Development of metamagnetic-type large magnetocaloric materials used in high performance magnetic refrigeration for cryogenic thermometry

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To obtain large magnetocaloric effects (MCEs) just above the Curie temperature Tc in La(Fe_xSi_<1-x>)_<L3> at low temperatures, it is necessary to control Tc to lower temperature regions. In (La_<1-z>R_z)(Fe_<0.88>Si_<0.12>)_<L3>(R=Ce), a lattice constant exhibit a remarkable shrinkage of about 1.5% and Tc is lowered to 173K. Variation of Tc was different between the partial substitutions of R=Ce and Pr, and it has been revealed that influence of hybridization among electrons in addition to the magnetovolume effect on Tc is important for the Ce-substituted compound s. Furthermore, a corresponding tendency that an increase (decrease) in 3d electron number results in the higher (lower) value of Tc has been observed in the partial substitution of Cr, Mn and Ni for Fe. From these results, the complex partial substitution in (La_<1-z>Ce_z)(Fe_<xy>Mn_ySi_<1-x-y>)_<13> has been carried out, resulting in variation of Tc from 0 to 180 K with maintaining the large MCEs by adjusting the concentration of Ce and Mn.In addition to these results, we have also investigated an influence of the energy barrier ΔE_b on the adiabatic temperature change. By examining the thermodynamic route in the adiabatic process, it has been revealed that the process is terminated when the energy barrier of 1kJ/kg can not be overcomed. Therefore, by taking the relation between ΔE_b and the hysteresis of metamagentic transition into account, we have investigated a control of the hysteresis loss and found a remarkable reduction of the loss by the partial substitution of Ce in the lower Fe concentration regions.We also confirmed that the thermal conductivity properties give no serious problem. Since these compounds are mainly composed of Fe which is cheap in the industrial market, and no constituent element is toxic, it is concluded that the present compound system is one of the promising candidates as new magnetic refrigerants working at low temperatures.