Thermopower of spin glasses

Abstract

We calculate the thermopowerS _d(T) of spin glasses on the basis of thesd exchange model with additional interactions between the impurity spins by means of time dependent perturbation theory. Similarly to the Kondo effect, the observed giant thermopower can only be explained if one also takes into account the spin independent interactionV between the magnetic impurities and the conduction electrons. We obtain a “Kondo” termS ⁽¹⁾_d (T) (which reduces for vanishing spin interactions to the thermopower of Kondo systems) and in addition a “resonance” termS ⁽²⁾_d (T) of opposite sign which vanishes for vanishing interactions. The superposition of both terms leads to an additional extremum of the total thermopower and to a change of sign at a temperatureT ₀ which depends on the Kondo temperature and on the magnetic excitations of the spin glass and therewith on the freezing temperatureT _f. We obtain at low temperatures for oscillating and relaxing spin glass modesS ⁽¹⁾_d ∝T² andS ⁽²⁾_d ∝T. At high temperaturesS ⁽¹⁾_d (T) decreases as |lnT|⁻³ andS ⁽²⁾_d asT ⁻¹. A close relationship between the temperature dependent resistivity in Born approximation andS ⁽¹⁾_d (T) is pointed out.