Strong anisotropies in MBE-grown Co/Cr(001): Ferromagnetic-resonance and magneto-optical Kerr-effect studies

Ferromagnetic-resonance (FMR) and magneto-optical Kerr-effect (MOKE) studies of molecular-beam-epitaxy grown Co/Cr(001) superlattices and single layers are presented. The FMR measurements were performed in different sample orientations and in a broad frequency range (9--92 GHz). MOKE hysteresis loops were detected in polar and in longitudinal configuration. The in-plane anisotropy parameters, K₁ and K₂, the out-of-plane anisotropy (i.e., the effective magnetization including the surface anisotropy, Kₛ), and the g factor were determined for these samples, which exhibit an anomalous out-of-plane lattice expansion of the hcp Co(112{}0) on bcc Cr(001) as a precursor of a structural phase transition. Due to the strong in-plane anisotropy the switching of the magnetization to out-of-plane easy axis can occur at lower thicknesses ({}14 {}) than the change of sign of the effective magnetization ({}21 {}). In the intermediate thickness range a peculiar situation can be found with the orientation of the magnetization along the surface normal lying energetically between the in-plane easy and hard axis. Taking into account these effects as well as a reduced magnetic moment, Kₛ{}0.76 erg/cm² is found. For thin Co layers the g factor is found to be increased in comparison with the bulk value. This indicates modifications of the electronic properties accompanied with the considerable structural changes.