Based on density functional calculations, we propose a possible orbital ordering in MnV2O4 which consists of orbital chains running along crystallographic a and b directions with orbitals rotated alternatively by about 45• within each chain. We show that the consideration of correlation effects as implemented in the local spin density approximation (LSDA)+U approach is crucial for a correct description of the space group symmetry. This implies that the correlation-driven orbital ordering has a strong influence on the structural transitions in this system. Inclusion of spin-orbit effects does not seem to influence the orbital ordering pattern. We further find that the proposed orbital arrangement favours a noncollinear magnetic ordering of V spins, as observed experimentally. Exchange couplings among V spins are also calculated and discussed.PACS numbers: 71.15.Mb, 71.70.Ej, The spinel compounds with a chemical formula of AB 2 X 4 where B sites are usually transition metal ions, form a frustrated pyrochlore lattice with corner-sharing tetrahedra. These compounds show a complex behavior including structural transitions from cubic to tetragonal symmetries which are often accompanied by an orbital order-disorder transition as well as complicated magnetic orderings at low temperatures 1 .The spinel MnV 2 O 4 has experienced a recent surge in activities due to new experimental observations in single crystals 2 revealing a lower symmetry structure than previously suggested 3 . This has important implications for the related orbital order at low temperatures which is still unclear. The presence of two magnetic ions in MnV 2 O 4 (Mn with spin 5/2 and V with spin 1) translates into more complex magnetic phase transitions in this system than in other vanadium spinel oxides such as ZnV 2 O 4 , MgV 2 O 4 or CdV 2 O 4 with nonmagnetic Asite ions. Recent experimental findings 2,4 indicated that MnV 2 O 4 undergoes a phase transition from paramagnetic to a collinear ferrimagnetic phase at 56K where the Mn and V spin moments point in opposite directions. At T = 53K a second magnetic phase transition to noncollinear ferrimagnetism follows accompanied by a structural transition from cubic to tetragonal phase.The cubic to tetragonal structural transition in MnV 2 O 4 is, similar to other vanadium spinels, associated with a compression of the VO 6 octahedron (c T /a T = 0.98). The octahedral environment of V (VO 6 ) splits the d states into lower t 2g and higher e g . Since V +3 is in a 3d 2 configuration, the t 2g orbitals are partially filled and possible orbital orderings may occur. Earlier experimental observations 3 indicated the tetragonal space group to be I4 1 /amd. However, recent precise measurements on a single crystal 2,4 showed that the tetragonal space group is I4 1 /a. Since the orbital order and, accordingly, the magnetic order are closely related to the underlying space group symmetry, it is very important to establish the space group symmetry unambiguously.The I4 1 /a space group breaks the mirror and glide symmetr...

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 ...

The three dimensional perovskite manganites R 1−x A x MnO 3 in the range of holedoping x > 0.5 are studied in detail using a double exchange model with degenerate e g orbitals including intra-and inter-orbital correlations and near-neighbour Coulomb repulsion. We show that such a model captures the observed phase diagram and orbital-ordering in the intermediate to large band-width regime. It is argued that the Jahn-Teller effect, considered to be crucial for the region x < 0.5, does not play a major role in this region, particularly for systems with moderate to large band-width. The anisotropic hopping across the degenerate e g orbitals are crucial in understanding the ground state phases of this region, an observation emphasized earlier by Brink and Khomskii. Based on calculations using a realistic limit of finite Hund's coupling, we show that the inclusion of interactions stabilizes the C-phase, the antiferromagnetic metallic A-phase moves closer to x = 0.5 while the ferromagnetic phase shrinks in agreement with recent observations. The charge ordering close to x = 0.5 and the effect of reduction of band-width are also outlined. The effect of disorder and the possibility of inhomogeneous mixture of competing states have been discussed.

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The pyrochlore oxides Dy2Ti2O7 and Ho2Ti2O7 are well studied spin ice systems and have shown the evidences of magnetic monopole excitations. Unlike these systems, Dy2Zr2O7 is reported to crystallize in a distorted fluorite structure instead of the pyrochlore structure. We present here the magnetic and heat capacity studies of La substituted Dy2Zr2O7. Our findings suggest the absence of spin ice state in Dy2Zr2O7 but the emergence of the magnetic field induced spin freezing near T ≈ 12 K in ac susceptibility measurements which is similar to Dy2Ti2O7. The magnetic heat capacity of Dy2Zr2O7 shows a shift in the peak position from 1.2 K in zero field to higher temperatures in the magnetic field, with the corresponding decrease in the magnetic entropy. The low temperature magnetic entropy at 5 kOe field is Rln2 -(1/2)Rln(3/2) which is same as for the spin ice state. Substitution of non magnetic, isovalent La 3+ for Dy 3+ gradually induces the structural change from highly disordered fluorite to weakly ordered pyrochlore phase. The La 3+ substituted compounds with less distorted pyrochlore phase show the spin freezing at lower field which strengthens further on the application of magnetic field. Our results suggest that the spin ice state can be stabilized in Dy2Zr2O7 either by slowing down of the spin dynamics or by strengthening the pyrochlore phase by suitable substitution in the system.

Using first-principles density-functional calculations, we perform a comparative study of two Fe-based spinel compounds, FeCr 2 S 4 and FeSc 2 S 4 . Though both systems contain an orbitally active A site with an Fe 2+ ion, their properties are rather dissimilar. Our study unravels the microscopic origin of their behavior driven by the differences in hybridization of Fe d states with Cr/ Sc d states and S p states in the two cases. This leads to important differences in the nature of the magnetic exchanges as well as the nearest-versus next-nearestneighbor exchange parameter ratios, resulting into significant frustration effects in FeSc 2 S 4 which are absent in FeCr 2 S 4 .

A double exchange model for degenerate e g orbitals with intra-and inter-orbital interactions has been studied for the electron doped manganites A 1−x B x MnO 3 (x > 0.5). We show that such a model reproduces the observed phase diagram and orbital ordering in the intermediate bandwidth regime and the Jahn-Teller effect, considered to be crucial for the region x < 0.5, does not play a major role in this region. Brink and Khomskii have already pointed this out and stressed the relevance of the anistropic hopping across the degenerate e g orbitals in the infinite Hund's coupling limit. From a more realistic calculation with finite Hund's coupling, we show that inclusion of interactions stabilizes the C-phase, the antiferromagnetic metallic A-phase moves closer to x = 0.5 while the ferromagnetic phase shrinks. This is in agreement with the recent observations of Kajimoto et. al. and Akimoto et. al.

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