First order to second order transition crossover in the magnetocaloric compounds HoCo_2-xNi_x (0 ≤ x ≤ 2): neutron diffraction study of HoCoNi

Magnetic and magnetocaloric properties of cubic Laves phase rare earth intermetallic HoCo_2-xNi_x (0≤x≤2) compounds (space group, Fd3-barm, No. 227) have been studied. Lattice parameter decreases with increasing Ni-concentration in HoCo_2-xNi_x compounds. The parent compound HoCo₂ orders ferrimagnetically at ∼ 80 K followed by a spin-reorientation transition at 14 K. The Ni-substituted compounds HoCo_2-xNi_x (x = 0.5, 0.75, 1.0, 1.25, 1.5, 1.75 and 2.0) order ferromagnetically at ∼ 42 K, 40 K, 42 K, 36 K, 28 K, 22 K and 14 K (T_C), respectively. All the Ni substituted compounds are soft ferromagnets at 2 K. The isothermal magnetic entropy change, ΔS_m, has been estimated from the magnetization vs field data around the magnetic transition. All these compounds show giant magnetic entropy change near T_C and the large values of relative cooling power make them potential magnetic refrigerants at low temperatures. The order of magnetic transition in all these compounds have been ascertained by the construction of universal master curve using the isothermal magnetic entropy change data in addition to making the conventional Arrott plots. The ΔS_m vs T curves of the compounds follow a universal magnetocaloric behavior illustrating the second order nature of the magnetic transition in HoCo_2-xNi_x (02-xNi_x (0C whereas it shows T² dependence below T_C suggesting dominant magnon scattering in the ordered state. While the powder neutron diffraction data on HoCoNi compound confirm collinear ferromagnetic ordering of holmium sublattice below ∼ 42 K and the magnetic moment reaches maximum value of 8.45(7) μ_B at 11 K, the temperature at which a spin reorientation-like transition is observed in the macroscopic magnetization data for this HoCoNi compound and this value continues to be 8.45(7) μ_B until the lowest measured temperature, 1.5 K. (author)