Electronic structure and exchange interactions in the manganese-based pyrochlore oxides

Abstract

We describe the ferromagnetism in the manganese pyrochlores, such as Tl 2 Mn 2 O 7 and Sc 2 Mn 2 O 7 , in terms of the interplay between superexchange, Zener double exchange, and indirect exchange, the first being antiferromagnetic ͑AF͒ while the last two are ferromagnetic. The tendency towards the antiferroalignment of the localized Mn spins is significantly weakened due to ͑i͒ the presence of spin frustration in the Mn sublattice and ͑ii͒ the large bend in the Mn-O-Mn bond, which reduces the magnitude of the AF superexchange. This helps the ferromagnetic interaction terms, viz., the indirect exchange and the Zener double exchange, to dominate. The indirect exchange on the Mn-O-Mn bond, which is modeled by a coupling of the O 2p electrons to the conduction-band states, produces a weak ferromagnetism in the insulating pyrochlores. In the metallic pyrochlores, the indirect exchange is also present, but it is now supplemented by the Zener double exchange, making the ferromagnetism more robust, as indicated from a higher value of the Curie temperature T c . Density-functional calculations for the metallic Tl 2 Mn 2 O 7 pyrochlore, show the itinerant Zener carriers mediating double-exchange to be electrons in the ⌫ 1 minority spin band crossing the Fermi energy, with predominantly Mn 3d, Tl 6s, and O 2p characters. These Zener carriers move in a lattice of localized Mn t 2g spins, with the carrier spins aligned antiparallel to the localized spins. ͓S0163-1829͑98͒05335-1͔

Charge dynamics in the colossal magnetoresistance pyrochloreTl2Mn2OOkamura1, Koretsune2, Matsunami3 et al. 2001Phys. Rev. B200151View full textAdd to dashboardCite

“…1͑b͔͒ and the measured density of nϭ0.006 per f.u., 4,10 we obtain m*ϳ0.6m 0 , which is indeed small and close to the calculated m*. Hence, the band calculations [7][8][9] are successful in predicting the basic electronic structures of FM Tl 2 Mn 2 O 7 near E F . Figures 2 and 3 show R() and (), respectively, in external magnetic fields Bр6 T, applied normal to the sample surface.…”

“…The peaks in the spectra above 2 eV can be attributed to charge transfer excitations from O 2p to Mn 3d bands ͑the strong peak near 2.5 eV͒ and excitations to other higher-lying states. [7][8][9] Figure 1͑b͒ shows R() and () below 0.5 eV at several T's. 14 Upon cooling below T c ϭ120 K, both R() and () increase rapidly, with R() approaching 1 and () showing a sharp rise toward the lower energy end.…”

“…Although these features appear very similar to those for the perovskites, various studies have suggested that the underlying mechanism should be very different: In Tl 2 Mn 2 O 7 the electric conduction takes place in a conduction band having strong Tl 6s and O 2 p characters, as shown by band calculations. [7][8][9] The spontaneous magnetization below T c is produced by the Mn 4ϩ sublattice through superexchange interaction, independently from the conduction system. The CMR results from changes in the conduction system caused by the magnetization in the Mn 4ϩ sublattice.…”

Investigation of the presence of charge order in magnetite by measurement of the spin wave spectrum

“…However, in this instance, we only require knowledge of the small corrections to the superexchange relative to the experimentally determined cubic values. Exact-diagonalization calculations and perturbation theory 29 have shown that for the transition metal oxides, the superexchange is approximately proportional to…”

Magnetic properties and magnetoresistance of the Ru-substitutedTl2Mn2−xSenis1, Martı́nez2, Fontcuberta3 et al. 2000Phys. Rev. B

“…The small, but finite, number of Zener carriers may contribute to the reinforcement of FM interactions via DE, leading to the higher T c observed experimentally. 19 Finally, as we mentioned in the experimental section, the observation of magnetic irreversibility in the ZFC-FC process even in the xϭ0 sample points to the existence of AF interactions in the pure Tl 2 Mn 2 O 7 compound that exhibit long-range FM ordering. This fact is difficult to reconcile with a simple DE or superexchange ferromagnetic picture of the magnetic coupling.…”