Ground states and thermodynamics of the spin–electron model with mixed spins: Application to rare-earth tetraborides

Abstract

A simple, but very realistic, spin–electron model is used to study the influence of mixed spins (from electron and spin subsystems) on a formation of fractional magnetization plateaus in rare-earth tetraborides. The spin subsystem is described by the ordinary Ising model and the electron subsystem by the ordinary Hubbard model. Moreover, both subsystems are coupled by the anisotropic spin-dependent interaction of the Ising type. To model the most realistically situation in rare-earth compounds we study the following combination of mixed spins: S = 1 , 3 / 2 and 2 in the spin subsystem and s d = 1 / 2 in the electron subsystem. It is shown that the mixed spins, in a combination with the Coulomb interaction U , have the significant impact on the formation and stabilization of different magnetization plateaus. In particular, we have found that the Coulomb interaction for S = 1 and S = 3 / 2 stabilizes the 1/2 plateau and strongly suppresses the 1/3 plateau which is in perfect agreement with the experimental measurements in TmB 4 and ErB 4 compounds. At nonzero temperatures our numerical results predict one high-temperature peak (of the Ising type) in the temperature dependence of the specific heat capacity, which accords with specific heat capacity measurements in TmB 4 and ErB 4 , and one low-temperature electron peak, which is not, however, seen in experiments. The reason is probably the fact that the electron peak appears at a several times lower temperature than the spin peak and thus the experiments, made only to 2 K, did not catch it. • A new spin–electron model with mixed spins is proposed. • Mixed spins in a combination with the Coulomb interaction stabilize 1/2 plateau. • Specific heat curves exhibit single- and double-peaked structure. • Predictions of our model are in a very good agreement with experiments in TmB 4 and ErB 4 .