Effect of Annealing on Magnetostrictive Properties of Fe­Co Alloy Thin Films

Takashi Nakajima1,+, Teruaki Takeuchi2, Isamu Yuito2, Kunio Kato2, Mikiko Saito2,Katsuhiro Abe1, Toshio Sasaki1, Tetsushi Sekiguchi2 and Shin-ichi Yamaura1

1Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan2Institute for Nanoscience & Nanotechnology, Waseda University, Tokyo 162-0041, Japan

The effect of the annealing temperature on the magnetostrictive properties of Fe­Co alloy thin films on quartz glass was systematicallyinvestigated. The saturation magnetostriction was 56 ppm for the Fe32Co68-sputtered thin film quenched from 673K, and it increased to 159 ppmwith increasing annealing temperature up to 1073K. The magnetostriction above 1093K significantly decreased with the emerging fcc phase inthe Fe­Co alloy. SEM images showed that the crystal grains present in the fcc phase aggregated to form a discontinuous surface, which resultedin a decrease in the effective magnetostriction. Measurements of the magnetic properties by vibrating sample magnetometer and magnetic forcemicroscopy revealed the enhancement of the magnetization at the crystal grain boundaries of Fe­Co alloy thin films with large magnetostriction.Thus, the heterogeneity of the magnetization can play a key role in inducing the large magnetostrictive effect.[doi:10.2320/matertrans.MBW201315]

(Received October 16, 2013; Accepted November 11, 2013; Published December 28, 2013)

Keywords: iron cobalt, thin films, sputtering film deposition, magnetostriction, magnetization, magnetic force microscopy

1. Introduction

In recent years, there has been strong interest in introduc-ing magnetostrictive thin films for microelectromechanicalsystems (MEMS) as novel actuators, sensors, and vibrationenergy harvesting devices.1­5) A key contribution of MEMStechnology is the integration of multifunctional elements ona single chip, which enhances the performance, reliability,and competitive pricing. Rare-earth-based magnetostrictivematerials, represented by Tb­Dy­Fe, have played a centralrole in developing such applications owing to their largemagnetostriction of more than 2000 ppm.6­11) However,there has also been demand for developing rare-earth-freemagnetostrictive materials as a result of the depletion ofnatural resources.

Recently, the strong magnetostrictive effect originatingfrom the heterogeneity at the phase boundaries of Fe-basedalloys has attracted great research interest. Extraordinarymagnetostrictive behaviors have been observed in Fe­Ga andFe­Al alloys, exhibiting large magnetostriction values (3/2)­100 of more than 400 ppm for Fe81.3Ga18.712,13) and 200 ppmfor Fe83.4Al16.6.14) In each case, the increase of magneto-striction occurred in the vicinity of ordered DO3/disorderedA2 phase boundaries, and several mechanisms such asmagnetic domain rotation15) and large elastic deformation ofDO3 nanoclusters16) have been proposed.

Much effort has also been devoted to developing themagnetostriction of Fe­Co alloys17­19) ever since the firstreport on the magnetostrictive properties over the entirecomposition range by Masiyama.20) More recently, Hunteret al. prepared compositional gradient Fe­Co alloy thin filmsusing a co-sputtering technique, and they found significantmagnetostriction enhancement at the (fcc + bcc)/bcc phaseboundary.21) The effective magnetostriction was 260 ppmfor an Fe34Co66 thin film quenched from 1073K, and theintrinsic magnetostriction ­100 was estimated to be more

than 1000 ppm. However, it is difficult to understand theinterfacial effect on magnetostriction by focusing solely on itscomposition dependence because the contribution of Joulemagnetostriction, which becomes prominent in the vicinity ofFe50Co50,21) should also be included. Furthermore, from apractical standpoint in terms of the manufacturing process, itis of special importance to investigate the effect of annealingon the magnetostrictive properties of Fe­Co alloys.

In this study, we report on the annealing temperaturedependence of the magnetostriction of Fe­Co alloy thin films.Thin films of Fe32Co68 were deposited on quartz glass usinga sputtering technique. The samples were then quenchedfrom 673 to 1173K. The magnetostrictive properties of thesefilms are discussed in detail based on the characterizationof the structures and their magnetic properties.

