Biquadratic coupling in antiferromagnetically coupled magnetic recording media

Using spinstand and magnetometry measurements we have investigated the effective coupling between the two ferromagnetic layers in antiferromagnetically coupled (AFC) longitudinal magnetic recording media. For AFC media with soft-magnetic CoCr and Co lower layers we find that the average equilibrium magnetization direction of the lower layer is a function of transition density in the upper layer. In particular for medium-sized bit cells, biquadratic coupling is observed, for which the average magnetization of the lower layer is perpendicular to that of the upper layer.