Structure, optical and varying magnetic properties of insulating MCr2O4 (M= Co, Zn, Mg and Cd) nanospinels

Chromite nanospinels of MCr2O4 (M= Co, Zn, Mg and Cd) have been synthesized using a microwave combustion method. The spinel phase and average particles size have been investigated by powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and transmission electron microscopy (TEM). Both XRD and TEM reveal an overall average particle size of about 12 nm. UV-Vis optical absorbance measurements at room temperature indicate common wide optical energy gap of about 3.1-3.4 eV. Magnetization processes have been studied via precise field cooled (FC) and zero-field cooled (ZFC) magnetization measurements. The magnetic transition temperatures found in the nanoparticles are slightly below those of corresponding bulk crystals. Survived spiral spin textures below similar to 26 K and coexisting spin-glassy state are observed in the Neel-type ferrimagnetic CoCr2O4 nanocrystals with Curie temperature of 92 K. While the antiferromagnetic state of chromites with nonmagnetic divalent cations is observed in ZnCr2O4 nanocrystals below Neel temperature of 11 K, it is significantly suppressed in MgCr2O4 and CdCr2O4 nanocrystals of identical average particle size. Our results indicate increased surface spins contribution with increasing lattice parameter. ESR linewidth and resonance field of MCr2O4 also show M-cation type dependence. The coexisting spin-glassy-like state in long-range ordered CoCr2O4 and a possible superparamagnetic state in MCr2O4 (M = Zn, Mg and Cd) nanospinels are discussed as finite-size and Cr-Cr separation effects. (C) 2019 Elsevier B.V. All rights reserved.