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Register a new userStrong competition between orbital-ordering and itinerancy in a frustrated spinel vanadate
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The crossover from localized- to itinerant-electron behavior is associated with many intriguing phenomena in condensed-matter physics. In this paper, we investigate the crossover from localized to itinerant regimes in the spinel system Mn$_{1-x}$Co$_x$V$_2$O$_4$. At low Co doping, orbital order (OO) of the localized electrons on the V3+ ions suppresses magnetic frustration by triggering a tetragonal distortion. With Co doping, electronic itinerancy melts the OO and suppresses the structural phase transition while the reduced spin-lattice coupling produces magnetic frustration. Neutron scattering measurements and first-principles-guided spin models reveal that the non-collinear state at high Co doping is produced by weakened local anisotropy and enhanced Co-V spin interactions.
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Neutron inelastic scattering and diffraction techniques have been used to study the MnV2O4 spinel system. Our measurements show the existence of two transitions to long-range ordered ferrimagnetic states; the first collinear and the second noncollinear. The lower temperature transition, characterized by development of antiferromagnetic components in the basal plane, is accompanied by a tetragonal distortion and the appearance of a gap in the magnetic excitation spectrum. The low-temperature noncollinear magnetic structure has been definitively resolved. Taken together, the crystal and magnetic structures indicate a staggered ordering of the V d orbitals. The anisotropy gap is a consequence of unquenched V orbital angular momentum.
We perform ultrasound velocity measurements on a single crystal of nearly-metallic spinel Co$_{1.21}$V$_{1.79}$O$_4$ which exhibits a ferrimagnetic phase transition at $T_C sim$ 165 K. The experiments reveal a variety of elastic anomalies in not only the paramagnetic phase above $T_C$ but also the ferrimagnetic phase below $T_C$, which should be driven by the nearly-itinerant character of the orbitally-degenerate V 3$d$ electrons. In the paramagnetic phase above $T_C$, the elastic moduli exhibit elastic-mode-dependent unusual temperature variations, suggesting the existence of a dynamic spin-cluster state. Furthermore, above $T_C$, the sensitive magnetic-field response of the elastic moduli suggests that, with the negative magnetoresistance, the magnetic-field-enhanced nearly-itinerant character of the V 3$d$ electrons emerges from the spin-cluster state. This should be triggered by the inter-V-site interactions acting on the orbitally-degenerate 3$d$ electrons. In the ferrimagnetic phase below $T_C$, the elastic moduli exhibit distinct anomalies at $T_1sim$ 95 K and $T_2sim$ 50 K, with a sign change of the magnetoresistance at $T_1$ (positive below $T_1$) and an enhancement of the positive magnetoresistance below $T_2$, respectively. These observations below $T_C$ suggest the successive occurrence of an orbital glassy order at $T_1$ and a structural phase transition at $T_2$, where the rather localized character of the V 3$d$ electrons evolves below $T_1$ and is further enhanced below $T_2$.
muSR experiments on the geometrically frustrated spinel oxide, Li2Mn2O4, show the development of spin correlations over a range of length scales with decreasing temperature. Increased relaxation below 150 K is consistent with the onset of spin correlations. Below 50 K, spin order on a length scale, which is long range for the muSR probe, appears abruptly in temperature, consistent with prior neutron diffraction results. The oscillations in the zero field asymmetry are analyzed using a three frequency model. By locating the muon site this is shown to be consistent with the unexpected 2D q = root 3 x root 3 structure on the Kagome planes proposed originally from neutron data. Longitudinal field data demonstrate that some spin dynamics persist even at 2 K. Thus, a very complex magnetic ground state, featuring the co-existence of long length scale 2D ordering and significant spin dynamics, is proposed. This is unusual considering the 3D topology of the Mn3+ spins in this material.
Nuclear magnetic resonance (NMR), neutron diffaction (ND), x-ray diffraction, magnetic susceptibility and specific heat measurements on the frustrated A-site spinel CoAl2O4 compound reveal a collinear antiferromagnetic ordering below Tn = 9.8(2) K. A high quality powder sample characterized by x-ray diffraction that indicates a relatively low Co-Al inversion parameter x = 0.057(20) in (Co1-xAlx)[Al2-xCox]O4, shows a broad maximum around 15 K in magnetic susceptibility and a sharp peak at Tn in heat capacity. The average ordered magnetic moment of Co^2+ (S = 3/2) ions at the A-site is estimated to be 2.4(1) Bohr magneton from NMR and 1.9(5) Bohr magneton from ND which are smaller than the expected value of 3 Bohr magneton for S = 3/2 and g = 2. Antiferromagnetic spin uctuations and correlations in the paramagnetic state are revealed from the magnetic susceptibility, NMR and ND measurements, which are due to spin frustration and site inversion effects in the system. The ND data also show short-range dynamic magnetic ordering that persists to a temperature that is almost twice Tn.
We consider the superexchange in `frustrated Jahn-Teller systems, such as the transition metal oxides NaNiO_2, LiNiO_2, and ZnMn_2O_4, in which transition metal ions with doubly degenerate orbitals form a triangular or pyrochlore lattice and are connected by the 90-degree metal-oxygen-metal bonds. We show that this interaction is much different from a more familiar exchange in systems with the 180-degree bonds, e.g. perovskites. In contrast to the strong interplay between the orbital and spin degrees of freedom in perovskites, in the 90-degree exchange systems spins and orbitals are decoupled: the spin exchange is much weaker than the orbital one and it is ferromagnetic for all orbital states. Due to frustration, the mean-field orbital ground state is strongly degenerate. Quantum orbital fluctuations select particular ferro-orbital states, such as the one observed in NaNiO_2. We also discuss why LiNiO_2 may still behave as an orbital liquid.
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