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

为了在La（Fe_xSi_）_2中获得高于居里温度Tc的低温磁热效应<1-x><L3>，必须将Tc控制在较低的温度范围内。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降低到173 K。部分取代的R=Ce和Pr的Tc的变化是不同的，它已被揭示，除了磁体积效应对Tc的电子之间的杂化的影响是重要的Ce取代化合物s。此外，在Cr、Mn和Ni部分替代Fe时，观察到3d电子数增加（减少）导致Tc值升高（降低）的相应趋势。在此基础上，对（La_<1-z>Ce_z）（Fe_<xy>Mn_ySi_<1-x-y>）_<13>进行了复杂的部分替代，通过调节Ce和Mn的浓度，在保持大的MCE的同时，使Tc在0 ~ 180 K范围内变化.此外，我们还研究了能垒ΔE_B对绝热温度变化的影响.通过对绝热过程热力学途径的考察，揭示了当不能越过1 kJ/kg的能垒时，绝热过程终止。因此，考虑到ΔE_B与相变滞后的关系，我们研究了滞后损耗的控制，发现在低Fe浓度区，Ce的部分替代可显著地降低滞后损耗，并证实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廉价的Fe组成，并且没有组成元素是有毒的，因此可以得出结论，本化合物系统是在低温下工作的新的磁制冷剂的有希望的候选者之一。

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.