The magnetic state and its macroscopic anisotropy in amorphous rare-earth alloys (invited)

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Abstract
The magnetic state and anisotropy of amorphous rare-earth transition-metal systems with a microscopic random uniaxial anisotropy are discussed. Both sputtered Tb-Fe and rapidly quenched Gd65−xRxCo35 alloys (R is a rare earth) were studied. The low-temperature magnetic state of these alloys depends on the anisotropy, exchange, and applied field strength. For the case of weak anisotropy (alloys with small concentrations of anisotropic rare earth) the low-field results can be explained in terms of a correlated spin-glass state and the higher-field magnetization curves by a ferromagnet with wandering axis state. It is also shown that a weak anisotropy such as that found in Gd65−xErxCo35 (x<8) can lead to double-transition behavior as a function of temperature. The microscopic anisotropy in Gd-Tb-Co, Gd-Er-Co, and Tb-Fe alloys leads to a large macroscopic anisotropy and hysteresis is observed in the torque at 2 K. The macroscopic anisotropy strength depends on the particular rare-earth component present in the alloy and shows a strong temperature dependence. None of the alloys of this work show a significant perpendicular anisotropy. In alloys where the macroscopic anisotropy is weak (Gd65Co35) a fixed in-plane anisotropy is found which is ascribed to anisotropic structural ordering in the plane of the sample. Finally, possible explanations for the movable macroscopic anisotropy found in some Tb-Fe alloys are discussed.