Theory of record thermopower near a finite temperature magnetic phase transition in IrMn

Abstract

The effect of scattering of conduction electrons by dynamical spin fluctuations on the thermopower in metals near a thermal phase transition into an antiferromagnetic phase is considered. We are interested in a transition at room temperature, as has been studied in a heterostructure involving layers of IrMn. We show that the electrical resistivity exhibits a narrow but low peak at the transition, which may be difficult to detect on top of the main contributions induced by phonons and impurities. By contrast, the thermopower is found to exhibit a prominent peak both as a function of temperature $T$ for fixed layer thickness ${t}_{\text{AFM}}$ and as a function of ${t}_{\text{AFM}}$ for fixed $T.$ We conjecture that the transition temperature ${T}_{c}$ is a function of both ${t}_{\text{AFM}}$ and the Fermi energy ${\ensuremath{\epsilon}}_{F}$. Both dependencies give rise to a sharp peak of the thermopower as a function of $T$ or ${t}_{\text{AFM}}$ near the transition. The estimated magnitude and width of the peak for the case of either three- or two-dimensional longitudinal spin fluctuations is in good agreement with experiment.