Temperature invariable magnetization in Co-Al-Fe alloys by a martensitic transformation

Recently, it was reported that in a dual-phase ferromagnetic Fe-Ga alloy, highly thermally stable magnetization is achieved by a diffusional transformation between the two phases with different magnetizations, which compensates for the inevitable magnetization reduction. Contrasting with the compensation mechanism through a diffusional transformation, here we report that a diffusionless martensitic transition can also be used as a compensation mechanism for achieving nearly temperature invariable magnetization. We found that ferromagnetic Co-Al-Fe alloys with a non-thermoelastic martensitic transformation exhibit almost unchanged magnetization up to 830 K (about 81% T_C). In-situ heating transmission electron microscopy reveals that the gradual transformation from a hexagonal close-packed (hcp) martensite phase with lower magnetization into a face-centered-cubic (fcc) parent phase with higher magnetization occurs over a wide temperature range, giving rise to such temperature invariable magnetization. The electrical resistivity measurement reveals that the fcc → hcp transformation can be repeated during heating-cooling cycles, which can be used to design temperature invariable magnets with a long cycling life.