Magnetoelectronic effects in pyrochlore Tl2 Mn2 O7 : Role of Tl-O covalency

Abstract

First principles calculations are used to elucidate the magnetic and electronic structure of pyrochlore ${\mathrm{Tl}}_2$ ${\mathrm{Mn}}_2$ ${\mathrm{O}}_7$ . It is found that the bonding has an important covalent contribution from Tl-O interactions. The high spin ferromagnetic ground state has a very large differentiation between minority and majority spin electronic properties. In particular, the minority spin channel contains a single, nearly spherical high velocity Fermi surface derived from strongly mixed combinations of Tl, Mn, and O orbitals, while the majority spin channel contains three smaller Fermi surfaces with much lower Fermi velocities. The proximity to a band edge in the majority channel may lead to localization of these states if sufficient disorder is present. In any case, highly spin differentiated transport results, with the conduction dominated by the minority channel.