, so the GMI ratios obtained for the present CFO-

coated ribbon samples are smaller than those previously

reported for some Co-based amorphous ribbons.1 If the opti-

mized Co-based ribbons are used, however, one should

expect to achieve higher values of GMI ratio for their CFO-

coated ribbons. An important consequence that emerges

from the present study is that we demonstrate the possibility

of using an additional coating layer of CFO to tune and opti-

mize the GMI response of a soft ferromagnetic amorphous

ribbon through controlling the crystallinity and thickness of

this magnetic coating layer, and that PLD is an excellent

technique for this purpose.

IV. CONCLUSIONS

We have systematically investigated the effect of deposit-

ing CoFe2O4 films of various thicknesses (d 0600 nm) onGMI response of an amorphous Co65Fe4Ni2Si15B14 ribbon.

We have observed a structural transformation of the CFO film

FIG. 8. Frequency dependence of (a) [DZ/Z]max and (b) field sensitivity forCFO-coated ribbons with varying CFO thickness.

FIG. 7. Frequency dependence of MImax for CFO-coated ribbons and their

control (uncoated) ribbons for CFO layer thicknesses of (a) 50 nm, (b)

300 nm, and (c) 600 nm.



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from amorphous to polycrystalline phases as the thickness of

the CFO film exceeded a critical value of 300 nm. Relative to

the uncoated ribbons, the ribbons coated with amorphous

CFO films showed significantly the enhanced GMI response.

On the contrary, those coated with polycrystalline CFO films

showed the reduced GMI response. The maximum GMI

response was achieved near the onset of the structural transi-

tion of the CFO film. The work highlights the PLD as an in-situ annealing and coating method for improving the GMIresponse of soft ferromagnetic amorphous ribbons for high-

performance sensor applications.

ACKNOWLEDGMENTS

The research was supported by Florida Cluster for

Advanced Smart Sensor Technologies (FCASST) and by

USAMRMC through Grant Nos. W81XWH-07-1-0708 and

W81XWH1020101/3349. Metglas/Hitachi Metals America

was acknowledged for providing Cobalt based METGLASVR

2714A ribbons.

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