Measurement of the antiferromagnet activity in exchange bias systems

Abstract

In this work, we describe the results of a recent set of experimental measurements on samples of CoFe/IrMn ferromagnet/antiferromagnet (AF) bilayer exchange bias materials. Careful magnetic measurement protocols and detailed particle size measurements have enabled us to confirm the link between the exchange field (Hex) and the distribution of grain volumes in the IrMn AF layer. This can be achieved by fitting the blocking temperature curve to obtain an effective value for the Néel temperature (TN). The value of (TN) is then used to determine the stable and set fraction of the grain volume distribution. Through a comparison of samples with different seed layer structures, we demonstrate control of the exchange bias where almost all the AF grains contribute to Hex. These samples have an AF with a mean grain diameter of 8.7 nm and an anisotropy constant of 7.8×106 ergs/cc. Our results indicate a reduction in the value of the Néel temperature (TN) of up to 100 K.