Diluted orbital degeneracy and large orthorhombic distortions in ferrimagnetic spinel CuxMn3-xO4.

We report large orthorhombic distortions in the ferrimagnetic tetragonal (c > a) spinel Cux Mn3-x,O4 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 x 10 -3 for x = 0 .1 9 , which is three times larger than the saturated value under external magnetic fields. For high doping (x ≳ 0 .1 7 ) , 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 Mn3M/sub>O4. [ABSTRACT FROM AUTHOR]