Exchange Bias in Manganese Alloys with Mixed Magnetic Behaviour

Abstract

Exchange bias has been a well known property of materials consisting of a ferromagnet interfaced with an antiferromagnet for many years, and the anisotropy resulting from the interaction has recently been nominated as a possible means for overcoming the superparamagnetic limit in high-density magnetic information storage media. However, a generalized theoretical description of the phenomenon remains elusive. More recently, analogues between the exchange bias eect and the behaviours of spin glass materials have become increasingly recognized. In this thesis, the analogues between exchange bias and spin glass systems are explored in a set of concentrated spin glass samples that exhibit a strong sensitivity to short range order of magnetic entities. A number of models were developed to analyze and account for the measurements of magnetization made on these samples. The rst was a simple, empirical model, based on an approximation of non-interacting, monodisperse, superparamagnetic clusters against a paramagnetic background to t single branches of minor hysteresis loops. In this model, the parameter to describe the coercive eld was added ad hoc. Another, more sophisticated, model was based on a modied Stoner-Wohlfarth expression for magnetic clusters and applied to the data using Monte Carlo simulations. The best ts resulted from this model when the cluster easy axis orientations were allowed to have a Gaussian distribution.