Magnetic and Orbital Ordering in the Spinel ${\mathrm{MnV}}_2{\mathrm{O}}_4$

Neutron inelastic scattering and diffraction techniques have been used to study the ${\mathrm{MnV}}_2{\mathrm{O}}_4$ 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.

Figure 1

(a) The zero-field-cooling (ZFC) and the field-cooling (FC) temperature dependence of magnetization in ${\mathrm{MnV}}_2{\mathrm{O}}_4$ single crystal. (b) Temperature dependence of the (400) and (200) Bragg peaks integrated intensities showing the existence of two magnetic transitions. The solid lines are guides to the eye. (c) Splitting of the (400) cubic peak into two tetragonal peaks $(220{)}_T$ and $(004{)}_T$.

Figure 2

(a) Projection of the ${\mathrm{V}}_4$ tetrahedron in the $ab$ plane. V–O bonds are arranged in an alternating pattern giving rise to a staggeredlike orbital ordering. (b), (c) Graphical representations of the low-temperature noncollinear ferrimagnetic structure of the ${\mathrm{MnV}}_2{\mathrm{O}}_4$. The Mn moments are aligned parallel to the $c$ axis, while the V moments are canted by approximately 65°. (d) Projection of the magnetic structure on the basal plane.

Figure 3

(a) Energy scans measured at different temperatures, at $Q=(220)$. Solid lines are fits to the data as described in the text. Inset: dispersion of the low-energy spin-wave branch along the $c$ axis, measured at 1.5 K. (b) Variation with the temperature of the energy gap (●) and (220) peak intensity (○). The gap mode extrapolates to zero at the structural transition temperature. The solid lines are power-law fits.