Ferro-to-antiferro orbital variation in vanadium spinel Fe1-xMnxV2O4

We have synthesized the Fe_1-xMn_xV₂O₄ (0 ≦ x ≦ 1) system, and have investigated its structural and magnetic properties through synchrotron powder diffraction, magnetization and specific heat measurements. We have examined the local distortion for both the FeO₄ tetrahedron and VO₆ octahedron on the basis of structural refinements using the synchrotron diffraction data. We found that the behaviour of these local distortions and the temperature dependent magnetization changes continuously with respect to the Mn²⁺ concentration, x. The local distortion of the FeO₄ tetrahedron derived from the orbital order of the x²-y² type below the ferrimagnetic transition temperature becomes smaller for x ≦ 0.6, and is absent for x > 0.6. The ferro-orbital order, leading to compressed local distortion of VO₆ below the non-collinear ferrimagnetic order temperature, is gradually suppressed with increasing x and changes to the antiferro-orbital order for x > 0.6, for which the long range orbital order of Fe²⁺ disappears. We suggest that the two types of V³⁺ orbital orders are strongly correlated with the non-collinear magnetic order, and that the ferro-orbital order is possibly stabilized by the orbital degrees of freedom of the Fe²⁺ ions located at the A-site.