Magnetic field-induced transition in co-doped Ni₄₁Co ₉Mn_31.5Ga_18.5heusler alloy

Thermal strain, magnetostriction and magnetization measurements of Ni ₄₁Co₉Mn_31.5Ga_18.5polycrystalline ferromagnetic shape memory alloy (FSMA) were performed across the martensitic transition temperature, TM, and the reverse martensitic transition temperature, TR, at atmospheric pressure. When cooling from the austenite phase, a steep decrease in thermal expansion due to the martensitic transition at TM was found. When heating from the martensitic phase, a steep increase in the thermal expansion due to the reverse martensitic transition at TR was observed. These transition temperatures decreased gradually with increasing magnetic field. The field dependence of the martensitic transition temperature, dTM/dB, is -4.2K/T and that of the reverse martensitic transition temperature, dTR/dB, is -7.9K/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 TM. The region above TM or TR is the ferromagnetic austenite phase and that below TM or TR 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.