Low-Temperature Griffiths Phase in Chemically Synthesized Comn2o4 Nanocubes

Abstract

Mixed transition metals and their oxides have gained a myriad of research interests because of their wide applications in electronics, magnetism, and electrochemical energy storage and conversion devices. In this study, we have synthesized CoMn2O4 nanocubes through the auto-combustion method. At room temperature, the X-ray diffraction technique confirms the tetragonal crystal structure and phase purity of annealed nanocubes. The Fourier transform infrared spectroscopy investigated that the oxide and hydroxyl groups are the major active sites. The absorption bands around 544 cm-1 and 638 cm-1 correspond to the atomic vibration of Co-O and Mn-O in octahedral and tetrahedral sites, respectively. The bandgap confirmed from the Uv-Vis spectrum is 1.3 eV, indicating that synthesized nanocubes are near-infrared light active material. The electron microscopies show that CoMn2O4 is made of regular cube-shaped nanostructures with an average particle size of ~180 nm. X-ray photoelectron spectra suggest a mixed spinel structure of nanocubes comprised of Co and Mn cations. The magnetic investigations revealed that the synthesized nanocubes exhibit a ferrimagnetic response. However, the emergence of large exchange bias is observed well above the transition temperature of ~172 K due to the Griffiths phase. It is proposed that the appearance of Griffiths phase arises due to the presence of antiferromagnetic impurities, which become active at low temperature.

Keywords: CoMn2O4, Auto-combustion method, Curie-Weiss law, Exchange bias, Griffiths phase