Magnetic and low temperature phonon studies of CoCr2O4 powders doped with Fe(III) and Ni(II) ions

Extensive temperature-dependent phonon studies and low-temperature magnetic measurements of CoCr2-xFexO4 (for x=0.5, 1 and 2) and Co0.9Ni0.1Cr2O4 polycrystalline powders are presented. The main aim of these studies was to obtain information on phonon and structural properties of these compounds as well as strength of spin-phonon coupling in the magnetically ordered phases. IR and Raman spectra show that doping of CoCr2O4 with Fe(III) ions leads to broadening of bands and appearance of new bands due to the formation of inverted spinel structure. In contrast to this behavior, doping with 10 mol% of Ni (II) ions leads to weak increase of band width only. Magnetization measured as a function of temperature and external magnetic field showed that magnetic properties of Co0.9Ni0.1Cr2O4 sample are similar to those reported for pure CoCr2O4, i.e., partial substitution of Ni(II) for Co(II) leads to slight shift of the ferrimagnetic phase transition at T-c and spiral spin order transition at T-s towards lower values. The change of crystallization preference induced by incorporation of increasing concentration of Fe(III) ions in the spinel lattice causes significant increase of T-c and decrease of T-s. The latter transition disappears completely for higher concentrations of Fe(III). The performed temperature-dependent IR studies revealed interesting anomalous behavior of phonons below T-c for CoCr1.5Fe0.5O4 and Co0.9Ni0.1Cr2O4, which was attributed to spin-phonon coupling. (C) 2013 Elsevier Inc. All rights reserved.