Changes in electronic states and magnetic free energy in La _1-zCe_z(Fe_xSi_1-x)₁₃ magnetic refrigerants

The influence of partial substitution of Ce on the electronic structure and magnetic free energy has been investigated for La_1-zCe _z(Fe_xSi_1-x)₁₃. From the Mossbauer spectroscopy of La_0.7Ce_0.3(Fe_0.88Si _0.12)₁₃, the distribution of the electric field gradient is found to be scarcely changed, therefore, the volume reduction by partial substitution is regarded as isotropic. The change of the isomer shift to positive sign after the partial substitution is closely correlated with the 5d and/or 4f electrons of Ce. The change in magnetic free energy has been examined for La_0.7Ce_0.3(Fe_0.86Si_0.14) ₁₃ having a large magnetic entropy change ΔS_m and a small hysteretic behaviour. From the results analysed by the Landau expansion theory, the large ΔS_m and the small hysteresis of this compound are attributed to the magnitude and thermal variation of the fourth-order Landau coefficient in magnetic free energy. Consequently, the combination of partial substitution and control of Fe concentration is useful for highly efficient magnetic refrigerants.