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

Abstract

We have theoretically studied successive transitions in vanadium spinel oxides with (t2g) 2 electron configuration. These compounds show a structural transition at ∼ 50K and an antiferromagnetic transition at ∼ 40K. Since threefold t2g orbitals of vanadium cations are occupied partially and vanadiums constitute a geometrically-frustrated pyrochlore lattice, the system provides a particular example to investigate the interplay among spin, orbital and lattice degrees of freedom on frustrated lattice. We examine the models with the Jahn-Teller coupling and/or the spin-orbital superexchange interaction, and conclude that keen competition between these two contributions explains the thermodynamics of vanadium spinels. Effects of quantum fluctuations as well as relativistic spin-orbit coupling are also discussed. §1. IntroductionIn this paper, we review our recent studies on phase transitions in vanadium spinel compounds, AV 2 O 4 (A=Zn, Mg or Cd). 1)-5) This family of materials is antiferromagnetic insulator with spin S = 1 on pyrochlore sublattice in the spinel structure (Fig. 1). Recent experiments showed the presence of two successive phase transitions at low temperatures around 50K. 6) As temperature is lowered, a structural transition occurs first, then a magnetic transition follows. As we will discuss later, simple spin exchange mechanism does not explain the lowest temperature phase, and we need to take account of two important factors in order to explain basic character of these transitions. One is geometrical frustration and the other is strong correlation effects, which lead to keen competition of various degrees of freedom. As widely known, the geometrical frustration often leads to huge degeneracy of the ground-state manifold, and suppresses a long-range ordering. On the other hand, the strong electron correlation opens several channels in the low energy sector, such as spin and orbital degrees of freedom. A key question is how these channels affect the manifold -do they result in some nontrivial phenomena through enhanced fluctuations, or lift the degeneracy in some nontrivial manner? We would like to clarify this intriguing problem by studying effects of both frustration and correlation in the vanadium spinels.Vanadium spinels are a 3d-electron system with orbital degrees of freedom. In the last decade, various transition-metal materials have been intensively studied to investigate the effects of orbital degrees of freedom. 7) In particular, manganites showing colossal magnetoresistance (CMR) are the most typical example. Vanadium spinels have different features from these materials. A distinct difference is the lattice structure. In manganites showing CMR, the lattice structure is perovskite; atomic bonds are connected with right angle and the network of magnetic ions is close to a typeset using PTPT E X.cls Ver.0.9

Suppression of spin frustration due to orbital selection

“…It is an intriguing issue to clarify the influence of orbital ordering on magnetic properties in geometrically frustrated systems. In fact, significance of t 2g -orbital degree of freedom has been discussed to understand the mechanism of two phase transitions in spinel vanadium oxides AV 2 O 4 (A=Zn, Mg, and Cd) [7,8,9]. It has been proposed that orbital ordering brings a spatial modulation in the spin exchange and spin frustration is consequently relaxed.…”

Orbital ordering and one-dimensional magnetic correlation in vanadium spinel oxides AV2O4(AMotome1, Tsunetsugu2 2005Physica B: Condensed MatterSelf Cite6030View full textAdd to dashboardCite

“…Since quantum nature exists only in the spin operators in HSO, we approximate them by classical vectors, and apply a standard metropolis MC algorithm. Details of MC calculations will be reported elsewhere [3].…”