Dynamic magnetic hysteresis and anomalous viscosity in exchange bias systems

A theory for thermally driven time dependent magnetization processes in exchange-coupled magnetic films is presented. Bilinear and biquadratic exchange coupling are used to describe compensated, partially compensated, and uncompensated interfaces between a ferromagnet and an antiferromagnet. Thermal properties of exchange bias are simulated using a Monte Carlo technique. Exchange coupling between a ferromagnetic and an antiferromagnetic film is shown to result in hysteresis without the need for anisotropies or thermal processes in the ferromagnet. Time dependence for the coercive field is calculated for coupled systems and hysteresis is studied as a function of field rate. Competition between demagnetizing effects and intergrain exchange coupling an shown to give rise to anomalous viscosity.