Anomalous Hysteresis Loops in Permalloy Thin Films near 4.2°K

Abstract

Mitchell and Briscoe have reported that the uniaxial anisotropy, H_k, 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 H_k 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.