Journal of Magnetism and Magnetic Materials 250 (2002) L1–L5

Letter to the Editor

The magnetic hysteresis of NiFe-31% thin films underisotropic in-plane stress

Patrick Hollanda, Mary Kemptona, Dustin Raganb, Steve Riosa,Anup K. Bandyopadhyaya, Archana Dubeyc, Wilhelmus J. Geertsa,*

aDepartment of Physics, Southwest Texas State University, San Marcos, TX 78666, USAb Department of Physics, Trinity University, San Antonio, TX 78212, USA

c University of Central Florida, Orlando, FL 32828, USA

Received 1 April 2002

Abstract

We investigated the magneto-elastic properties of 500 nm thick sputtered NiFe-31% thin films. The hysteresis curves

were measured as function of an externally applied isotropic in-plane stress. Coercivity and hysteresis loss decrease

linearly as a function of tensile stress. Reduced pinning of the domain walls is believed to be responsible for the stress

dependence. r 2002 Elsevier Science B.V. All rights reserved.

PACS: 75.80; 85.70; 75.60.G; 87.20.L; 68.60.B

Keywords: Magneto-elastic effects; Magnetic hysteresis; Coercivity; NiFe; Thin films

1. Introduction

It has been long known that the properties offerromagnetic objects depend on the stress state ofthe material [1]. Coercivity, hysteresis loss, andpermeability, among other factors, change as afunction of an externally applied stress. This effecthas been successfully exploited in pressure, accel-eration, and force sensors. Recently, Rissing et al.showed that a sensor using a magnetic thin filmmight have a much larger sensitivity than thetraditional piezoelectric force gauges [2]. Although

a lot of work has been done on bulk magneto-elastic properties, we still lack a thorough under-standing of these properties for thin films. Inparticular, the contribution of different effectssuch as magnetostriction and domain wall pinningis still not very clear.

We investigated the magneto-elastic propertiesof 500 nm NiFe-31%. This material is the textbookexample of a stress sensitive magnetic material anda most promising choice for application in a stresssensor. While preparing the sample and theexperiment, we tried to keep a high level ofsymmetry and to not introduce any in-planeanisotropy axis; we used circular substrates,applied no magnetic fields to the sample duringdeposition, and took measurements with anisotropic in-plane stress applied.

LETTER TO THE EDITOR

*Corresponding author. Tel.: +512-245-1821; fax: +512-

245-8233.

E-mail address: wjgeerts@swt.edu (W.J. Geerts).

URL: http://www.swt.edu/Bwg06/.

0304-8853/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved.

PII: S 0 3 0 4 - 8 8 5 3 ( 0 2 ) 0 0 3 6 9 - 4

2. Sample preparation

The NiFe films were grown by DC magnetronsputtering in a vacuum chamber with a back-ground pressure better than 4� 10�7 Torr. Circu-lar glass microscope slides (thickness=0.2 mm,diameter=22 mm) were used as substrates. Priorto loading them into the vacuum chamber, thesubstrates were cleaned ultrasonically in 5%micro, acetone and isopropyl alcohol and driedwith pressurized nitrogen gas. The substrates weremounted on the sample-holder with two stainless-steel spring blades which guaranteed a goodthermal contact with the holder. Just beforedeposition, the substrates were heated to 3501Cto remove absorbed gases. After the substrates hadcooled down to below 401C, a titanium seed-layerof 2 nm was applied to improve the adhesion of themagnetic thin film, and to gather possible remain-ing contaminants on the substrate. The NiFe wassputtered from a NiFe-35% target using a Torus2C source (sputter pressure=4 mTorr argon;sputter power=70 W). The thickness of oursamples, as measured by a stylus profilometer,was 500 nm. All samples were covered with a 2 nmtitanium caplayer to avoid oxidation of theferromagnetic material. No magnetic field wasapplied to the substrates during the depositionprocess. A Ti target located away from the samplewas left on during NiFe deposition to act as agetter pump for impurities.

The chemical composition of the samples waschecked by Energy Dispersive Spectroscopy (ac-celeration voltage 15 keV). The iron content of oursamples is 31%, which is a little lower than theconcentration of the target. X-ray measurementswere performed with a Bede D1 high-resolutiondiffractometer. The 2y scans show that our filmshave an FCC structure and are polycrystallinewith a (1 1 1) texture.

