Diluted orbital degeneracy and large orthorhombic distortions in ferrimagnetic spinel CuxMn3-x O4

We report large orthorhombic distortions in the ferrimagnetic tetragonal (c>a) spinel CuxMn3-xO4 stabilized by a few percent of Cu doping. The orthorhombic strains of the ferrimagnetic phases increased linearly to the doping and reached up to ε≈8.2×10-3 for x=0.19, which is three times larger than the saturated value under external magnetic fields. For high doping (x≳0.17), the distortions first appeared in the paramagnetic phases and underwent further enhancement simultaneously with the onset of the noncollinear ferrimagnetic ordering. We present the rich magnetostructural phase diagram of CuxMn3-xO4, and argue that the diluted t2 orbital degeneracy of Cu2+ under tetrahedral crystal field breaks the global symmetry and triggers the orthorhombic instability inherent in Mn3O4.