Investigation of local structure by X-ray absorption fine structure and magnetic properties of CoMn2O4 nanoparticles

Local structure and magnetic properties of CoMn$_2$O$_4$ nanoparticles prepared by co-precipitation method have been examined. X-ray diffraction pattern shows mixed phase of tetragonal and cubic phases at room temperature with crystallite size of 42 nm, while Fourier transform infrared spectrum shows bands corresponding to tetrahedral and octahedral sites of spinel. Pre-edge feature from XANES and Fourier transform (FT) EXAFS spectra at Co and Mn K-edges confirm that majority of Co and Mn occupy A and B sites, respectively, while small amount of Co and Mn occupies the B and A sites, respectively. The spin dynamics of these nanoparticles show the dispersion of ac susceptibility (χ″) with frequency, and confirm the spin-glass like behaviour. Surprisingly, we observe a progressive increase in vertical magnetization shift (VMS), while measuring magnetization under field-cooled condition. Remarkably, a maximum VMS of 2.45 emu g$⁻¹$ and exchange bias (EB) of 888.5 Oe were observed under a 10,000 Oe cooling field. Such a shift is intricately linked to the Yafet–Kittel spin structure of Mn$³⁺$ localized in B site and the EB could be due to the interaction between ferrimagnetic and spin-glass like phase.