Local symmetry breaking and orbital glass like behaviour in CoFe2O4

In the present manuscript, structural distortions, orbital correlations, and electronic states
in cobalt ferrite (CoFe2O4) were investigated using several synchrotron-based techniques,
including temperature-dependent X-ray diffraction, hard X-ray photoelectron
spectroscopy, and temperature-dependent extended X-ray absorption fine structure
measurements, along with Raman spectroscopy and magnetisation measurements.
Synchrotron X-ray diffraction confirms the phase purity of CoFe2O4 and reveals a
temperature-dependent superlattice reflection between 200 K and 100 K, consistent with
the emergence of short-range orbital ordering driven by cooperative Jahn-Teller distortion.
The disappearance of this feature below 100 K suggests the freezing of short-range orbital
correlations into an orbital-glass-like regime. Raman spectroscopy further supports these
observations through phonon anomalies and enhanced spin-phonon coupling in the same
temperature range where the superlattice reflection appears. Hard X-ray photoelectron
spectroscopy measurements reveal multiple spectral features along with charge-transfer
satellites in the Co and Fe core levels, indicating mixed valence states and strong electron
correlations. Temperature-dependent extended X-ray absorption fine structure
measurements indicate increasing local disorder, particularly in the Fe-O and Fe-Fe
octahedral bonds, as evidenced by enhanced Debye-Waller factors. These static and
symmetric distortions, attributed to cation redistribution and oxygen vacancies,
predominantly affect the octahedral sublattice. Magnetisation measurements reveal spin
reorientation and exchange interaction anomalies that correlate with the observed orbital
behaviour. Altogether, these results point toward a frustrated orbital regime in CoFe2O4,
possibly involving cooperative Jahn-Teller distortions and the freezing of short-range
orbital correlations without the development of long-range orbital order.