Magnetization reversal with variation of the ratio of the anisotropy energies in exchange bias systems

We systematically vary the ratio of the exchange and ferromagnetic anisotropies in a single multilayered system in finally unravelling the mysteries of magnetization reversal of exchange coupled systems. This is particularly possible due to increasing unidirectional exchange anisotropies in our multilayer system which we saw earlier from sequential switching of exchange coupled layers along the stack with increasing applied field strengths. Here, by introducing different directions (theta) of the applied field with respect to the unidirectional anisotropy direction, we have varied two different energy parameters: (i) the exchange anisotropy for each layer, (ii) the ferromagnetic anisotropy. Our polarized neutron measurements thus show a gradual crossover from layer flipping (domain-wall motion) for low torque regimes of the effective field to coherent rotation for high torque regimes. We explain these findings within a general and simple model where the angular dependencies for reversal are guided by the relative strengths of Zeeman, exchange, and anisotropy energies.