Antiferromagnetic layer thickness dependence of noncollinear uniaxial and unidirectional anisotropies in NiFe/FeMn/CoFe trilayers

Abstract

We have investigated the dependence of magnetic anisotropies of the exchange-biased NiFe/FeMn/CoFe trilayers on the antiferromagnetic (AF) layer thickness $({t}_{\text{AF}})$ by measuring in-plane angular-dependent ferromagnetic resonance fields. The resonance fields of NiFe and CoFe sublayers are shifted to lower and higher values compared to those of single unbiased ferromagnetic (F) layers, respectively, due to the interfacial exchange coupling when ${t}_{\text{AF}}\ensuremath{\ge}2\text{ }\text{nm}$. In-plane angular dependence of resonance field reveals that uniaxial and unidirectional anisotropies coexist in the film plane, however, they are not collinear with each other. It is found that these peculiar noncollinear anisotropies significantly depend on ${t}_{\text{AF}}$. The angle of misalignment displays a maximum around ${t}_{\text{AF}}=5\text{ }\text{nm}$ and converges to zero when ${t}_{\text{AF}}$ is thicker than 10 nm. Contributions from thickness-dependent AF anisotropy and spin frustrations at both F/AF interfaces due to the structural imperfections should be accounted in order to understand the AF-layer thickness dependence of noncollinear magnetic anisotropies.