Magnetic characterization of exchange-coupled composite media

Abstract

In this work we compare the magnetization reversal processes in a set of exchange-coupled composite (ECC) media and a similar set of continuous granular coupled (CGC) media. We have studied the reversal processes in both systems as a function of the thickness (ts) of the soft overlayer. We find that the soft overlayer contributes a reversible contribution to the magnetization in both types of media, lowering the coercivity in both cases but that there is an optimum value of ts in terms of the resulting squareness. This has important implications for the output signal in both cases. We have also studied the activation volume of reversal which will ultimately determine the limit to the data density achievable in both types of media. We find that in each case the activation volume increases monotonically with the thickness of the soft layer, indicating that nucleation of reversal in the soft layer will determine the ultimate data density achievable. Although absolute values for the activation volume cannot be determined due to the uncertainty in the value of Ms, our data indicate unexpectedly that an exchange-coupled soft layer, i.e., the ECC media, has smaller activation volumes than is the case for CGC media. However, we cannot discount the possibility that increased anisotropy from the hard layer may contribute to this effect.