Orbital Glass State of the Nearly Metallic Spinel Cobalt Vanadate

Abstract

Strain, magnetization, and unpolarized and polarized neutron diffraction measurements were performed to study the magnetic and structural properties of spinel CoV2O4. Magnetostriction measurements indicate that a subtle distortion of the crystal along the direction of magnetization, ∆L/L ∼ 10 −4 , exists and varies from elongation to contraction in a second order fashion upon cooling. Unpolarized and polarized single-crystal neutron experiments indicate that upon cooling the ferrimagnetic structure changes from collinear to noncollinear at T ∼ 90 K, where the elongation of the crystal is maximized. These results imply the existence of an orbital glassy state in the nearly metallic frustrated magnet CoV2O4.

Variety of elastic anomalies in an orbital-active nearly itinerant cobalt vanadate spinel

“…For CoV 2 O 4 , the previous reports of the experimental studies in the single-crystalline and polycrystalline samples suggested the presence of additional transition within the ferrimagnetic phase [ 34,35,38,39 ]. However, the reported transition temperatures, also including the ferrimagnetic transition temperature T C , differ from study to study.…”

“…Fig. 1(*) depicts the ferrimagnetic transition temperature T C and two other transition temperatures T 1 and T 2 in Co 1+x V 2−x O 4 as functions of off-stoichiometry x (the solid lines), which were suggested from the magnetization and magnetostriction measurements [ 39 ]. By increasing the amounts of the off-stoichiometry x, T C increases but T 1 and T 2 decrease.…”

“…In the crystal growth of the cobalt vanadate spinel, the Co : V ratio of the grown crystal is prone to deviate from the stoichiometric ratio of 1 : 2 to 1+x : 2-x with excess Co x. Very recently, the measurements of the magnetization, magnetostriction, neutron scattering, and dielectric constant for the single-crystalline and polycrystalline Co 1+x V 2−x O 4 with various x extracted the intrinsic magnetic and structural properties irrespective of the off-stoichiometry [ 39 ]. It was suggested that, in addition to the ferrimagnetic transition at T C , two other possible transitions occur within the ferrimagnetic phase [ 39 ].…”

“…By increasing the amounts of the off-stoichiometry x, T C increases but T 1 and T 2 decrease. T 1 corresponds to a spin canting temperature, where a collinear-to-noncollinear ferrimagnetic ordering occurs upon cooling [ 39 ]. In Fig.…”

“…For AV 2 O 4 (A = Zn, Mg, Cd, Mn, and Fe), the structural phase transition is considered to arise from a longrange ordering of the V t 2g orbitals, where the lowering of the lattice symmetry should result in the release of frustration (magnetic ordering). The orbital order to explain the structural and magnetic orders for AV 2 O 4 is still under debate both theoretically [ [25][26][27][28][29][30][31] and experimentally [ 4,7,12,15 ]; it is considered to be driven by the competition among Jahn-Teller coupling, Kugel- ) is a unique vanadate spinel with its highest electrical conductivity among the insulating or semiconducting AV 2 O 4 , which is considered to arise from the nearly-itinerant character of the V 3d electrons [ [33][34][35][36][37][38][39]. CoV 2 O 4 exhibits a ferrimagnetic order below T C ∼ 150 K, which is similar to AV 2 O 4 with magnetic A = Mn (T C = 57 K) and Fe (T C = 110 K).…”