Evidence of the oxygen vacancies-induced room temperature ferromagnetism in multiferroic Co-doped LiNbO3 films

Local Co structures and magnetic properties of the Co-doped LiNbO3 films deposited by RF-magnetron sputtering have been systemically studied by different experimental techniques. The detailed structural analysis using XRD, XPS, XANES and EXAFS as well as full multiple-scattering ab initio theoretical calculations of Co K-edge XANES spectra show that the doped Co ions substitute for the Li sites of the LiNbO3 lattice with +2 oxidation state and form CoLi + VO complexes with oxygen vacancies in the nearest coordination shell of Co. No extra impurity peaks, such as Co metal clusters and Co oxide secondary phases were observed with the XRD detection limit. All the Co-doped LiNbO3 films display obvious room temperature (RT) ferromagnetism with a Curie temperature of about 630 K. The saturation magnetic moments (Ms) decreases monotonously with increasing Co concentration, which is attributed to an increase in the antiferromagnetic coupling between Co ions. It can concluded that the observed RT ferromagnetism is intrinsic and originates from electrons bound in defect states associated with oxygen vacancies, producing an effective ferromagnetic interaction and aligning the magnetic moments of BMPs. This work provides strong evidence that the ferromagnetism and ferroelectricity can coexist in the Co-doped LiNbO3 systems.