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Hotta, Takashi
Progress in Ferromagnetism Research, p.19 - 38, 2006/00
In manganites, the appearance of the ferromagnetic (FM) metallic phase has been usually rationalized by the so-called double-exchange mechanism, based on a strong Hund's rule coupling between mobile 
electrons and localized 
spins. Then, it has been believed for a long time that hole-doping is indispensable for explaining the FM metallic phase in manganites. However, it has been theoretically pointed out that a novel metal-insulator transition in the FM state can occur even for undoped manganites based on the -orbital degenerate Hubbard model tightly coupled to Jahn-Teller distortions. In this article, the result is reviewed in detail.
Hotta, Takashi; Moraghebi, M.*; Feiguin, A.*; Moreo, A.*; Yunoki, Seiji*; Dagotto, E.*
Physical Review Letters, 90(24), p.247203_1 - 247203_4, 2003/06
Times Cited Count：83 Percentile：91.21(Physics, Multidisciplinary)Novel ground-state spin structures in undoped and doped manganites are here investigated based on the orbital-degenerate double-exchange model, by using mean-field and numerical techniques. In undoped manganites, a new antiferromagnetic (AFM) state, called the E-type phase, is found adjacent in parameter space to the A-type AFM phase. Its structure is in agreement with recent experimental results. This insulating E-AFM state is competing with a ferromagnetic metallic phase as well, suggesting that large magneto-resistant effects could exist even in undoped manganese oxides. For doped layered manganites, the phase diagram includes another new AFM phase of the 
-type. Experimental signatures of the new phases are discussed.
