Cation distribution dependent magnetic properties in CoCr 2-x Fe x O 4 (x = 0.1 to 0.5): EXAFS, Mössbauer and magnetic measurements.

In this report, we have examined the evolution of the structure and rich magnetic transitions such as a paramagnetic to ferrimagnetic phase transition at the Curie temperature (TC ), spiral ordering temperature (TS ) and lock-in temperature (TL ) observed in the CoCr2 O4 spinel multiferroic after substituting Fe. The crystal structure, microstructure and cation distribution among the tetrahedral (A) and octahedral (B) sites in the spinel lattice are characterised by X-ray diffraction, transmission electron microscopy, Extended X-ray Absorption Fine Structure (EXAFS) and Mössbauer spectroscopy. Due to the same radial distances of the first coordination shell in both tetrahedral and octahedral environments observed in EXAFS spectra, the position of the second coordination shell specifies the preference of more Fe ions towards the A site at x = 0.1. At x = 0.5, more Fe ions favour the B site. The cation distribution quantitatively obtained from the Mössbauer spectral analysis shows that while 60% of Fe ions occupy the A site in x = 0.1, 40% occupy it in x = 0.5. Surprisingly at x = 0.3, Fe ions are distributed equally among the A and B sites. dc magnetization reveals an increase in TC from 102 K to 200 K and in TS from 26 to 40 K with an increase in Fe concentration, indicating an enhancement in A-B exchange interaction at the expense of B-B. No report has until now demonstrated such an enhancement in TS either in pure or in doped CoCr2 O4 . Furthermore, frequency-dependent ac susceptibility (χ) data fitted with different phenomenological models such as the Néel-Arrhenius, Vogel-Fulcher and power law confirm a spin-glass and/or cluster-glass behaviour in nanoparticles of CoCr2-x Fex O4 .