Metamagnetic transition and magnetocaloric properties in antiferromagnetic Ho2Ni2Ga and Tm2Ni2Ga compounds

The structure, magnetic transition and magnetocaloric properties in the ternary gallium intermetallic compounds of Ho2Ni2Ga and Tm2Ni2Ga have been systematically investigated by using X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) and magnetization measurements. As found from powder XRD and Rietveld refinement, Ho2Ni2Ga and Tm2Ni2Ga crystallized in an orthorhombic structure of the W2B2Co type belonging to the Inunm space group. Both compounds are ordering anti-ferromagnetically at low temperatures and under low magnetic fields together with a field-induced metamagnetic transition from antiferromagnetic (AFM) to ferromagnetic (FM). Peculiar and similar magnetocaloric properties are observed for both compounds, i.e., an inverse magnetocaloric effect (positive magnetic entropy change, Delta S-M) under low magnetic field changes (AM and at low temperatures together with a large normal reversible magnetocaloric effect under high Delta H. The positive Delta S-M below/at the Neel temperatures (T-N) is ascribed to first order transition from antiferromagnetic (AFM) to ferromagnetic (FM) states, whereas the negative Delta S-M above TN is due to the fact of the magnetic transition from paramagnetic (PM) to ferromagnetic (FM) states. Based on the field-dependent magnetization data, the maximum magnetic entropy change (-Delta S-M(max)), relative cooling power (RCP) and refrigerant capacity (RC) are established, and the corresponding values are 9.6 J/kg K, 276 J/kg and 206 J/kg for Ho2Ni2Ga, and are 4.3 J/kg K, 60 J/kg and 46 J/kg for Tm2Ni2Ga, respectively, for a magnetic field change of 0-70 kOe.