2. Experimental Procedure

Fe­Co alloy thin films 240 nm thick were sputter-depositedon quartz glass substrates using RFS-200 (ULVAC). Thesubstrates were 20mm long, 5mm wide, and approximately60­70 µm thick. The films were fabricated at a substratetemperature of 623K under an Ar pressure of 1.0 Pa and asputtering power of 100W, which yielded a deposition rateof 2 nm/min. In this study, the composition of the films waschosen to be Fe32Co68 so as to obtain an fcc and/or bcc phaseby annealing between 673 and 1173K.22) The samples wereannealed for 60min in an evacuated glass tube and quenchedby immersing the tube in ice water.

The crystallographic phases of the Fe­Co alloy thin filmswere characterized by X-ray diffraction (XRD; Ultima-III,Rigaku) analysis using Cu K¡ radiation with a monochro-mator. The microstructure observation and the compositionanalysis were carried out using a field-emission scanningelectron microscope (FE-SEM; S-4800, Hitachi) equippedwith an energy dispersive X-ray spectrometer.

The magnetostriction was evaluated by using the cantileverdeflection technique, which allows the magnetostriction ­+Corresponding author, E-mail: tnakajima@imr.tohoku.ac.jp

Materials Transactions, Vol. 55, No. 3 (2014) pp. 556 to 560©2013 The Japan Institute of Metals and Materials

to be determined from the field-induced bending of thecantilevered substrate.23) The displacement of the cantileverwas induced by an alternating magnetic field parallel (H¬)and perpendicular (H?) to the longitudinal direction of thesubstrate but always parallel to the film plane. The saturationmagnetostriction ­s was calculated using the followingexpression:

­ s ¼ð�sk ��s?ÞEsts

2ð1þ vfÞ18EftfLlð1� vsÞ

ð1Þ

where ¦S¬ and �S? are the maximum position change of thelaser spot on the position sensitive detector (PSD) inducedby H¬ and H?, respectively, E is the Young’s modulus, t thethickness, l the distance between the clamping edge to thelaser spot on the sample, and L the optical travel distancefrom the incident point at the sample. The subscripts “s” and“f ” denote “substrate” and “film”, respectively. The Young’smoduli were determined by a nanoindentation techniqueusing a mechanical properties tester for thin films (NECMH4000).24)

The magnetic properties were measured by a vibratingsample magnetometer (VSM; VSM-5-10, TOEI). Themagnetic microstructure was investigated by magnetic forcemicroscopy (MFM; SPI3800N/SPA400, SII) employing asilicon cantilever coated with a CoCr alloy (MFMR-10,NanoWorld).

3. Results and Discussion

Figure 1 shows the XRD patterns of the Fe­Co alloy thinfilms quenched from various temperatures. As shown inFig. 1(a), the reflections from the bcc and/or the fcc phasewere observed in wide-range XRD patterns, depending onthe annealing temperature. Detailed data for the crystallinephase of the Fe­Co alloy thin films was furthermoreexamined by narrow-range XRD patterns and the resultsare shown in Fig. 1(b). The double-peak feature is due tothe K¡1 and K¡2 X-rays, but this feature is not detectable at673K owing to the significant broadening of the XRD peak.A single-phase bcc structure was observed when theannealing temperature was less than 1093K. As the temper-ature increased, the structure exhibited a coexisting phaseof bcc and fcc between 1113 and 1143K and single-phasefcc above 1153K. Thus, the crystal structures, which areconsistent with the conventional phase diagram of the Fe­Cobinary system,22) were successfully obtained.

Figure 2 shows changes in surface morphologies byannealing. The film annealed at 673K exhibits a uniformsurface that consists of tiny grains. Annealing at 1073Kresulted in growth of the crystalline grains. The increase ofthe grain size is supported by the XRD patterns in Fig. 1(b)showing the decrease of the peak width. Annealing at 1133Kcaused the grains to aggregate, thus forming a discontinuoussurface. The aggregation was further developed at 1153K.One of the conceivable causes of the aggregation is theinterfacial reaction of the Fe­Co layer with the quartzsubstrate. Bendersky et al. reported the formation of anFe2SiO4 mixed silicate at the FeCo/SiO2 interface byannealing at 1073K,25) where the silicate may have actedas the crystal-growing nucleus. Differences in the crystal-

lization kinetics between the bcc phase and the fcc phasecould be another cause because of the discontinuous surfaceobserved in the film with an fcc phase.

