Magnetic Frustration Driven by Itinerancy in Spinel CoV2O4

Abstract

Localized spins and itinerant electrons rarely coexist in geometrically-frustrated spinel lattices. They exhibit a complex interplay between localized spins and itinerant electrons. In this paper, we study the origin of the unusual spin structure of the spinel CoV₂O₄, which stands at the crossover from insulating to itinerant behavior using the first principle calculation and neutron diffraction measurement. In contrast to the expected paramagnetism, localized spins supported by enhanced exchange couplings are frustrated by the effects of delocalized electrons. This frustration produces a non-collinear spin state even without orbital orderings and may be responsible for macroscopic spin-glass behavior. Competing phases can be uncovered by external perturbations such as pressure or magnetic field, which enhances the frustration.

Figure 1

NC spin states in cubic CoV₂O₄ compared to tetragonal MnV₂O₄. Temperature dependence of the (111) (triangles), (220) (circles), and (002) (squares) Bragg peak intensities for CoV₂O₄ (a) and MnV₂O₄ (b) measured by neutron diffraction at HB-3A. The peak intensities of (111) and (220) above the magnetic transition temperature are fully from the nuclear structure and were subtracted. The (002) peak is not allowed from the structural symmetry and fully originated from the magnetic scattering. The background was subtracted. All the magnetic peaks observed by our neutron diffraction are instrument resolution limited besides the peak broadening caused by the structural transition for MnV₂O₄, and thus indicate the long range ordered magnetic moments. The strongly-reduced intensity of (002) peak in CoV₂O₄ indicates that only tiny amount of V spin orders, which is caused by enhanced itinerancy.

Figure 2

Reduced single-ion anisotropy (SIA) of V in CoV₂O₄ compared with that in MnV₂O₄. (a) Total energy versus angle and associated SIA (meV) of V³⁺ in ambient (circle) and 10 GPa (square) pressure for bulk CoV₂O₄ and MnV₂O₄ (diamond). (b) NC spin configurations of V³⁺ and Co²⁺/Mn²⁺ spins pointing along local [111] and global [001] directions, respectively. The round bold (dotted) arrows close to V spins depict the rotational flexibility in CoV₂O₄ (MnV₂O₄). (c) SIA of Co²⁺ in bulk CoV₂O₄ under 10 Gpa compared to SIA of Mn²⁺ in bulk MnV₂O₄. (d) Orbital occupation configuration of Mn²⁺ (d ⁵) and Co₂₊ (d ⁷). (e) R _V−V in CoV₂O₄ and MnV₂O₄ are compared with R _V−V in other vanadates from ref..

Figure 3

Origin of the enhanced magnetic ordering temperature in CoV₂O₄. Projected density-of-states of CoV₂O₄ (a) compared to MnV₂O₄ (b) in unit of eV⁻¹. Dotted arrows denote the energy differences, Δ between V and Co/Mn for possible AFM super-exchange (J _A−V ~ −t ²/Δ). t is the hopping parameter between orbitals.

Figure 4

Evolution of magnetic couplings and competing ground states driven by Co-doping and pressure. (a) Change of all magnetic interactions with Co doping and external pressure (GPa) calculated by LSDA + U for the ground states at zero temperature. Points represent DFT results and the connecting lines are a guide for eye. $J_{\mathrm{V}-\mathrm{V}}^{\mathrm{in}}$ and $J_{\mathrm{V}-\mathrm{V}}^{\mathrm{out}}$ are expected to be degenerate at x = 0.8 (cubic) in Mn_1−xCo_xV₂O₄ from experiments^,. Bold (dotted) lines represent the exchange (J) and SIA (D) interactions. (b) The anisotropic J _V−V prefers the TI/TO state and the antiferromagnetic J _Mn−V also stabilizes TI/TO state in Mn-rich region. (c) The isotropic J _V−V prefers the AI/AO state but the antiferromagnetic J _Co−V tries to stabilizes the TI/TO state in Co-rich region. Thus, two states of the isosymmetric one-angle TI/TO state and the two-angle state that evolves from the AI/AO state compete with each other. (d) The energy landscape of CoV₂O₄ with the disappearance of SIA at high pressures. The massive degeneracies in the energy landscape of two angles (θ ₁ and θ ₂) may induce spin glass or liquid phases as explained in the text.

Figure 5

Prediction of novel phases driven by magnetic field in Mn_0.2Co_0.8V₂O₄. Experimental results for the field-dependent (020) (a) and (220) (b) Bragg peak intensities. (c) Field dependence of spin angle θ calculated by spin models using DFT parameters. First-order phase transition from TI/TO to AI/AO-derived state with magnetic field is indicated by the jump in θ. Black open squares are from the neutron scattering measurement up to 10 T. (d) One-angle state based on TI/TO (left) and two-angle state based on AI/AO (right). The latter is driven by the competition between J _Co−V and magnetic energy.