Single-ion anisotropy, Dzyaloshinskii-Moriya interaction, and negative magnetoresistance of the spin-12pyrochloreR2V

Abstract

The electronic and magnetic properties of spin-1/2 pyrochlores R 2 V 2 O 7 were investigated on the basis of density-functional calculations. Contrary to the common belief, the spin-1/2 V 4+ ions are found to have a substantial easy-axis single-ion anisotropy. We show that the magnon quantum Hall effect of Lu 2 V 2 O 7 is a combined consequence of the easy-axis single-ion anisotropy and the Dzyaloshinskii-Moriya interaction of the spin-1/2 V 4+ ions. We also show that the negative magnetoresistance observed for R 2 V 2 O 7 arises from a new mechanism, i.e., the band gap decreases as the spin alignment becomes more parallel to each other.

Spin-Wave Spectrum of the Quantum Ferromagnet on the Pyrochlore LatticeLu2V2O7Mena1, Perry2, Perring3 et al. 2014Phys. Rev. Lett.554423View full textAdd to dashboardCite

“…The data are found to be in excellent overall agreement with a minimal model that includes a nearestneighbour Heisenberg exchange J = 8.22(2) meV and a Dzyaloshinskii-Moriya interaction (DMI) D = 1.5(1) meV. The large DMI term revealed by our study is broadly consistent with the model developed by Onose et al to explain the magnon Hall effect they observed in Lu2V2O7 [1], although our ratio of D/J = 0.18(1) is roughly half of their value and three times larger than calculated by ab initio methods [2].…”

“…Its value is roughly half the ratio of ∼1/3 used in the analysis of the magnon Hall effect data [1]. It is clearly substantially larger than the value 0.05 obtained from DFT calculations [2]. However, our determination of the leading term in the Hamiltonian, J=8.22(2) meV, is in reasonable accord with the corresponding value of J=7.1 meV from DFT taking into account the accuracy of the computational method.…”

“…1(b), S i are spin operators, H is the magnetic field, and <ij > runs over all pairs of nearest-neighbours [1] The magnitude of the anomalous contribution to the thermal conductivity due to magnons is determined by the ratio D/J, which was estimated in Lu 2 V 2 O 7 to be D/J 1/3 by fitting the transport data [1] and hence would require strong spin-orbit coupling. In contrast, DFT calculations obtained a much lower ratio D/J 1/20, with the authors instead emphasising the importance of single ion anisotropy [2]. There is thus a prima facie case for accurately determining the Hamiltonian of Lu 2 V 2 O 7 both in terms of interpreting the transport data, and more generally of understanding the spindynamics of this elusive example of a pyrochlore lattice in the quantum limit.…”

Crystal-field, exchange interactions and magnetism in pyrochlore ferromagnet R2V2O7 (R3+=Y, Lu)

“…Such an orbital structure is expected in a trigonal (D 3d symmetry) crystal-field arising due to the distortion of VO 6 octahedra. Under trigonal CF, the lower t 2g electronic states are split into the lowest a 1g state 90S and two e 0 g states 9 þS and 9 À S, and the higher e g states d x 2 Ày 2 and d z 2 remain degenerate [9,11]. It is found that at each V-site of a given V 4 -tetrahedron, only a 1g level is occupied by an electron.…”

“…Furthermore these vanadate pyrochlores are unique and intriguing, because they are 3d ferromagnets with magnetic ordering temperature T C E 68 K [6] and 70 K [6][7][8], and at the same time Mott insulators having band gap D bg ¼0.33 eV [9], contrary to the common belief that ferromagnetism leads usually to metallic character [1]. In these materials, electronic configuration of V 4 þ (3d 1 , S ¼1/2) ions is (t 2g ) 1 (e g ) 0 and the orbital ordering pattern determined by the neutron diffraction [10] showed that each orbital is extended along the four local /111S directions towards the center-of-mass of the elementary V 4 -tetrahedron.…”