Effects Of Adsorption O₂ On The Magnetism Of Defective SrTiO₃

ABO₃-type perovskite is a traditional functional material,and its diverse structure and different element ratios endow it with rich physical and chemical properties.Sr Ti O₃（STO）,as one of the representatives,is an incipient ferroelectric or quantum paraelectric,which has attracted much attention due to its high dielectric constant and low dielectric losses.The strained STO films and nanocrystalline plates along the crystallographic direction could exhibit ferroelectricity even at room temperature,and the perfect STO with 3d⁰Ti ions is nonmagnetic due to the absence of the unpaired electron in the empty d-orbital.Therefore,the introduction of magnetism into the nonmagnetic STO to form the multiferroics has become the current research hotpot.At present,a large number of experimental and theoretical studies have found that the lattice defects and doping with 3d transition metals can make non-magnetic STO have room temperature ferromagnetism,but there are differences on the source of magnetism and the magnetic coupling type.In this paper,we selected the Ti O₂-treminated cubic STO（001）surface as the research object and analyzed the density of states,spin density and Bader charge by first principles calculations.The influence of oxygen molecule adsorption on the magnetic properties of undoped STO（001）surface and Fe/Mn-doped STO（001）surface and the magnetic coupling of their stable model are also studied.The main conclusions are as follows:1.The magnetic properties of STO（001）surface with single vacancy and effects of O₂gas adsorption on their magnetism were studied.For the cubic STO（001）surface,four kinds of single vacancies were considered,namely the surface O vacancy(V_OI),the subsurface O vacancy(V_OII),Ti vacancy(V_Ti)and Sr vacancy(V_Sr).STO（001）surface without vacancy and with strontium vacancy is non-magnetic.However,the oxygen vacancy and titanium vacancy can introduce the magnetic moment in STO（001）surface.We considered the single oxygen molecule and the double oxygen molecule adsorb each defective STO（001）surface.After adsorbing one O₂molecule,the magnetic moment of the STO（001）surface with V_Oor V_Tidecreases,while the surface with V_Srcannot adsorbed O₂molecule.With the addition of the second adsorbed O₂molecule,the magnetic moment of the STO（001）surface with V_Oor V_Tiis enhance significantly.Moreover,the STO（001）surface with surface oxygen vacancy show ferromagnetic coupling,the STO（001）surface with subshurface oxygen vacancy show antiferromagnetic coupling,and the STO（001）surface with titanium vacancy show ferromagnetic coupling.After adsorbing one oxygen molecule,the magnetism of STO（001）surface with oxygen vacancy disappears,and the STO（001）surface with titanium vacancy still show ferromagnetic coupling.After adsorbing two oxygen molecules,all the above surfaces show antiferromagnetic coupling.2.The magnetic properties of STO（001）surface with complex defects and effects of O₂gas adsorption on the magnetism of defective STO（001）surface were investigated.For the complex defects,we considered V_Ti-O(V_Ti+V_Ocomplex)and V_Sr-O(V_Sr+V_Ocomplex).By comparing the vacancy formation energy,the complex defect is easier to form compared to the corresponding single cation vacancy.For the V_Ti-Ocomplex defect,the closer the distance between V_Tiand V_O,the more stable the complex defect formed.For the V_Sr-Ocomplex defect,the distance between vacancies is not the decisive condition for whether the V_Sr-Ocomplex defect is easy to form.The V_Ti-Ocan introduce the magnetic moment in the STO（001）surface and show ferromagnetic coupling,while the STO（001）surface with V_Sr-Ois non-magnetic.After adsorbing one O₂molecule,the magnetic moment of the STO（001）surface with V_Ti-Odecreases but the magnetic coupling of the surface does not change.The magnetic moment of the STO（001）surface with V_Sr-Oincreases and show antiferromagnetic coupling.Moreover,the STO（001）surface with complex defects that has adsorbed one O₂molecule cannot adsorb more O₂molecule.3.Effects of O₂gas adsorption on the magnetism of Fe/Mn-doped STO（001）surface with oxygen vacancy were investigated.The results are as follows:The doping can make the non-magnetic STO（001）surface have a large magnetic moment and show the ferromagnetic coupling.For the STO（001）surface with Fe _Ti+V_Oand Mn _Ti+V_Ocomplex defects,the surface oxygen vacancy(V_OI)is easier to form.However,the introduction of surface oxygen vacancy does not necessarily enhance the magnetic moment of Fe/Mn-doped STO（001）surface.The change of the magnetic moment is related to the electron arrangement of the doping ion d-orbital.Due to the enhancement of p-d hybridization effect,the introduction of V_OIstrengthens the surface ferromagnetic coupling.Oxygen molecule could be adsorbed chemically on the Fe/Mn-doped STO（001）surface with oxygen vacancy.The chemically adsorbed O₂molecule restores the surface magnetic moments.At the same time,the surface ferromagnetic coupling will be further enhanced due to the emergence of d-d hybridization effect.In conclusion,the magnetic properties of undoped and Fe/Mn-doped STO（001）surface are affected by vacancies and chemisorbed O₂molecules.In this paper,the effect of chemical adsorption of oxygen molecules on the surface magnetism of STO（001）was systematically studied,which provided a new method for the magnetic generation of perovskite oxides and a new idea for the design of single-phase room-temperature multi-iron materials.