Physical Properties Of The Sm_1-xGd_xAl₂ Polycrystalline Samples At Low Temperatures

In this paper, we report the results of the magnetization ,electrical resisitivity, thermal conductivity and thermoelectric power in applied magnetic field at low temperatures of Sm_1-xGd_xAl₂(in this paper, 0﹤x﹤0.1) polycrystalline samples. Magnetization measurements were performed on the Sm_1-xGd_xAl₂ polycrystalline samples in applied magnetic fields. The materials are ferromagnetism and the paramagnetic-ferromagnetic transition occurs at Tc about 125K. The spin and orbital moments have different temperature dependence and coupled antiparallel by the spin–orbital interaction below ordering temperature. The difference temperature dependence of spin and orbital moments come from the complex thermal admixture of nearly degenerate J multiplets in which the Sm³⁺ ion exist (the ground state J=5/2 multiplets is only 1400K from the first excited state J=7/2). The admixture arises from the crystalline electric field effect on the degeneracy of the J states. Therefore, the net moments dip to zero at a compensation temperature. At this temperature, the spin and orbital moments are equal to each other. In strong applied magnetic field, the spin-orbital flip occurs at compensation temperature. The electrical resistivity of ferromagnetic materials Sm_1-xGd_xAl₂ consists ρ₀, caused by lattice defects etc.; ρ_g(T), arising from the electron-phonon interaction and ρ_m(T), arising from scattering processes of conduction electrons due to disorder in the arrangement of the magnetic moments. In the high temperature limit, ρ_g(T) is proportional to T. When temperature is above ordering temperature, ρ_m(T) does not change with temperature. There is a sudden bend appears on the curve of ρ_m(T) with decreasing temperature through ordering temperature. It is assumed that the total thermal conductivity is given by the sum of two contributions: the electronic thermal conductivity λ_e and the lattice thermal conductivity λ_l . A little change of the curvature is observable at ordering temperature. It imply that magnetic scattering processes have an influence on the thermal conductivity. For all the isostructural magnetic REAl₂ compounds, λ_l is sufficient small to be neglected, then λρis proportional to T. there is no anomaly in the thermal conductivity at compensation temperature and the applied magnetic field has no effect on the thermal conductivity in the measured temperatures. The thermoelectric power of Sm_1-xGd_xAl₂ compounds is given by the sum of two contributions: Se(T), duing to thermal diffusion of conduction electrons in the thermal gradient and Smag(T), depending on the partical state densities of conduction electrons of the sample. When temperature is below ordering temperature, the contribution of thermoelectric power is mainly due to ferromagnetic ordering, denoted by Smag. When temperature is above ordering temperature, S(T) is due to Se(T). The spin-wave fluctuation is suppressed in magnetic field. This is responsible for the increasing of thermoelectric power in magnetic field. At low temperatures, the temperature dependence of thermoelectric power is governed by the electron-magnon scattering. Therefore, the change of thermoelectric power between in zero and 10T magnetic field is small at low temperatures. Spin-wave fluctuation contribution term becomes more important from about 50K to Tc. The change of total thermoelectric power between in zero and 10T magnetic field becomes bigger.