Coexistence of tetragonal and cubic phase induced complex magnetic behaviour in CoMn <sub>2</sub> O <sub>4</sub> nanoparticles
Sanjna Rajput, Manish Yadav, Taranga Dehury, Akhilesh Kumar Yadav, Pratap K. Sahoo, Chandana Rath
Abstract
Abstract CoMn 2 O 4 , known for its extensive range of applications, has been subject to limited investigations regarding its structure dependent magnetic properties. Here, we have examined the structure dependent magnetic properties of CoMn 2 O 4 nanoparticles synthesized through a facile coprecipitation technique and are characterized using x-ray diffractometer, x-ray photoelectron spectroscopy (XPS), RAMAN spectroscopy, transmission electron microscopy and magnetic measurements. Rietveld refinement of the x-ray diffraction pattern reveals the coexistence of 91.84% of tetragonal and 8.16% of cubic phase. The cation distribution for tetragonal and cubic phases are (Co 0.94 Mn 0.06 )[Co 0.06 Mn 1.94 ]O 4 and (Co 0.04 Mn 0.96 )[Co 0.96 Mn 1.04 ]O 4 , respectively. While Raman spectra and selected area electron diffraction pattern confirm the spinel structure, both +2 and +3 oxidation states for Co and Mn confirmed by XPS further corroborate the cation distribution. Magnetic measurement shows two magnetic transitions, Tc 1 at 165 K and Tc 2 at 93 K corresponding to paramagnetic to a lower magnetically ordered ferrimagnetic state followed by a higher magnetically ordered ferrimagnetic state, respectively. While Tc 1 is attributed to the cubic phase having inverse spinel structure, Tc 2 corresponds to the tetragonal phase with normal spinel. In contrast to general temperature dependent H C observed in ferrimagnetic material, an unusual temperature dependent H C with high spontaneous exchange bias of 2.971 kOe and conventional exchange bias of 3.316 kOe at 50 K are observed. Interestingly, a high vertical magnetization shift (VMS) of 2.5 emu g −1 is observed at 5 K, attributed to the Yafet–Kittel spin structure of Mn 3+ in the octahedral site. Such unusual results are discussed on the basis of competition between the non-collinear triangular spin canting configuration of Mn 3+ cations of octahedral sites and collinear spins of tetrahedral site. The observed VMS has the potential to revolutionize the future of ultrahigh density magnetic recording technology.