Anion vacancy-driven magnetism in incipient ferroelectric SrTiO₃ and KTaO₃ nanoparticles

Abstract

We predict that undoped nanoparticles (size≤10–100 nm) 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). In particular, for a particle radii R less than the critical radius R_c(T) ferromagnetic long-range order appears in a shell region of thickness 5–50 nm once the concentration of magnetic defects N exceeds the magnetic percolation threshold N^p. At vacancy concentrations N<N^p and radiiR<R_c(T), short-range ferromagnetic order and consequently a spin glass state may appear. For particle radii R>R_c(T), only the paramagnetic phase is possible.