Magnetic Field-Induced Transition in Co-Doped Ni₄₁Co₉Mn_31.5Ga_18.5 Heusler Alloy

Abstract

Thermal strain, magnetostriction and magnetization measurements of Ni₄₁Co₉Mn_31.5Ga_18.5 polycrystalline ferromagnetic shape memory alloy (FSMA) were performed across the martensitic transition temperature, T_M, and the reverse martensitic transition temperature, T_R, at atmospheric pressure. When cooling from the austenite phase, a steep decrease in thermal expansion due to the martensitic transition at T_M was found. When heating from the martensitic phase, a steep increase in the thermal expansion due to the reverse martensitic transition at T_R was observed. These transition temperatures decreased gradually with increasing magnetic field. The field dependence of the martensitic transition temperature, dT_M/dB, is −4.2 K/T and that of the reverse martensitic transition temperature, dT_R/dB, is −7.9 K/T. The metamagnetic transition appeared between 330 and 390 K. The results of thermal strain and magnetization measurements indicate that a magneto-structural transition occurred at T_M. The region above T_M or T_R is the ferromagnetic austenite phase and that below T_M or T_R is the paramagnetic or weak ferromagnetic martensitic phase. At constant temperature, a magnetic field-induced strain was observed with a value of 1.0 × 10⁻³, which indicates that this alloy is sensitive to magnetic fields. Strong magneto-structural coupling was revealed by the magnetic properties and phase transitions.