Emergence of a diffusive metal state with no magnetic order near the Mott transition in frustrated pyrochlore-type molybdates

Tailoring magnetism in spinel vanadate CoV2O4epitaxial thin films

Near itinerant cubic bulk CoV₂O₄is at variance with other spinel vanadates by not showing orbital ordering down to low temperature, albeit it displays fragile anomalies related to spin, and lattice structure, signaling a spin/orbital glass transition around 95 K. We investigate tetragonal-like epitaxial CoV₂O₄films on SrTiO₃and (La_0.3Sr_0.7)(Al_0.65Ta_0.35)O₃substrates that exhibit pronounced signature of spin reorientation transition from toa/bplane around 90 K unlike its bulk counterpart. Using in-plane and out-of-plane magnetic measurements, we demonstrate the intricate link between Co²⁺and V³⁺sublattice magnetizations that give rise to anisotropic magnetic switching. In-plane magnetic measurements reveal a wasp-waist shapedM(H) loop below reorientation transition temperature, while the out-of-plane follows antiferromagnet-likeM(H) response. The wasp-waist shaped feature could be linked to in-plane spin-canted (anti)ferromagnetism induced by canting away of V-spins away from antiferromagnetically coupled Co-spin direction below reorientation transition temperature. Further, we uncover the evidence for slow relaxation over a period of ∼10⁴ s at 20 K and memory effect that indicates the possible existence for magnetic glassy phase in the low temperature regime. Using epitaxial strain as a control knob, our results inspire future study to manipulate orbital states, spin texture and itinerant electron character in tailored CoV₂O₄films away from cubic lattice symmetry.

Orbital Mott transition in two dimensional pyrochlore lattice

We study orbital Mott transition in two dimensional pyrochlore lattice, using a two orbital Hubbard model with only inter-orbital electronic hopping. We use a real space Monte Carlo based approach to study the model at finite temperature, and establish temperature-interaction phase diagram that highlights the Mott transition, orbital ordering, and spectral trends, and possible window of pseudogap. Due to only inter-orbital hopping, the Mott insulator 'generates' ferro exchange resulting in ferro-orbital ordering, with T _corr/t peaked at ≈0.2 around U/t ≈ 6. The optical conductivity shows unusual two peak feature due to two dimensional pyrochlore lattice.

First-principles studies of spin-orbital physics in pyrochlore oxides

The pyrochlore oxides [Formula: see text]O₇ exhibit a complex interplay between geometrical frustration, electronic correlations, and spin-orbit coupling (SOC), due to the lattice structure and active charge, spin, and orbital degrees of freedom. Understanding the properties of these materials is a theoretical challenge, because their intricate nature depends on material-specific details and quantum many-body effects. Here we review our recent studies based on first-principles calculations and quantum many-body theories for 4d and 5d pyrochlore oxides with B  =  Mo, Os, and Ir. In these studies, the SOC and local electron correlations are treated within the local density approximation (LDA)  +  U and LDA  +  dynamical mean-field theory formalisms. We also discuss the technical aspects of these calculations.

Magnetic Frustration Driven by Itinerancy in Spinel CoV2O4

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.

Topological Hall effect in pyrochlore lattice with varying density of spin chirality

The three-site spin correlation, S(i)·(S(j)×S(k)) on the neighboring triangular sites i, j and k, termed scalar spin chirality, can endow the conduction electron with a quantum Berry phase and resultant transverse (Hall) transport. The paramagnetic barely metallic state was prepared in hole-doped Y2Mo2O7 with pyrochlore lattice using a high-pressure synthesis method, which is further endowed with the spin chirality by partially replacing Y site with Tb (content x). The local spin chirality formed by the adjacent three Tb Ising moments on the pyrochlore lattice can couple to the conduction electrons to give rise to the topological Hall effect whose magnitude increases in proportion to x3 or the density of the Tb-moment triangular clusters.

Frustration-induced insulating chiral spin state in itinerant triangular-lattice magnets

We study the double-exchange model at half-filling with competing superexchange interactions on a triangular lattice, combining exact diagonalization and Monte Carlo methods. We find that in between the expected itinerant ferromagnetic and 120° Yafet-Kittel phases a robust scalar-chiral, insulating spin state emerges. At finite temperatures the ferromagnet-scalar-chiral quantum critical point is characterized by anomalous bad-metal behavior in charge transport as observed in frustrated itinerant magnets R2Mo2O7.

Phase competition in the double-exchange model on the frustrated pyrochlore lattice

Competition between the ferromagnetic double-exchange interaction and the superexchange antiferromagnetic interaction is theoretically studied in the presence of geometrical frustration. As the superexchange interaction increases, the ferromagnetic metal becomes unstable and is taken over by a cooperative paramagnetic metal, in sharp contrast to a discontinuous transition to the antiferromagnetic insulator in the absence of frustration. In the critical region, the system exhibits a peculiar temperature-independent behavior with highly incoherent transport, suggesting a large residual entropy at low temperatures. We discuss the relevance of the results to the pressure-induced behaviors in Mo pyrochlore oxides [S. Iguchi et al., Phys. Rev. Lett. 102, 136407 (2009)].