Pair Ordering Anisotropy In Amorphous Tb-Fe Thin Films
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"**StanfordSynchrotron
Radiation Laboratory, Stanford, CA 94309-0210
ABSTRACT We have studied the structural origins of perpendicular magnetic anisotropy in amorphous Tb-Fe thin films by employing high energy x-ray scattering. The as-deposited films show a clear structural anisotropy, with a preference for Fe-Tb near-neighbors to align in the out-of-plane direction. Upon annealing, the magnetic anisotropy energy drops significantly, and we see a corresponding reduction in the structural anisotropy. The radial distribution functions indicate that the number of Fe-Tb near-neighbors increases in the in-plane direction, but does not change in the out-of-plane direction. Therefore, the distribution of Fe-Tb near-neighbors becomes more uniform upon annealing. We conclude that the observed reduction in perpendicular magnetic anisotropy energy is a result of this change in structure.
INTRODUCTION Amorphous RE-TM alloy thin films are in widespread use as magnetooptic recording media. One important property that these materials have is a strong perpendicular magnetic anisotropy; that is, an easy axis of magnetization perpendicular to the plane of the film. While the physical origins of magnetic anisotropy (such as dipolar interactions and single-ion anisotropy) are reasonably well understood, each of these mechanisms requires an underlying structural anisotropy to produce a macroscopic magnetic anisotropy. The nature of this structural anisotropy in amorphous RE-TM films has been the subject of considerable debate. A variety of different theories have been proposed, including pair ordering anisotropy[l] and bond orientation anisotropy[2]. Recently, two independent observations of atomic-scale structural anisotropy in amorphous RE-TM thin films have been reported. The first compared x-ray scattering from an amorphous Tb.26 Fe.6 2 Co. 12 in the symmetric reflection geometry (which gives in-plane structural information) with grazing-incidence scattering (which gives out-of-plane structural information) [2]. While no real-space structural information was presented, the difference in scattering between the two geometries lead the authors to conclude that bond orientation anisotropy was present in their sample. Bond orientation anisotropy is characterized by a different near-neighbor spacing and coordination number in the in-plane and out-of-plane directions, but not by any difference in chemical ordering between the two directions. The presence of pair-ordering anisotropy in amorphous Tb.26Fe.7 4 has been reported by Harris and coworkers[3]. These authors measured the polarization-dependent EXAFS 239 Mat. Res. Soc. Symp. Proc. Vol. 384 ©1995 Materials Research Society
from their samples and then fit calculated EXAFS spectra based on a structural model to the experimental data. Their results showed that there was a slight preference for Fe-Tb near-neighbors to align in the out-of-plane direction. They did not see any difference in overall coordination numbers or near-neighbor spacings between the two directions. The present e
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