Phase competition in the double-exchange model on the frustrated pyrochlore lattice

Competition between the ferromagnetic double-exchange interaction and the superexchange antiferromagnetic interaction is theoretically studied in the presence of geometrical frustration. As the superexchange interaction increases, the ferromagnetic metal becomes unstable and is taken over by a cooperative paramagnetic metal, in sharp contrast to a discontinuous transition to the antiferromagnetic insulator in the absence of frustration. In the critical region, the system exhibits a peculiar temperature-independent behavior with highly incoherent transport, suggesting a large residual entropy at low temperatures. We discuss the relevance of the results to the pressure-induced behaviors in Mo pyrochlore oxides [S. Iguchi et al., Phys. Rev. Lett. 102, 136407 (2009)].