Anisotropic Damping in Exchange Bias Systems*

Current experimental broadband investigations of the magnetization dynamics have shown that the damping mechanism in exchange bias systems exhibits a strong unidirectional contribution, caused in part by two-magnon scattering. However, a detailed analysis strongly suggests the presence of a previously undescribed unidirectional relaxation mechanism [1]. To describe this phenomenon theoretically we use the formalism of an anisotropic Gilbert damping tensor that takes the place of the (scalar) Gilbert damping parameter in the Landau-Lifshitz-Gilbert equation of motion. Each component of the symmetric tensor can depend implicitly on the magnetization direction. While for single crystals the anisotropy of the damping tensor is expected to be small, making experimental confirmation difficult, the broken symmetry in exchange bias systems provides an excellent testing ground to study the modified magnetization dynamics as well as the dynamic response under the influence of unidirectional damping.
References: [1] J. Beik Mohammadi et al., Phys. Rev. B 95, 064414 (2017).