Crystal structure, magnetic and transport properties, and electronic band structure of colossal magnetoresistance Tl2Mn2O7 pyrochlore

A thallium–manganese pyrochlore, Tl2Mn2O7, shows magnetotransport properties including a colossal magnetoresistance (CMR) quite similar to those observed in the hole-doped perovskite manganese oxides. The pyrochlore material, however, differs both structurally and electronically from the perovskites. Neither Zener’s double-exchange nor the phonon-mediated mechanism, which is usually applied to the perovskite compounds, seems to explain the CMR in the pyrochlore. A distinctive electronic band structure, in which a hybridized band of Tl(6s)–O(2p)–Mn(3d) plays an essential role in magnetic and transport properties, is described. The CMR mechanism for Tl2Mn2O7 is discussed in terms of a drastic change in the electronic band structure.