Orbital ordering and one-dimensional magnetic correlation in vanadium spinel oxides AV2O4(A

Abstract

We present our theoretical results on the mechanism of two transitions in vanadium spinel oxides AV2O4 (A=Zn, Mg, or Cd) in which magnetic V cations constitute a geometrically-frustrated pyrochlore structure. We have derived an effective spin-orbital-lattice coupled model in the strong correlation limit of the multiorbital Hubbard model, and applied Monte Carlo simulation to the model. The results reveal that the higher-temperature transition is a layered antiferro-type orbital ordering accompanied by tetragonal Jahn-Teller distortion, and the lower-temperature transition is an antiferromagnetic spin ordering. The orbital order lifts the magnetic frustration partially, and induces spatial anisotropy in magnetic exchange interactions. In the intermediate phase, the system can be considered to consist of weakly-coupled antiferromagnetic chains lying in the perpendicular planes to the tetragonal distortion.Key words: vanadium spinel oxides, geometrical frustration, orbital ordering, antiferromagnetic ordering, Monte Carlo simulation Vanadium spinel oxides AV2O4 with nonmagnetic cations A=Zn, Mg, or Cd are one of the most typical geometrically-frustrated systems; the magnetic V cations constitute the pyrochlore lattice, which is a three-dimensional network of corner-sharing tetrahedra. The geometrical frustration strongly suppresses development of magnetic correlations, and the antiferromagnetic (AF) ordering sets in at a much lower temperature ∼ 40K than the Curie-Weiss temperature ∼ 1000K [1]. Besides the AF transition, at a slightly higher temperature ∼ 50K, the system shows another phase transition with the structural change from high- temperature cubic to low-temperature tetragonal symmetry [1]. The origin of the two successive transitions is under debate. In particular, theories based on spin degree of freedom only or the spin-lattice coupling appear to be insufficient to describe the two transitions in a comprehensive manner [2].In the present study, we focus on the t2g orbital degree of freedom in these vanadium spinels to understand the microscopic mechanism of the two successive transitions. Each V 3+ cation has two 3d electrons in threefold t2g orbitals, and therefore the orbital degree of freedom is active. Starting from the standard multiorbital Hubbard model, we consider the perturbation in the strong correlation limit to describe the low energy physics of these insulating materials, and derive the effective spin-orbital-lattice coupled model in the form [2] H = HSO + HJT,

Molecular Spin Resonance in the Geometrically Frustrated MagnetMgCr2O4by Inelastic Neutron Scattering

“…The candidates for the high-order modes can appear in the paramagnetic phase similarly to other types of frustrated magnets, e.g., like the heptamer. Second, the high symmetry of the lattice is dynamically restored in the magnetically ordered phase with lattice distortion probably by the spin-lattice coupling, often caused by the geometric frustration [22][23][24]. In addition, as shown in Figs.…”

Theory of Successive Transitions in Vanadium Spinels and Order of Orbitals and Spins

“…We also examine a model with spin-orbital superexchange interaction alone in Sec. 3. In these two sections, we show that both two models fail to explain the experimental results.…”

Nonadiabatic small polarons, positive magnetoresistance, and ferrimagnetism behavior in the partially inverse Mn2−xV1+xO4 spinels

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