The thermopower of transition-metal double-perovskites: A sensitive probe of their electronic structures around the Fermi level

We report on a comparative study of S(T) for a series of transition-metal double-perovskites A2BB′O6 (A – Ca, Sr, Ba, and B, B′ = transition metal ions), some of them known to have half-metallic ground states. For Sr2BB′O6 with BB′ = CrMo, CrW, CrRe, FeMo, and FeRe (ferrimagnetic with high Curie temperatures), S(T) is metallic, for B′ = Mo and W it is n-type and for B′ = Re, p-type. For A2FeMoO6 (A = Ca, Sr, Ba), the crystallographic differences (monoclinic, tetragonal and cubic space-groups, respectively) are accompanied by prominent differences in their (metallic) S(T). For the insulating Sr2MnReO6 and Ba2MnReO6, the onset of ferromagnetic order below Tc ∼ 120 K is marked by a steep drop of S(T) accompanied by only a slight change in the slope of ln ρ versus 1/T1/2. Significant conclusions were drawn from the experimental results without the need for elaborate models.