Stabilization of orthorhombic distortions in Cu-doped and Co-doped ferrimagnetic Mn3 O4

Using synchrotron X-ray diffraction, we investigate how the tetragonal-to-orthorhombic phase transition in the ferrimagnetic (c>a) spinel Mn3O4 is driven by the t2 orbital degeneracy of Cu2+(3d9) ions doped in its tetrahderal sites. At high temperatures where the orthorhombic phase initially appears, we observe that the local elongations of Cu-doped tetrahedra cause the unit cells to contract along the same direction. The signs of the local and global lattice strains finally agree with each other when the orthorhombic phase transition is completed below TN=42.5 K. Using neutron diffraction, we report that Co2+(3d7) doping also stabilizes the orthorhombic phase but without enhancing the associated lattice strains. These results are consistent with the scenario that the orthorhombic instability of the undoped Mn3O4 is driven by the spin-lattice coupling.