Magnetic Anisotropy Induced by Magnetic and Stress Annealing in Co, Co–Ni and Co–Fe Alloys

Abstract

The uniaxial magnetic anisotropy induced by magnetic annealing has been studied for Co, Co–Ni alloys with the γ\\rightleftarrowsε and for Co–Fe, Fe–Ni alloys with the γ\\rightleftarrowsα martensitic phase transition. An anomalously large uniaxial magnetic anisotropy was found in both pure Co and Co–Ni alloys, while a much smaller anisotropy was observed in the Co–Fe and Fe–Ni alloys. The necessary and sufficient condition to induce such an anomalously large magnetic anisotropy is the existence of a magnetic field during the martensitic phase transition on cooling from a temperature much higher than the phase transition point. A large uniaxial magnetic anisotropy was also observed in Co by annealing under a small stress of about 50 g/mm2 during cooling. No preferred crystal orientation was detected by X-ray diffraction. Therefore, it is suggested that a directional alignment of structural imperfections, such as stacking faults, which relieve the magnetostrictive stresses during the phase transition, could be responsible for the induced magnetic anisotropy for cobalt and cobalt-based alloys.