Exchange Anisotropy in Oxidized Permalloy Thin Films at Low Temperatures

Abstract

Oxidized Permalloy thin films exhibit uniaxial anisotropy at room temperature, but at low temperatures develop a mixture of uniaxial and unidirectional anisotropies. Exchange coupling between the ferromagnetic permalloy and the antiferromagnetic oxide, at temperatures below the latter's Néel temperature, account for this phenomenon. New observations and interpretations of low-temperature hysteresis-loop effects in oxidized Permalloy films are reported in this paper. The initial susceptibility in the original uniaxial hard direction has been measured at 4.2°K for various combinations of Permalloy and oxide-layer thicknesses. While the results are found to be in qualitative agreement with a model based on a simple mixture of anisotropies, a semiquantitative understanding is found possible only after assuming that reversal of the Permalloy magnetization direction also reverses the spin orientations in a large fraction of the antiferromagnetic material. For very thin (∼10 Å) oxide layers, this fraction is larger than 9/10, but it drops to about ½ for 60-Å layers. The observed reduction in hysteresis loss as the oxide thickness is increased is also consistent with this model. An example is given where the unidirectional anisotropy is sufficiently large so that the magnetic structure has only one stable remanent state. This same structure is shown to exhibit a new property of ferromagnetic-antiferromagnetic coupling recently reported by Paccard et al. Finally, the temperature dependence of the hysteresis loops above 4.2°K is discussed. It is concluded that exchange anisotropy serves as a basis for understanding the many anomalous effects which have been observed.