Large magnetocaloric effects and Landau coefficients of itinerant electron metamagnetic La(FexSi1-x)13 compounds

The paramagnetic-ferromagnetic (P-F) transition, also known as itinerant electron metamagnetic transition, is accompanied by large magnetocaloric effects (MCEs) such as the isothermal entropy change and the adiabatic temperature. In this work, the relation between MCEs and Landau coefficients of itinerant electron metamagnetic La(Fe_xSi_1-x)₁₃ (x = 0.87, 0.88) compounds is experimentally examined. Expressions describing the Helmholtz magnetic free energy, magnetic entropy and the equation of state H(M,T) are derived. The coefficients A(T), B(T) and C(T) are then experimentally determined from the magnetization curves by curve fitting of H(M,T), and are found to exhibit temperature dependence reflecting the thermal variation on the amplitude of spin fluctuations. The first-order P-F transition at Curie temperature appears in the region A(T) > 0, C(T) > 0, B(T) < 0 and 5/28 < A(T)C(T)B(T) < 3/16. From these Landau coefficients, the energy barrier between the F and P states and its influence in the demagnetisation process is calculated.