Electronic transitions and multiferroicity in transition metal oxides

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Zhou, Haidong

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Four systems have been studied for the localized-itinerant electronic transition in transition-metal oxides: (i) In CaV1-xTixO3, substitution of Ti(IV) introduces Anderson-localized states below a mobility edge µ c that increases with x, crossing F ε in the range 0.2 < x< 0.4 and also transforms the strong-correlation fluctuations to localized V(IV): t1e0 configurations for x ≥ 0.1. (ii) The properties of LaTiO3+δ reveal that a hole-poor, strongly correlated electronic phase coexists with a hole-rich, itinerant-electron phase. With δ ≤ 0.03, the hole-rich phase exists as a minority phase of isolated, mobile itinerant-electron clusters embedded in the hole-poor phase. With δ ≥ 0.08, isolated hole-poor clusters are embedded in an itinerant-electron matrix. As δ > 0.08 increases, the hole-poor clusters become smaller and more isolated until they are reduced to super-paramagnetic strong-correlation fluctuations by δ = 0.12. (iii) The data of Y1-xLaxTiO3 appears to distinguish an itinerant-electron antiferromagnetic phase in the La-rich samples from a localized-electron ferromagnetic phase with a cooperative Jahn-Teller distortion in the Y-rich phase. (iv) The transition at Tt in Mg[Ti2]O4 is a semiconductor-semiconductor transition associated with Ti-Ti dimerization instabilities. The dimerization is caused by lattice instabilities resulting from a double-well Ti-Ti bond potential at a crossover from localized to itinerant electronic behavior. RMn1-xGaxO3 (R = Ho, Y) and Ho1-xYxMnO3 have been studied for the multiferroicity of RMnO3. Ga doping raises the ferrielectric Curie temperature TC and the Mn-spin reorientation temperature TSR while lowering TN of the Mn spins and the Ho magnetic ordering temperature T2. The data show an important coupling between the Mn3+-ion and Ho3+-ion spins as well as a TSR that is driven by a cooperative MnO5 site rotation and R3+-ion displacements that modify the c lattice parameter. The data also support an enhanced spin-lattice interaction in the geometrically frustrated (GF) Mn-spin system. Y doping enhances the temperature region for the P6’3cm’ magnetic phase and thereby increases TSR for Ho1- xYxMnO3. The studies of several oxygen non-stoichiometric Fe4+/Fe3+ oxoperovskite show that two mechanisms, the formation of Fe3+-O-Fe4+ pair and the disproportionation reaction 2Fe(IV)O6/2 = Fe3+ + Fe(V)O6, dominate the electronic behavior. The properties of DyBaCo2O5.5 reveal a spin-state transition from the low-spin t 6 e 0 ground state to higher spin-state at octahedral-site Co3+, which is also accounted for the metamagnetism in the sample.