Effects Of Adsorbed O₂ And Intrinsic Vacancy On The D⁰ Magnetism Of BaTiO₃

BaTiO₃（BTO）is a kind of perovskite ferroelectric with stable ferroelectric properties at room temperature.It has a wide range of application prospect in the field of microelectronics and optoelectronics.However,at room temperature and atmosphere pressure,BTO is non-magnetic,which limits its application in spintronic devices.A large number of experiments and theoretical calculations on the origin of the magnetic properties of BTO d⁰ magnetism are still controversial,In this paper,the effects of adsorbed oxygen and intrinsic vacancy on the magnetic properties of BTO have been studied by first principles calculation.The main work and conclusions are as follows:For the BTO（001）surface of theTiO₂terminal,four types of single vacancies were constructed on the surface according to the type of atoms and the positions where oxygen atoms were located,which were surface oxygen vacancy,subsurface oxygen vacancy,titanium vacancy and barium vacancy respectively.Both low concentration（vacancy toO₂ molecule ratio of 1:1）and high concentration（vacancy toO₂ molecule ratio of 1:2）ofO₂were adsorbed on the surface containing each vacancy.With the increase ofO₂ concentration,the magnetic moment generated on the surface containing surface oxygen vacancy and the surface containing subsurface oxygen vacancy first decrease from 1.80μ_B and 1.85μ_B to 0,and then increase sharply to 1.97μ_B and 2.0μ_B,respectively.The magnetic moment generated by the surface containing titanium vacancy decreases from 3.85μ_B to-0.54μ_B continuously,while the surface containing barium vacancy can not adsorbO₂ at both low and high concentrations.For BaO terminal of BTO（001）surface,surface oxygen vacancy,subsurface oxygen vacancy,titanium vacancy and barium vacancy are constructed in the same way on the surface,and different concentrations ofO₂ were adsorbed on each defect surface.The results showed that the surface with oxygen vacancy and with subsurface oxygen vacancy can only adsorb a small amount of oxygen molecules,making their magnetic moment drop to zero.Under the condition of highO₂ concentration,neither defect surface can adsorbO₂.The surface of with barium vacancy can adsorbO₂ under both low oxygen and high oxygen coverage,which weakens the magnetic moment of the surface.For the surface with titanium vacancy,the magnetic moment decreases from 3.93μ_Bto 1.67μ_B after adsorbed low concentrationO₂.Under the condition of high concentrationO₂,oneO atom is obtained from the defect surface to form oneO₃ molecule far away from the surface by adsorptionO₂,which weakens the magnetic moment introduced by titanium vacancy.On the basis of the above research,the case of complex defect is further studied.At theTiO₂ terminal of BTO（001）surface,titanium-oxygen complex defect and barium-oxygen complex defect are constructed.Four types of titanium-oxygen complex defect and three types of barium-oxygen complex defect were obtained respectively,according to the distance between the oxygen vacancy and the titanium vacancy,or oxygen vacancy and the barium vacancy.After calculating the formation energy of complex defect,both the titanium-oxygen complex defect surface and the barium-oxygen complex defect surface with lower energy were selected to adsorbO₂ at different concentrations.The results show that with the increase of adsorptionO₂ concentration,the magnetic moment generated on the defect surface containing titanium-oxygen complex defect decreases from 2.0μ_Bto 0.17μ_Band then increases to 1.98μ_B.The magnetic moment on the defect surface containing barium-oxygen complex defect increased from 0 to 0.7μ_B after adsorption of low concentrationO₂,and theO₂ cannot be adsorbed on the defect surface at high concentration.For BaO terminal of BTO（001）surface,four kinds of titanium-oxygen complex defect and three kinds of barium-oxygen complex defect are also constructed.According to the result of the formation energy calculation,selected to the containing titanium-oxygen complex defect surface and containing the barium-oxygen complex defect surface with lower energy to adsorbO₂ of different concentrations.The results show that the defect surface containing titanium-oxygen complex defect can not adsorbO₂ at both low concentration and high concentration ofO₂,However,the surface containing barium-oxygen complex defect can adsorbO₂at low concentration,which makes the magnetic moment increase to 0.7μ_B,and the chemisorption can not take place on the surface at high concentration ofO₂.In conclusion,for the undoped BTO（001）surface,the results show that the magnetic properties of the material are the result of the interaction of single vacancy defects(V_O,V_Tiand V_Ba)and compound vacancy defects(V_Ti-O and V_Ba-O)and the adsorption of oxygen molecules in the environment.In this paper,the influence mechanism of defect type on the concentration of adsorbed oxygen and the influence mechanism of synergistic effect of adsorbed oxygen and intrinsic vacancy defect on the magnetic properties of BaTiO₃ thin films were discussed,and the source of ferromagnetism of undoped perovskite oxide was elucidated,providing a new idea for the design and preparation of single-phase room-temperature ferromagnetic materials.It plays an active role in exploring the magnetic origin of ferroelectric materials.