Orbital Dilution Effect on Structural and Magnetic Properties of FeMn_XV₂O₄

Structural and magnetic properties are investigated for Fe_1?xMn_xV₂O₄ (0≤x ≤1) spinels. As orbital-active Fe²⁺ is substituted with Mn²⁺, the cubic-to-tetragonal transition T_S1 and the tetragonal-to-orthorhombic transition T_S2 gradually decrease. These structural transitions originate from the Fe²⁺ ferro-orbital order (F-OO). Below Yafet–Kittel (YK) magnetic transition T_N2, V ³⁺ orbital order (V-OO) plays an important role on global structure. Here x=0.6 is a critical point. Fe²⁺ F-OO and V ³⁺ F-OO coexist for 0≤x ≤0.5. For x≥0.6, the orbital pattern of V ³⁺ is antiferro (AF)-OO, and Fe²⁺ F-OO disappears. Structural transition T_S3, accompanied by YK magnetic transition T_N2, decreases initially, and then increases at x=0.6. A scenario for the complex phase diagram arising from the cooperation or competition of Fe²⁺ and V ³⁺ orbitals is proposed.