Influence of twin-crystal structures on the temperature dependence of magneto-optic Kerr effect and magnetic anisotropy in epitaxial Ni thin films

Abstract

We report on the enhanced Curie temperature and magnetic anisotropy observed in the temperature dependence of magneto-optic Kerr effect (MOKE) in epitaxial Ni films grown on single-crystal c-cut sapphire Al2O3 (0001) substrates. X-ray diffraction (XRD) revealed the epitaxial growth of Ni (111) on Al2O3 (0001) substrate while XRD azimuthal scans indicated the presence of 60°-rotated twin-crystal structures of fcc Ni in ABC…and ACB… stacking in the Ni/Al2O3 (0001) film. Twin-crystal structures were also revealed from the surface morphology of the Ni (111) layer in the Ni/Al2O3 (0001) film. MOKE hysteresis loops indicated higher Kerr remanence with lower coercivity for the Ni/Al2O3 (0001) film as compared to the polycrystalline Ni film grown on Si (100) substrate under the same conditions. Azimuthal MOKE hysteresis loops confirmed the presence of 2-fold in-plane magnetic anisotropy in the Ni/Al2O3 (0001) film. Temperature dependent MOKE measurements in the range of 300– 680 K revealed a power-law dependence of the Kerr remanence similar to magnetization. The Curie temperature (TC) of the Ni/Al2O3 film was found to be almost 10 K higher than bulk Ni. In- and out-of-plane magnetization revealed a uniaxial perpendicular magnetic anisotropy with the easy axis along the Ni/Al2O3 film plane. The calculated values of the anisotropy constants revealed a high degree of magnetocrystalline anisotropy in the epitaxial Ni/Al2O3 (0001) film. A close correlation of MOKE and magnetization hysteresis loops indicated similar surface and volume magnetic behaviors in the thin films. The enhanced temperature dependent MOKE, magnetocrystalline anisotropy and magnetization could be attributed to the twin-crystal structures in the epitaxial Ni/Al2O3 film.