Coupled magnetostructural transition in Ni-Mn-V-Ga Heusler alloys and its effect on the magnetocaloric and transport properties

In the present work, the magnetocaloric and transport properties of Ni2.2Mn0.72-xVxGa1.08 (x = 0.0, 0.04, 0.08, 0.12) magnetic shape memory alloys are investigated. The alloys show a coupled magnetostructural transition from paramagnetic austenite to ferromagnetic martensite in a composition range of 0 <= x <= 0.08. For higher V substitution (x = 0.12), the martensite transition is lower than the conventional ferromagnetic transition. Large magnetic entropy change values of about 12.4, 16.2 and 19 J kg(-1) K-1 and corresponding refrigeration capacities of 60.6, 82.5, and 103 J kg(-1) were observed for x = 0, 0.04 and 0.08 alloys, respectively. The above two parameters linearly increase with increasing magnetic field. The indirect adiabatic temperature change calculated from the heat capacity measurement is found to be at its maximum for x = 0.12 at H = 8 T. The magnetoresistance is observed to increase from 0 % (x = 0.12) to 28 % (x = 0) at the maximum field of 8 T. The Sommerfeld coefficients are almost the same for the parent and a V-doped sample, which reveals a low free electron density, and the Debye coefficients decrease with an increase in V doping, confirming the phenomenon of electron-phonon scattering. The critical exponents at second order magnetic transition for x = 0.12 are calculated as beta = 0.482, gamma = 1.056, delta = 3.021, which agrees closely with mean field theory.