Particle size effects on the magnetic and phonon properties of multiferroic CoCr2O4

Temperature-dependent magnetic and IR studies of two nanocrystlline CoCr2O4 samples with the mean crystallite size of 23.3 and 4.5 nm are presented. Magnetic studies show that transition temperature to collinear ferrimagnetic structure, which occurs in bulk CoCr2O4 at Tc=93–97 K, is weakly affected by reducing the size to about 8 nm. However, it strongly decreases, to about 75 K, by further reducing the size to 4.5 nm. We also report that the incommensurate spiral magnetic structure may easily develop below Ts=24 K in particles with the mean size of 23.3 nm but magnetization data do not show any evidence of this transition for the particles with the mean size of 4.5 nm. Temperature-dependent IR studies revealed pronounced low temperature phonon anomalies below Tc, which were attributed to strong coupling between the low wavenumber phonons and a ferrimagnetic collinear component of the magnetic order. Our results also revealed weak phonon anomalies below 25 K, which were attributed to weak spin–phonon and/or electromagnon–phonon coupling. Analysis of phonon damping also revealed some anomalies near expected magnetic phase transitions. The observed anomalous behavior is most likely dominated by spin–phonon coupling.