The total energy of exchange bias systems with biaxial anisotropy

Most of theoretical models describing the magnetic properties of ferromagnetic or anti-ferromagnetic thin films assume that they have uniaxial magneto-crystalline anisotropy. This is a convenient assumption for computational reasons, which also reproduces experimental results with reasonable accuracy. However, many magnetic materials have cubic symmetry with more than one easy axis. In this paper we examine and compare the energy profiles of an anti-ferromagnet with uniaxial and bi-axial magneto-crystalline anisotropy, when it is coupled to a ferromagnetic layer in a well-known exchange bias system. We show that the bi-axial symmetry case introduces an extra energy minimum corresponding to anti-ferromagnetic spins oriented transverse to the easy axis of the ferromagnet. The multiple symmetry of the anti-ferromagnet could be responsible for the irreversible change in the exchange bias systems, known as the “training effect”.