Theoretical study of longitudinal and transverse magneto-thermoelectric effects

We present a combined experimental and theoretical studies of the longitudinal and transverse thermoelectric properties of ferrimagnetic spinels CuCr₂X₄ (X = S, Se, Te). The thermoelectric power of all studied phases is positive and consists of two contributions; an almost linear diffusive thermopower starting from high temperature, and a strong enhancement by magnon drag below T_C. The absolute value of thermopower and resistivity decreases from X = S to Te. The thermopower was calculated by DFT method using GGA, GGA+U, GGA+oeeHyb and mBJ potentials. A good agreement with the experimental thermopower was achieved using GGA potential for X = S and with mBJ potential for X = Te. The mBJ potential, which was designed for sp-type semiconductors, better describes the valence bands of CuCr₂Te₄ that exhibit stronger sp-orbitals character than sulphide and selenide. The anomalous Nernst effect (ANE) is negative at room temperature for all phases, the highest absolute value ~1.5 µV/K is observed for X = Te and Se around room temperature, whereas ANE for X = S is much smaller. A sign change of ANE to positive is observed at 285 K for X = S and at 65 K for X = Se. The trend of anomalous Nernst conductivity (ANC), which is increasing from X = S to Te, is reproduced by Berry phase calculations.