3. Experimental procedure

The bulk magnetic properties of the sampleswere measured by a vibrating sample magnet-ometer (VSM). Hysteresis curves were measuredby applying a field parallel to the substrate and

monitoring the magnetic moment parallel to thefield direction. The magnetic properties of thesurface were measured via the magneto-opticalKerr effect using a polarization modulated ellips-ometer. The used setup was similar to thatdescribed in Ref. [3]. The magnetic field wasapplied parallel to the substrate and parallel tothe plane of incidence (longitudinal Kerr effect).Since the Kerr ellipticity is larger than the Kerrrotation, the former was monitored as a functionof the applied field. As a light source, an intensitystabilized HeNe laser was used (Melles Griot05STP-901). The angle of incidence was approxi-mately 301. The photo-elastic modulator (PEM)was placed at 01, the polarizer at 451 and theanalyzer at 901 (all angles with respect to the planeof incidence). The modulation depth of the PEMwas kept at 2.4 rad. In this configuration, the Kerrsignal is sensitive to in-plane and perpendicularcomponents of the net magnetization. Additionalmeasurements performed at perpendicular angle ofincidence (field parallel to the substrate) didnot reveal any significant Kerr signal. From this,we concluded that our films do not contain anet magnetic moment perpendicular to the filmsurface.

A nonmagnetic stress-fixture (see Fig. 1), similarto that of Callegaro et al. [4], was constructed. Thesample, a microscope slide supporting the thinNiFe-film, constituted the membrane of a pressur-ized vessel. Pressuring the vessel by applyingcompressed gas to the backside of the substrate,bent the sample in two dimensions and created aconvex-shaped mirror. This resulted in a tensile(compressive) isotropic in-plane stress when thefilm was on the outer (inner) surface of themicroscope slide. We chose not to glue the samplein the pressure cell, but let it be supported by theedge of the measurement window. Vacuum greasewas used to make a high-pressure seal between theglass substrate and the brass holder. The max-imum pressure that we could apply was limited bythe strength of the glass substrate and the qualityof the seal. Samples would break typically some-where between 15 and 80 psi. The elastic theory ofa one-side supported plate of Filonenko–Borodichwas used to estimate the distribution of the strainover the film surface [5]. The calculations showed

LETTER TO THE EDITOR

P. Holland et al. / Journal of Magnetism and Magnetic Materials 250 (2002) L1–L5L2

that for our sample holder the strain varies lessthan 5% within a radius of 1 mm of the center ofthe window. Since we used a lens to focus theincident beam to a spot smaller than 0.3 mm, thestrain can be considered to be constant over themeasurement area. The mathematical analysisfurthermore revealed that the strain in the thinfilm is linearly proportional to the pressure in thevessel. The strain–pressure relation was experi-mentally determined by measuring the change inthe diameter of a laser beam reflected from thesample as a function of the vessel pressure. Themeasured proportionality constant was approxi-mately 0.0001 psi�1.

4. Measurement results

The saturation magnetization as derived fromthe VSM data and the thickness measured by thestylus profilometer is in agreement with the valuereported by Bozorth et al. [6]. The in-plane VSMhysteresis curves measured at different anglesvaries less than 5%. The small variation is causedby a residual uniaxial in-plane anisotropy causedby a small bending of the glass substrates during

the deposition process; the sample holders in oursputtering system are not completely flat butslightly curved (made of stainless-steel sheetmetal). The Kerr hysteresis curves of the top-surface and the seed layer (measured through theglass substrate) are similar to the hysteresismeasured by VSM. This suggests that the magne-tization is very homogeneous throughout the filmthickness. The measurement data on the un-stressed samples is summarized in Table 1. Thesmall difference in Hc between top and surfacemight originate from the slightly different stress-state for both measurements; in the Kerr setupthe sample is secured on a flat surface by smallblade springs while for the VSM measurementsthe sample is glued to a stick with rubbercement.

Fig. 2 shows the hysteresis curves of the topsurface of the NiFe film for different values ofapplied pressure. This is raw data. The coercivityand hysteresis loss decrease with increasing strain

Table 1

Comparison of VSM and Kerr data on unstressed NiFe-31%

film

Sample

name

Film

thickness

Hc

(VSM)

Hc

(Kerr)

surface

Hc

(Kerr)

substrate

021700-01 500 nm 16.2Oe 18Oe 16Oe

Fig. 1. Stress fixture: pressurizing the brass vessel will bulge out

the sample and cause an isotropic in-plane tensile stress in the

thin film on top of the glass substrate.

