Monte Carlo Study of the Pyrochlore Double-Exchange Model –Frustration Effects in Molybdenum Pyrochlore Oxides–

where t is the transfer integral for nearest-neighbor sites hiji on the pyrochlore lattice, JH represents the Hund’s-rule coupling between the itinerant electron spin ~si and the localized spin ~ Si, while JAF is the super-exchange antiferromagnetic interaction between the neighboring localized spins. The first two terms induce the double-exchange ferromagnetic interactions through the kinetic motion of electrons, which compete with the third term. Antiferromagnetic interactions create frustration effects on the pyrochlore lattice. Details of the Monte Carlo calculations are given in Ref. [2]. The study is motivated by the experimental investigations of molybdenum pyrochlore oxides R2Mo2O7 which show an interesting competition between ferromagnetic metal and spin-glass insulator by changing the rear-earth element R [3, 4]. Recent experiments by applying external pressure revealed some new aspects on the phase competition. In particular, it was shown that the ferromagnetic metal turns into a peculiar diffusive metal under pressure, with showing an intervening spin-glass metallic state [5]. This is surprising because in many transition metal compounds the chemical pressure by