ISOTROPIC GIANT LINEAR MAGNETOSTRICTION AND LARGE MAGNETOCALORIC EFFECTS IN La(Fe_xSi_(1-x))_(13) ITINERANT-ELECTRON METAMAGNETIC COMPOUNDS AND THEIR HYDRIDES

摘要

In the present review, the itinerant-electron metamagnetic (IEM) transition of the La(Fe_xSi_(1-x))_(13) compounds and their hydrides are discussed from the practical viewpoints. The field-induced first-order magnetic transition from the paramagnetic (P) to the ferromagnetic (F) state, that is, the IBM transition takes place in the La(Fe_xSi_(1-x))_(13) compounds. Since the magnetization change due to the IEM transition is very large, the La(Fe_xSi_(1-x))_(13) compounds exhibit an isotropic giant linear magnetostriction and large magnetocaloric effects (MCEs). For the practical applications of the IEM transition in the La(Fe_xSi_(1-x))_(13) compounds, we have examined to increase T_C up to room temperature, because the IEM transition field increases with going away from T_C. The value of T_C increases, whereas the IEM transition becomes obscure with decreasing x. On the other hand, by hydrogen absorption into the La(Fe_xSi_(1-x))_(13) compounds, T_C increases up to room temperature with keeping the IEM transition above T_C. As a result, a giant linear magnetostriction of about 0.27 percent originated from an isotropic volume magnetostriction is observed in the La(Fe_(0.89)Si_(0.11))_(13)H_(1.3) compound by applying the magnetic field of 1 T at 292 K. This value is much larger than those of TbFe_2 and its quasi-binary compounds which are well-known as giant magnetostrictive compounds. In contrast to conventional anisotropic magnetostrictive materials, there is no need of controlling the crystallographic structures because of the isotropic volume magnetostriction. Consequently, the La(Fe_xSi_(1-x))_(13)H_y compounds are promising as new-type practical giant magnetostrictive materials. Large values of the isothermal magnetic entropy change (triangle open)S_m = - 30 J/kg K and the adiabatic temperature change (triangle open)T_(ad) = 12.1 K for the La(Fe_(0.90)Si_(0.10))_(13) compound are observed at T_C = 184 K in the magnetic field change from 0 to 5 T ((triangle open)H = 5 T). The La(Fe_(0.90)Si_(0.10))_(l3)H_(1.1) compound also exhibits (triangle open)S_m = - 31 J/kg K and (triangle open)T_(ad) = 15.4 K at T_C = 287 K in (triangle open)H = 5 T, because the IEM transition is induced in the vicinity of room temperature by controlling the hydrogen concentration y. The value of (triangle open)S_m is larger than those of Gd and Gd_5Si_2Ge_2 , and almost the same value as that of MnAs. The value of (triangle open)T_(ad) for the La(Fe_(0.90)Si_(0.10))_(13)H_(1.1) compound is also larger than those of Gd and MnAs, and almost the same value as that of Gd_5Si_2Ge_2. Since the IEM transition in the La(Fe_xSi_(1-x))_(13) compounds and their hydrides takes place without any crystallographic structural changes, these compound are stable against thermal cycles. Their thermal transport properties are also excellent from the standpoint of practical applications. These results make it clear that the La(Fe_xSi(1-x))_(13) compounds and their hydrides are promising as high-performance magnetic refrigerants working in a wide temperature range covering room temperature in relatively low magnetic fields. On these grounds, we arrive at the conclusion that the La(Fe_xSi_(1-x))_(13) compounds and their hydrides are promising in both fields of practical applications for magnetostrictive and magnetic refrigerant materials.