Coexistence of tetragonal and cubic phase induced complex magnetic behaviour in CoMnOnanoparticles.

CoMnO, 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 CoMnOnanoparticles 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 (CoMn)[CoMn]Oand (CoMn)[CoMn]O, 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, Tcat 165 K and Tcat 93 K corresponding to paramagnetic to a lower magnetically ordered ferrimagnetic state followed by a higher magnetically ordered ferrimagnetic state, respectively. While Tcis attributed to the cubic phase having inverse spinel structure, Tccorresponds to the tetragonal phase with normal spinel. In contrast to general temperature dependentobserved in ferrimagnetic material, an unusual temperature dependentwith 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 gis observed at 5 K, attributed to the Yafet-Kittel spin structure of Mnin the octahedral site. Such unusual results are discussed on the basis of competition between the non-collinear triangular spin canting configuration of Mncations 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.