Anomalous Hysteresis Loops in Permalloy Thin Films near 4.2°K

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Abstract
Mitchell and Briscoe have reported that the uniaxial anisotropy, Hk, in thin Permalloy films is 2 to 3 times larger at 4.2°K than it is at 300°K and that an additional doubling in Hk occurs in going from 4.2° to 1.46°K. Several new and interesting characteristics and a tentative explanation of this phenomenon are given in the present paper. The explanation is based on the formation of an antiferromagnetic oxide on the film surface. By oxidation in O2 and subsequent reduction in H2 (each for several hours at 120°C), the anomalous susceptibility at low temperatures has been introduced and removed repeatedly in the same sample. While in the low susceptibility state, measurement of the uniaxial anisotropy at 4.2°K by means of a rotating field magnetometer demonstrates that only the susceptibility, not the uniaxial anisotropy, is anomalous. A unidirectional anisotropy, arising from the exchange coupling between the ferromagnetic and antiferromagnetic materials, is thus proposed to explain the susceptibility. Several aspects of the hysteresis loops observed at 4.2°K support this explanation, and additional evidence is obtained from the temperature dependence of the anomalous loop behavior.

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