Magnetic refrigerant properties of Mn-Pnictides showing giant magnetocaloric effecits

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In recent years, there has been increasing interest in magnetic refrigeration as an environmentally friendly alternative to conventional vapor-cycle refrigeration. In order to realize magnetic refrigeration, new materials with giant magnetocaloric effects(MCEs) are strongly desired. In 2001, we have found that MnAs_<1-x>Sb_x compounds with 0【less than or equal】x【less than or equal】0.3 exhibit giant MCEs between 230 K and 315 K. In this research, we performed detailed measurements of magnetocaloric properties on MnAs_<1-x>Sb_x, which were prepared in various different conditions. Following results were obtained.1)We found that MnAs_<1-x>Sb_x forms a single phase with NiAs-type structure, when Mn was slightly enriched. The results suggest that some Mn atoms occupy the interstitial site of the NiAs-type structure, as observed in the case of Mn_<1+δ>Sb.2)Effects of heat treatment on the magnetic transition of Mn_<1+δ>As_<1-x>Sb_x were examined. It was found that quenching from a melted state and subsequent annealing are important to realize a sharp magnetic transition. X-ray diffraction measurements have revealed a first-order magnetic transition(FOMT) of the present compounds. We believe that FOMT is responsible for giant MCEs in Mn_<1+δ>As_<1-x>Sb_x.3)Direct measurements of the adiabatic temperature change, ΔT_<ad>, were carried out for Mn_<1+δ>As_<1-x>Sb_x compounds with x=0.1 and 0.25. For both compounds, we obtained ΔT_<ad> =10 K in a field change of 5 T. These are direct evidence of giant MCEs of Mn_<1+δ>As_<1-x>Sb_x.4)We examined MCEs of other compounds showing FOMT, such as Mn_3GaC,Ho(CoNi)_2 and Co(SSe)_2. From these results, we can conclude that the transition metal based compounds are essentially advantage for giant MCEs near room temperature.