Fig. 2. Kerr hysteresis curves for different values of the

pressure.

LETTER TO THE EDITOR

P. Holland et al. / Journal of Magnetism and Magnetic Materials 250 (2002) L1–L5 L3

(dHc=de � 5 � 106 Oe). Fig. 3 shows the change inthe coercivity as a function of the in-plane strain.The graph shows data obtained from two differentsamples. No detectable change in the HcðeÞ couldbe observed after cleaning the sample and measur-ing it again. This suggests that we operate thematerial in the elastic regime.

We also tried to measure the stress dependenceof the hysteresis curve as measured through thesubstrate. For this experiment the sample wasflipped in the pressure cell. Pressurizing the cellcaused a compressive strain to the thin film.Preliminary results show that the coercivityslightly increases for larger compressive strains.The data however was very scattered and moredetailed analysis need to be performed.

5. Discussion

The decrease of the coercivity for larger valuesof in-plane tensile strain is similar to that observedby Callegaro and Puppin for an electroplatednickel sample. As a first impression this seemsconfusing since the sign of the magnetostriction ofNiFe-31% is opposite to that of nickel. However,a more detailed look at Fig. 1 shows us that thehysteresis curve does not shear for a larger tensilestrain, as was the case for pure nickel. Apparently,application of tensile stress will not push/pull themagnetization out of/into the film plane. Althoughthe easy axis of the magnetization is the [1 0 0]direction the large magnetization (Ms ¼ 1000 emu/

cm3 [1]) and low crystal anisotropy (K1 ¼5 � 103 erg/cm3 [6]) of NiFe-30% will keep themagnetic moment in the plane of the film.Although it is not possible to determine unam-biguously from our data what is causing thedecrease in coercivity, reduced pinning of thedomain walls as described in Ref. [7] couldbe responsible for the observed effect. Such areduction in domain wall pinning can originatefrom a decrease of the gradient of the magnetiza-tion caused by the in-plane stress. Domain studiesof the stressed sample need to be performed inorder to reveal the exact nature of the observedeffects. Furthermore, we plan to investigatehow patterning will influence the magneto-elasticproperties.

6. Conclusions

NiFe-31% thin films with a thickness of 500 nmare very sensitive to an in-plane isotropic strain.The change of the coercivity for isotropic in-planestrain is linear and is approximately 1.5� 104 Oeper percentage of strain. Since the hysteresis curvedoes not shear, the hysteresis loss, WH, is alsolinearly proportional to the strain

WH ¼Z

m0H dM ¼ 2m0MsHcðeÞ;

where m0 is the permeability of vacuum, Ms issaturation magnetization of NiFe-31%, H is theapplied field, Hc is the coercivity field, and e is thestrain in the thin film. A coil or transformer thatuses this material in its yoke would make asensitive sensor. The real part of its impedancewill be linearly proportional to e; and below theproportionality limit linearly proportional to theapplied stress s:

Acknowledgements

The authors would like to thank Dr. CarlosGutierrez of SWT for his help with interpreting theEDS and X-ray data. Dustin Ragan would like tothank Dr. Hector Mireles of Trinity University forintroducing him to SWT. This work was funded in

Fig. 3. Coercivity as a function of the strain (tensile stress

configuration).

LETTER TO THE EDITOR

P. Holland et al. / Journal of Magnetism and Magnetic Materials 250 (2002) L1–L5L4

part by the National Science Foundation underGrant No. 0075372 and in part by an award fromResearch Corporation. Wilhelmus Geerts is aCottrell Scholar of Research Corporation.

References

[1] B.D. Cullity, Introduction to Magnetic Materials, Addison-

Wesley, Reading, MA, 1972.

[2] L.H. Rissing, S.A. Zielke, H.H. Gatzen, IEEE Trans.

Magn. 34 (1998) 1378.

[3] K. Sato, Jpn. J. Appl. Phys. 20 (1981) 2403.

[4] L. Callegaro, E. Puppin, IEEE Trans. Magn. 32 (1996)

4767;

L. Callegaro, D. Petrali, E. Puppin, Rev. Sci. Instrum. 68

(1997) 1796.

[5] M. Filonenko-Borodich, P. Groningen, Theory of Elasti-

city, Noordhoff, Leiden, 1964.

[6] R.M. Bozorth, Ferromagnetism, IEEE Press, Piscataway,

NJ, 1978.

[7] I.J. Garshelis, J. Appl. Phys. 73 (1993) 5629.

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