Size effect on magnetic properties of MnCr2O4 nanoparticles

Abstract

This paper studies the size effect on the magnetic properties of spinel MnCr2O4 powders synthesized by a bottom-up approach. The samples were synthesized by the co-precipitation method and calcined in 50 °C steps starting at 750 °C and ending at 1050 °C to increase the particle size. X-ray diffraction (XRD) analyses reveal that the samples calcined between 750 °C and 850 °C showed a Cr2O3 impurity phase. Thus, the results discussed here relate only to the MnCr2O4 nanoparticles calcined at 900 °C and 1050 °C. The average crystallite size of the synthesized samples determined by XRD analysis through the Scherrer equation shows an increase from 52 ± 4 nm at 900 °C to 70 ± 5 nm at 1050 °C. Particle size was found to increase from 88 ± 4 nm for the samples calcined at 900 °C to 127 ± 3 nm at 1050 °C as determined from the transmission electron microscopy (TEM) results. The elemental composition of the samples determined using energy dispersive X-ray spectroscopy (EDS) confirmed the presence of only Mn and Cr, while the selected area electron diffraction (SAED) showed the crystalline nature of the nanoparticles. Fourier transform infrared spectroscopy (FTIR) measurements show characteristic vibrations of different bonds. The magnetic properties were probed through temperature dependent magnetization measurements (M−T). From this, it was found that the MnCr2O4 particles show paramagnetic to ferrimagnetic transitions at the Curie temperatures (TC) of 43.45 ± 0.01 K and 42.66 ± 0.03 K for the samples calcined at 900 °C and 1050 °C, respectively. The short-range spiral order follows at transition temperatures (TS) of 17.03 ± 0.01 K and 17.71 ± 0.01 K for the samples calcined at 900 °C and 1050 °C, respectively. Thus, TC decreases while TS increases with an increase in calcination temperature and particle size. Magnetization as a function of the applied magnetic field studies corroborates the M−T results. The present results are compared with those previously obtained for MnCr2O4 prepared by the top-down approach.