Large scale micromagnetic simulation for the exchange interaction between a polycrystalline antiferromagnet and a ferromagnet

Exchange bias in biaxial and uniaxial systems has been micromagnetically simulated for a system containing more than 250000 elements. Each ferromagnetic element was coupled to a neighboring antiferromagnetic grain that rotated uniformly under the effect of thermal fluctuations. The time evolution of the ferromagnetic moments was determined by solving the Landau-Lifshitz-Gilbert equation for each degree of freedom. For a sample of NiFe (thickness of 250) coupled to polycrystalline NiMn (thickness of 500) we obtained an exchange bias of 118 Oe and an enhanced coercivity of 108 Oe. There is good agreement with experiment despite the absence of adjustable parameters. The simulation also correlates the symmetry of the antiferromagnet to the loop shape and training effect.