Figure 3 shows the annealing-temperature dependence of­s of the Fe­Co alloy thin films. The magnetostriction foreach sample was determined from the field-induced butterflyloops as a result of magnetostrictive behavior, as shown inthe inset of Fig. 3. As the annealing temperature increased,­s gradually increased, reaching a maximum of 159 ppm at1073K and then it decreased. The ­s value of the sampleannealed at 1073K is almost three times larger than that ofthe sample annealed at 673K. From the XRD results shownin Fig. 1, it is apparent that ­s is greatly enhanced in thevicinity of the bcc/fcc coexisting phase, but note that thephase exhibiting maximum ­s is single bcc. The ­s valuedecreased with the appearance of the fcc phase, and themagnetostrictive response became undetectable by ourmeasurement system for the fcc single-phase samplesannealed above 1143K. However, the significant decrease

Inte

nsity

(lo

g ar

b. u

nit)

1008060402θ (degree)

1153 K (200

)

1073 K

973 K

673 K

fccbcc

(a)(111

)

(311

)

(222

)

(110

)

(200

)

(211

)

(220

)

1113 K

Cu-Kα Fe32Co68 alloy thin films

Inte

nsity

(lo

g ar

b. u

nit)

47464544432θ (degree)

1173 K

1153 K

1143 K

1133 K

1113 K

1093 K

1073 K

1023 K

973 K

673 K

fcc(111)

bcc(110)

(b)

Cu-Kα Fe32Co68 alloy thin films

Fig. 1 (a) Annealing temperature dependence of XRD patterns from2ª = 30 to 110°; (b) fcc (111) and bcc (110) reflection peaks for Fe­Coalloy thin films.

Effect of Annealing on Magnetostrictive Properties of Fe­Co Alloy Thin Films 557

of ­s is considered to be caused by the discontinuous surfaceshown in Fig. 2 rather than by the magnetostrictive propertiesof the crystalline phase. For simplicity in this study, wecalculated ­s using eq. (1) on the assumption that the film isuniform, but the intrinsic ­s should be higher because theeffective magnetostriction is hindered owing to the discon-tinuous surface.

Next, we performed M­H curve measurements to evaluatethe magnetic properties at room temperature. Figure 4 showsthe in-plane hysteresis curves of thin films annealed at 673,1073, and 1153K. Every curve exhibits significant soft

magnetic properties, indicating that the easy-axis direction ofthe preferential magnetization is parallel to the film plane.The most striking feature in this figure is the annealingtemperature-dependence of the magnetization. Figure 5shows the plots of saturation magnetization Ms and coercivefield Hc against the annealing temperature. It is seen that Ms

monotonically increased with increasing annealing temper-ature up to 1093K and then it began to decrease. Thistendency is similar to that of the magnetostriction, and themagnetostriction enhancement is considered to be related tothe magnetization increase. Hc shows a two-step decreaseas the annealing temperature increases. This decrease isprobably due to the stress relief created in the material itself

Fig. 2 SEM images of Fe­Co alloy thin films annealed at (a) 673K, (b) 1073K, (c) 1133K, and (d) 1153K.

150

100

50

0

Sat

urat

ion

mag

neto

stric

tion

(ppm

)

120011001000900800700

Annealing temperature (K)

Fe32Co68 alloy thin films

-10

-5

0

5

10

Sig

nal (

V)

-50 0 50Magnetic field (kA/m)

HH

Fig. 3 Annealing temperature dependence of saturation magnetostrictionof Fe­Co alloy thin films. The inset shows the magnetostrictive behaviorof an Fe­Co alloy thin film on quartz substrate as detected by thecantilever deflection method.

-2

-1

0

1

2

Mag

netiz

atio

n (W

b/m

2 )

-1000 -500 0 500 1000

Magnetic field (kA/m)

673 K

1073 K

1153 K

Fe32Co68 alloy thin films

Fig. 4 In-plane magnetization curves of Fe­Co alloy thin films annealed at673, 1073, and 1153K.

