Martensitic transformation induced by magnetocrystalline anisotropy energy for Co-Fe alloys

Abstract

The magnetization process of Co-0.8 at. % Fe, Co-1.0 at. % Fe, and Co-1.2 at. % Fe alloy single crystals along the a axis and c axis was carefully investigated using a vibrating sample magnetometer. The easy axis of magnetization for Co-0.8 at. % Fe, Co-1.0 at. % Fe, and Co-1.2 at. % Fe alloy single crystals was found to be reoriented from the a axis to the c axis when the magnetic field was applied below 211, 132, and 56 \ifmmode^\circ\else\textdegree\fi{}C, respectively. This magnetic field was also found to work anisotropically for this reorientation of the easy direction. The critical-field strength and critical angle were precisely determined. The x-ray-diffraction study and the transmission electron microscope observations revealed that the reorientation of the easy direction originated in the martensitic transformation from double hexagonal close-packed structure (dhcp) to hexagonal close-packed structure (hcp) induced by a magnetic field. The main driving force of this field induced martensitic transformation was the large magnetocrystalline anisotropy energy difference between hcp and dhcp phases determined by thermodynamic analysis, contrary to the magnetostatic energy difference which was well understood as the driving force for an ordinary field induced martensitic transformation mainly observed in Fe-based alloys. It was found to be a type of field-induced martensitic transformation which was driven by the magnetocrystalline anisotropy energy difference between both phases.