Anion vacancy driven magnetism in incipient ferroelectric SrTiO3 and KTaO3 nanoparticles

Abstract: Based on our analytical results [http://arxiv.org/abs/1006.3670], we predict that undoped nanoparticles (size <10-100nm) of incipient ferroelectrics without any magnetic ions can become ferromagnetic even at room temperatures due to the inherent presence of a new type of magnetic defects with spin S=1, namely oxygen vacancies, where the magnetic triplet state is the ground state in the vicinity of the surface (magnetic shell), while the nonmagnetic singlet is the ground state in the bulk material (nonmagnetic core). Consideration of randomly distributed magnetic spins (S=1) had shown that magnetic properties of incipient ferroelectric nanoparticles are strongly size and temperature dependent due to the size and temperature dependence of their dielectric permittivity and the effective Bohr radius proportional to permittivity. The phase diagrams in coordinates temperature - particle radius are considered. In particular, for particle radii less that the critical radius ferromagnetic long-range order appears in a shell region of thickness 5 - 50 nm once the concentration of magnetic defects exceeds the magnetic percolation threshold. The critical radius is calculated in the mean field theory from the condition of the magnetic defects exchange energy equality to thermal energy. For particle radii higher than critical value only the paramagnetic phase is possible. The conditions of the super-paramagnetic state appearance in the assembly of nanoparticles with narrow distribution function of their sizes are discussed also.