Annealing influence on magnetic anisotropies of CoFe thin films

Post-annealing effect on the magnetic anisotropies of cubic CoFe films epitaxially grown on MgO (001) substrates are investigated. X-ray diffraction patterns reveal that the crystallization degree gradually improves with the increase of annealing temperature (T_a), which results in obvious enhancement in both the uniaxial (H_U) and biaxial magnetic anisotropy fields. Magnetic hysteresis loops of the unannealed sample show two equivalent easy axes along the CoFe [110] and [110] directions, being in accord with the observed fourfold symmetry of angular remanent magnetization (ARM) curve. Nevertheless, with the increase of T_a, the two easy axis loops become inequivalent due to the increased H_U along [110] direction, thus giving rise to a two-step switching behavior for the [110] direction loop at T_a>350°C and the ARM curve transformation from fourfold to twofold symmetry. This work provides a deep insight and an effective approach to control the magnetic anisotropies and magnetization states for practical applications.