T. Nakajima et al.558

and in the substrate interface, which is estimated to beprominent after the appearance of the discontinuous surfaceat higher annealing temperatures.

For further understanding of the magnetization increase,we conducted MFM measurements to investigate themagnetic microstructure. Figure 6 shows the results of theMFM measurements on the sample annealed at 1093K. TheAFM image (a) was obtained by tapping mode for the firstscan and the MFM image (b) was obtained for the secondscan by monitoring the phase shift in the resonant cantileveroscillation, keeping the cantilever height constant above thesample surface configuration. Comparing Figs. 6(a) and 6(b),the MFM image exhibits a dark-colored area in the vicinity ofthe grain boundaries, as indicated by the arrows. As seen inthe line profile of the height and the phase shift shown inFig. 6(c), the phase shift increases at the dark-colored area.The degree of the phase shift provides information on themagnetic field gradient between the cantilevered tip and theFe­Co surface. Therefore, these results suggest that the localmagnetization is enhanced around the grain interfaces of theFe­Co alloy thin film quenched from 1093K. The origin ofthe magnetostriction enhancement in Fe­Ga is thought toresult from martensitically transformed precipitates in Fe­Gaalloys acting as tetragonal defects that can be rotated byapplying a magnetic field.15) Hunter et al. also proposed asimilar scenario, in which the reorientation of the tetragonalprecipitates at the (fcc + bcc)/bcc phase boundary resultsin the magnetostriction enhancement in Fe­Co alloy thinfilms.21) Our findings of the magnetization increase at thegrain boundaries serve as strong evidence for the presence oftetragonal precipitates, which supports the magnetic domainrotation model as the mechanism for the magnetostrictionenhancement of the Fe­Co alloy system.

4. Conclusions

We investigated the effect of annealing temperature on themagnetostrictive properties of a Fe32Co68 alloy thin film onquartz glass. As the annealing temperature increased, thesaturation magnetostriction ­s increased, reaching a max-imum of 159 ppm at 1073K, which is almost three timeslarger than ­s of the sample annealed at 673K. We alsoobserved the increase of the magnetization at the grainboundaries of Fe­Co alloy thin films with large magneto-

2.0

1.8

1.6

1.4

1.2

Cor

cive

fiel

d (k

A/m

) 8

7

6

5

Sat

urat

ion

mag

netiz

atio

n (W

b/m

2 )

120011001000900800700

Annealing temperature (K)

Fe32Co68 alloy thin films

Fig. 5 Annealing temperature dependence of saturation magnetization andcoercive field for Fe­Co alloy thin films.

100

80

60

40

20

0

Heg

iht d

iffer

ence

(nm

)

2000150010005000distance (nm)

14

12

10

8

6

Phase shift (°)

from AFM image (a) from MFM image (b)

(c)

Fig. 6 (a) AFM and (b) MFM images of Fe­Co alloy thin film annealedat 1093K. (c) Profiles of height and phase shift upon dashed lines in (a)and (b).

Effect of Annealing on Magnetostrictive Properties of Fe­Co Alloy Thin Films 559

striction, which would support the magnetic domain rotationmodel as the mechanism of the magnetostriction enhance-ment at the (fcc + bcc)/fcc interface. After emerging fcc-phase annealing above 1113K, ­s significantly decreased.SEM observations suggested that the decrease of ­s is dueto the discontinuous surface structure rather than to themagnetostrictive properties of the crystalline phase. Thesefindings allow us to envision an ideal structure with largemagnetostrictive properties that consist of high density(fcc + bcc)/bcc phase boundaries with suppressed fccemergence to minimize the undesired discontinuous structure.

Acknowledgments

This research was supported by a Grant-in-Aid for ScienceResearch in a Priority Area, “Advanced Materials Develop-ment and Integration of Novel Structured Metallic Glassesand Inorganic Materials” from the Ministry of Education,Culture, Sports, Science and Technology, Japan. The authorsthank Professor T. Furukawa of Kobayashi Institute ofPhysical Research and Mr. H. Sagami of Ideal Star Inc. forSPM measurements. We are also grateful to Professors R. Y.Umetsu and Y. Mitsui of Tohoku University for valuablediscussions.

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