铁磁学

Evolution of magnetization reversal mechanism in Fe-Cr alloy films T. R. Gao, S. P. Hao, S. M. Zhou, and L. Sun

Citation: J. Appl. Phys. 100, 073909 (2006); doi: 10.1063/1.2357637

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Evolution of magnetization reversal mechanism in Fe-Cr alloyfilms T.R.Gao,S.P.Hao,and S.M.Zhou a͒

Surface Physics Laboratory(State Key Laboratory)and Department of Physics,Fudan University,

Shanghai200433,China

L.Sun

Department of Mechanical Engineering,University of Houston,Houston,Texas77204

͑Received8November2005;accepted29July2006;published online13October2006͒

A large thinfilm sample of Fe x Cr1−x alloy with a composition gradient͑x=0.38−0.52͒has been

prepared by co-sputtering to investigate magnetic anisotropy and magnetization effects on reversal

mechanisms.The single-phased Fe-Crfilms have a bcc structure with͓110͔preferred orientation.

Since the magnetization decreases as the Fe content is decreased and the uniaxial anisotropy energy

keeps at almost constant,the uniaxial anisotropicfield H K is increased.At the same time,the pinning

field H p͑0͒changes little since the lattice constant of Fe-Cr alloyfilm does not change much with

the composition.Consequently,the H K can be much larger than H p͑0͒for low Fe concentrations and

becomes comparable with increasing Fe concentration.As a result,the magnetization reversal

mechanism evolves from a mode based on the pinning and motion of domain wall to another mode

based on modified Kondorsky model as the Fe content is increased.©2006American Institute of

Physics.͓DOI:10.1063/1.2357637͔

I.INTRODUCTION

Metallic ferromagneticfilms are of crucial importance because of their potential applications in magnetoelectronics and spintronic devices.In spin valves,for example,ferro-magnetic layers with large asymmetry of spin-dependent scattering are required for high giant magnetoresistance ratio. In order to enhance the sensitivity of the device,small coer-civity and uniaxial anisotropy are desired.More importantly, the control of the magnetization reversal process in free fer-romagnetic layer can help to suppress the noise level caused by imperfect magnetization reversal.1

For magnetic materials with uniaxial anisotropy,there are three mechanisms to describe magnetization reversal in magnetic singlefilms,including coherent magnetization re-versal,pinning and motion of domain wall,and the Kondor-sky model.2In the coherent rotation model,the magnetiza-tion reversal is accompanied by the coherent rotation of magnetization under an external magneticfield.In the sec-ond model,the magnetization reversal is accomplished only through the motion of the domain wall when the pinning field of the domain wall H p͑0͒along the easy axis must be much smaller than the uniaxial anisotropicfield H K͑=2K U/M S͒,where the K U is the induced uniaxial aniso-tropic energy and M S is the saturation magnetization.H p͑0͒

is equal to the maximum of thefirst derivative of the areal domain wall energy with respective to the location.The switchingfield H S͑␾H͒is the magneticfield to overcome the irreversible motion of the domain wall and H S͑␾H͒=H p͑0͒sec͑␾H͒.3It is usually found in single crystal mate-rials with large intrinsic uniaxial anisotropy like CrO2, LaSrMnO3,barium ferrite,and some permanent magnetic materials.2,4–6In addition to these two mechanisms,the Kon-dorsky model is developed to describe the magnetization re-versal under the condition where the H p͑0͒and H K are com-parable,and the above two magnetization reversal mechanisms coexist.Up till now,in most studies,the mag-netization reversal mechanism has been studied case by case and few experimental studies have been performed on the evolution of the magnetization reversal mechanism in one system.

On one hand,in most of the binary alloys of transition metals,magnetization can be tuned continuously through composition control,and thus these systems can provide im-portant information on the evolution of magnetization reversal.7The uniaxial anisotropy can be induced in the alloy films by deposition magneticfield and have a maximal value at a certain composition.To achieve a wide enough expan-sion of H K,single phase alloys with a wide composition range and thus a wide magnetization range are desired.In this work,we have chosen metastable single-phased Fe-Cr alloyfilms as an example to study the evolution of the mag-netization reversal mechanism with the alloy composition.It is well known that the Fe-Cr alloy exhibits antiferromag-netic,spin glass and ferromagnetic phases in the Fe concen-tration range from0.5to30at.%.8,9For the Fe concentra-tion higher than30at.%,the ferromagnetic phase can usually be observed.As the magnetization is decreased with decreasing Fe content,the magnitude of H K varies sharply with the composition.10On the other hand,since the lattice constant of the Fe-Cr alloys does not change significantly as a function of the alloy composition,the H p͑0͒of the domain wall displacement is expected to change little.Therefore,the magnetization reversal mechanism can be expected to change through modification of the ratio H p͑0͒/H K as a func-tion of the alloy composition.

a͒Author to whom correspondence should be addressed.Electronic mail:

shimingzhou@

JOURNAL OF APPLIED PHYSICS100,073909͑2006͒

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