Orbital Order inZnV2O4

Abstract

In view of recent controversy regarding the orbital order in the frustrated spinel ZnV2O4 , we analyze the orbital and magnetic groundstate of this system within an ab initio density functional theory approach. While LDA+U calculations in the presence of a cooperative Jahn-Teller distortion stabilize an A-type staggered orbital order, the consideration of relativistic spin-orbit effects unquenches the orbital moment and leads to a uniform orbital order with a net magnetic moment close to the experimental one. Our results show that ab initio calculations are able to resolve the existing discrepancies in previous theories and that it is the spin-orbit coupling alongwith electronic correlations which play a significant role in determining the orbital structure in these materials.PACS numbers: 71.15.Mb, 71.70.Ej Correlated electronic systems involving transition metal oxides took the centrestage of condensed matter physics research for the last three decades because of their intriguing, often non-intuitive properties. Transition metal spinel oxides with an additional complexity of a geometrically frustated lattice provide an exciting ground for the study of several competing interactions among spin, orbital and lattice degrees of freedom 1,2 . Besides being of fundamental interest, spinels have been also proposed for spintronics applications 3,4 . In the present work on ZnV 2 O 4 we investigate the effect of competing spin, orbital and lattice degrees of freedom and show that density functional calculations provide an adequate and realistic ground to establish the dominant mechanism driving the orbital order in vanadium spinels.The orbital order in ZnV 2 O 4 as well as in other vanadium spinels such as MgV 2 O 4 and CdV 2 O 4 is presently a subject of considerable debate 5 . In order to understand the behavior of these compounds, various groups 6,7,8 have proposed alternative microscopic mechanisms which predict different orbital patterns. The ongoing debate has its origin in the complex nature of these systems with competing spin, orbital and lattice degrees of freedom. These systems have V 3+ ions in a spin 1 state characterized by double occupancy of the triply degenerate t 2g (d xy , d xz , d yz ) orbitals. These partially filled t 2g orbitals leave the orbital degrees of freedom unfrozen opening up the possibility of orbital order. Moreover, the V-sites in the cubic spinel structure form a pyrochlore lattice, which gives rise to frustrated antiferromagnetic interactions among these sites 9 . In ZnV 2 O 4 the interplay of all these degrees of freedom leads to two successive phase transitions which involve structural, orbital and magnetic changes. At T S = 51 K, ZnV 2 O 4 undergoes a structural phase transition where the symmetry is lowered from cubic to tetragonal with a compression of the VO 6 octahedron along the c axis 10 and the system possibly orbital orders. The structural transition also lifts the geometrical frustration of the cubic phase making a way for the second transition at T N = 40 K which is ...

Effect of spin–orbit coupling on magnetic and orbital order in MgV2O4

“…Also it explains the low magnetic moment per V ion observed in these systems as a large negative orbital moment is expected from a strong SO coupling. These findings were also corroborated by electronic structure calculations 5 for…”

“…Vanadium spinels AV 2 O 4 (A=Mg, Zn, Cd) are being studied extensively in recent years [1][2][3][4][5][6] as they provide a very interesting playground for the study of competing interactions on a frustrated lattice in 3-dimension. The Vanadium (V) ions at the B-sites of the spinel structure form a pyrochlore lattice, with corner sharing tetrahedra, which is geometrically frustrated.…”

Designing a robustly metallic noncenstrosymmetric ruthenate oxide with large thermopower anisotropy

“…Since SOI play a significant role in determining the orbital structure in 3d transition metal oxides 33,34 , we calculate the DOS with SOI (Fig. 3b).…”

The nature of itineracy in CoV2O4: a first-principles study

“…In Fig. 4 we show the effect of U on the vanadium d states in a range of U ef f values which are found relevant also for other vanadium spinels [7,25]. We discuss later in this section some important physical insights that one can get from Fig.…”

“…The SO coupling is known to lift many degeneracies and thus is likely to give rise to the desired gap in the energy spectrum of conduction electrons. Also, it is often found to be significant and has an appreciable effect on the orbital order in other vanadium spinels [13,25]. In Fig.…”