The origin of magnetism in perovskite ferroelectric ABO3 nanoparticles (A = K,Li; B = Ta,Nb or A = Ba,Sr,Pb; B = Ti)

Using spin Hamiltonian models and Green’s function techniques, we study the ferroic order parameters of ferroelectric nanoparticles, and show how multiferroic behavior can be achieved in such systems. We present a theoretical study suggesting that unexpected ferromagnetic properties of perovskite ferroelectric ABO3 nanoparticles (A = K, Li; B = Ta, Nb or A = Ba, Sr, Pb; B = Ti) observed recently at room temperatures can be explained by considering oxygen vacancies at the surface of the nanocrystalline materials. Such vacancies lead to the appearance of Ta4+ and/or Ta3+ (Ti3+ and/or Ti2+) ions at the surface with nonzero net spin. The resulting different valence compared to the Ta5+ (or Ti4+) with S = 0 in the bulk offers a nonzero magnetization which increases with decreasing particle size. The system shows a multiferroic behavior below a critical size of the nanoparticles and the related polarization tends to a saturation value when the particle size is enhanced.