Origin of ferromagnetism in BaTiO₃ nanoparticles

Abstract

Recently, there has been a great effort in studying magnetism in nonmagnetic semiconductors diluted with magnetic impurities due to possible applications in spinbased electronic systems. Moreover, nanoparticles (NP) of inorganic materials including otherwise nonmagnetic oxides such as CeO 2 , Al 2 O 3 , MgO, ZnO, In 2 O 3 and SnO 2 , nitrides, chalcogenides and other functional materials like superconductors and ferroelectrics were shown to be ferromagnetic at room temperature. The magnetism in these NP has been suggested to be intrinsic and originates from cation or anion vacancies at the surfaces of the NP. Very recently, it was reported from both experiments and firstprinciples calculations that typical ferroelectric materials such as BaTiO 3 (BTO) and PbTiO 3 (PTO) become multiferroic when they are prepared at the nanoscale [1 -8]. So, nanocrystalline BTO offers a room-temperature magnetic hysteresis as well as temperature-dependent dielectric constant and a polarization. Multiferroics that exhibit magnetoelectric coupling are widely discussed from quite different context, see [9 -12]. However, apart from a density functional calculation as vacancy-induced magnetism in BTO(001) thin films [13] a well accepted theoretical description of the ferroic properties of nanocrystalline BTO is still missing. In a previous paper [14] the static and dynamic properties of KH 2 PO 4 (KDP)-type and BTO-type ferroelectric NP have been reviewed. Based on that approach we propose a statistical model which reflects the multiferroic properties of BTO-NP. While the spontaneous polarization in a classic ferroelectric material like BTO is expected to diminish when the particle size is reduced [14-16], ferromagnetism cannot occur in bulk material [17]. It is observed experimentally that the multiferroic nature emerges in an intermediate size range of nanocrystalline BTO. Whereas the ferromagnetism is arising from the oxygen vacancies or point defects at the surface of the NP the ferroelectricity is originated from the core of the material [1][2][3].As demonstrated in [14] the Ising model in a transverse field is also appropriate to describe the properties of ferroelectric NP. The Hamiltonian reads e 1 2Since the ferroelectricity in BTO is originated from the off-centering of the Ti ions with respect to the cubic perovskite crystal we assume as the simplest model that there are two positions of the Ti atoms in a double-well potential. These two states are mapped on the S z -component of a pseudo spin-1/2 operator whereas the S x -component Based on a microscopic approach we demonstrate that the unexpected ferromagnetic properties of BaTiO 3 (BTO) or PTO observed recently at room temperature are due to oxygen vacancies at the surface of the nanocrystalline materials. Such vacancies lead to the appearance of Ti 3+ or Ti 2+ ions with nonzero net spin. The resulting different valence states composed of Ti 3+ or Ti 2+ offer a nonzero magnetization which decreases with increasing particle size. The system shows a multiferroic behavior belo...

Reduction of Ln2Ti2O7 Layered Perovskites: A Survey of the Anionic Lattice, Electronic Features, and Potentials

“…This nonzero magnetization called FM nano hereafter was already observed and discussed for nanometric BaTiO 3 35 and PbTiO 3 36 with similar amplitudes as in the title compounds. In agreement with theoretical calculations 37 , it was assigned to extrinsic surficial oxygen vacancies leading to Ti 3+ or Ti 2+ FM interactions. Then in both LTO-CaH 2 and LTO-H 2 it indicates a certain segregation of reduced species at the surface, together with the nanometrization upon reduction, leading to intrinsic weak ferromagnetic component.…”

Origin of ferromagnetism in transition metal doped BaTiO3

“…As already mentioned in the Introduction, we have shown 26,27 that the unexpected ferromagnetic and multiferroic properties observed at room temperature in ferroelectric ABO 3 , such as BaTiO 3 , SrTiO 3 , or PbTiO 3 nanoparticles can be due to the oxygen vacancies at the surface leading to the appearance of Ti 3þ and/or Ti 2þ ions with S 6 ¼ 0. Therefore, we assume that the transition metal ion doping effects in ABO 3 nanostructures would be stronger compared to the bulk case because to the magnetization due to the doping ions there is also the additive contribution of the surface effects.…”

“…In our previous papers, 26,27 we have proposed a model for pure perovskite ferroelectric BTO and ABO 3 nanoparticles, respectively, which allows finding out the multiferroic properties in these nanostructures. We have shown that the unexpected ferromagnetic properties observed recently at room temperatures can be explained by considering oxygen vacancies at the surface of the nanocrystalline materials.…”

Enhancement of the magnetoelectric effect in doped BaTiO3 nanoparticles

“…But the situation is changed in case of BTO‐NP. As shown in the unexpected ferromagnetic and multiferroic properties observed at room temperature in ferroelectric ABO, such as BaTiO, SrTiO, or PbTiO nanoparticles could be due to the oxygen vacancies at the surface leading to the appearance of Ti and/or Ti ions with non‐zero amount of spin. However, the related magnetization is very small.…”

“…Moreover, in Fe doped PTO nanostructures one has observed an increase () or a decrease () of magnetization M with reduction in particle size or film thickness, respectively. In two previous papers we have studied theoretically the origin of ferromagnetism in pure ABO nanostructures and transition metal ion doped bulk ABO materials (). The aim of the present paper is to investigate the magnetic properties of Fe‐doped BTO nanoparticles and to compare them with those of Fe‐doped bulk BTO materials.…”