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

为了在La(Fe_xSi_<1-x>)_<L3>中获得居里温度Tc以上的大磁热效应（MCEs），有必要将Tc控制在较低的温度区域。在（La_<1-z>R_z）（Fe_<0.88>Si_<0.12>）_<L3>(R=Ce)中，晶格常数显著收缩约1.5%，Tc降至173K。在R=Ce和Pr的部分取代中，Tc的变化是不同的，揭示了除磁体积效应外，电子间杂化对Ce取代化合物Tc的影响是重要的。此外，在Cr、Mn和Ni的部分取代中，观察到三维电子数的增加（减少）导致Tc值的升高（降低）的相应趋势。根据这些结果，在（La_<1-z>Ce_z）（Fe_<xy>Mn_ySi_<1-x-y>）_<13>中进行了复部分取代，导致Tc从0到180 K变化，并通过调整Ce和Mn的浓度来保持较大的MCEs。除了这些结果外，我们还研究了能量势垒ΔE_b对绝热温度变化的影响。通过研究绝热过程的热力学路线，发现当不能克服1kJ/kg的能垒时，绝热过程终止。因此，考虑到ΔE_b与超磁过渡迟滞之间的关系，我们研究了迟滞损失的控制，并发现在较低铁浓度区域，Ce的部分取代显著降低了损失。我们还确认导热性能没有严重的问题。由于这些化合物主要由工业市场上价格便宜的铁组成，且其组成元素无毒性，因此认为该化合物体系是一种有前景的新型低温磁性制冷剂。

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.