Brillouin Light Scattering study of the rotatable magnetic anisotropy in exchange biased bilayers of Ni81Fe19Ir20Mn80

It is known that when a ferromagnet (FM) is in atomic contact with an antiferromagnet (AF) the exchange coupling between the FM and AF spins at the interface induces a unidirectional anisotropy in the ferromagnetic film. This effect is known as exchange bias (EB). Despite the large amount of research on this topic there are still several aspects of the EB mechanism that are not well understood. One of this aspects is the origin of the rotatable anisotropy in polycrystalline AFs. By means of Brillouin Light Scattering (BLS) measurements, we investigated the dependence of the rotatable anisotropy field H$_{\mathrm{RA}}$ and exchange field H$_{\mathrm{E}}$ with the magnitude of the external magnetic field (H$_{\mathrm{o}})$ in FM/AM bilayers of Ni$_{\mathrm{81}}$Fe$_{\mathrm{19}}$(10nm)/Ir$_{\mathrm{20}}$Mn$_{\mathrm{80}}$(t$_{\mathrm{AF}})$. We developed an algorithm to numerically fit the in-plane angular dependence of the magnon frequency, at a fixed value of H$_{\mathrm{o}}$ measured by BLS. From the fit parameters we were able to investigate H$_{\mathrm{RA}}$ and H$_{\mathrm{E}}$ dependency on H$_{\mathrm{o}}$. The results reveal that H$_{\mathrm{RA}}$ value depends on H$_{\mathrm{o}}$, so we argue that AF grain distribution at the interface is partially modified by the applied field strength. Contrary to this, the relation between H$_{\mathrm{E}}$ and H$_{\mathrm{o}}$ is not straightforward, remaining constant at high values of H$_{\mathrm{o}}$.