Electronic And Magnetic Properties In Titanium Oxide Based Hetero And Amorhpous Structures:A First Principle Study

The transition-metal oxide is very promising as it offers novel functionalities and device concepts.The transition-metal oxide heterostructure(HS)creates an opportunity for even greater tunability due to the rich physics of the d-orbital electrons.Using computational modeling we can obtain more insights of these phenomena and more excitingly,guidance to experiments to achieve or optimize a desirable property or functionality.In the present work the electronic and magnetic properties in titanium based oxides HSs and amorphous structures are investigated by the DFT calculations.It is known that a conducting electronic state can form at the interface between two insulating oxides due to the polar discontinuity.Novel phenomena appear when two different oxide materials are combined together to form an interface.For example,at the interface of LaAlO3/SrTiO3,two dimensional conductive states form to avoid the polar discontinuity and magnetic properties are found at such interface.Further more,the electronic properties at the interface of HS are sensitive to the interfacial crystal structure and external field.It creates an opportunity for tunability,which is requisite for application.In this work,we propose a new type of interface between two nonmagnetic and nonpolar oxides that could host a magnetic state,where it is the ferroelectric polarization discontinuity instead of the polar discontinuity that leads to the charge transfer,forming the interfacial magnetic state.As a concrete example,we investigate the HSs made of ferroelectric perovskite oxide PbTiO3 and non-ferroelectric polarized and non-magnetism oxide TiO2 or SrTiO3.We show that charge transferred to the interfacial layer forming an interfacial ferromagnetic ordering that may persist up to room temperature.Especially,the strong coupling between bulk ferroelectric polarization and interface ferromagnetism represents a new type of magnetoelectric effect,which provides an ideal platform for exploring the intriguing interfacial multiferroics.The two types of HS,anatase/perovskite type TiO2/PbTiO3 and the perovskite/perovskite type SrTiO3/PbTiO3 HS are further investigated for their remarkable difference in magnetic ordering as the ferroelectric changes.We find that the ferroelectric polarization discontinuity at the interface leads to partially occupied Ti-3d states and the magnetic moments.The magnitude of the magnetic moments and the ground state magnetic coupling are sensitive to the polarization strength of PbTiO3.As the ferroelectric polarization of PbTiO3 increases,the two HSs show different magnetic ordering that strongly depends on the population of the Ti-t2g orbitals.For the TiO2/PbTiO3 interface,the magnetic moments are mostly contributed by degenerated dyz/dxz orbitals of interfacial Ti atoms,and the neighboring interfacial Ti atoms form ferromagnetic coupling via double exchange interaction.For SrTiO3/PbTiO3 interface,the interfacial magnetic moments are mainly contributed by occupied dxy orbital because of the increased polarization intensity,and as the occupation increases up to a critical value,the magnetic coupling between neighboring Ti atoms become antiferromagnetic via the superexchange interaction.Our study suggests that the manipulating the interfacial geometry and ferroelectric polarization is one effective way to control interfacial magnetic ordering for enhanced functionalities in oxide heterostructures.The findings here are important not only for fundamental science but also for promising applications in nanoscale electronics and spintronics.TiO2 films have been grown on Si(100)substrates via pulsed laser deposition.Significant magnetization(up to 180.4 emu/cc)has been obtained for the film in complete amorphous state.Compared to crystallized anatase and rutile phases,the amorphous structure contains more oxygen vacancies(Vo)which are beneficial to achieve large room temperature ferromagnetism of the TiO2 films.Our calculations indicate that the magnetic moments are distributed around the Ti3+ ions.The average local magnetic moment is 0.36-0.51?B per Ti atom for amorphous structure with Vo concentration of 10%-20%.The value is in good agreement with the experimental result.The net magnetic moment just arised in an amorphous system contained partialy filled Ti-3d orbital,means some Ti atoms in Ti3+ state,as the results of the oxygen vacancy.For the amorphous-crystal interface system,the spin polarized charge distribute dominatly in the amouphous part.These calculated results confirmed the experimental found that the amorphous sample has the larger saturation magnetization than that of the crystallized samples.And the boundary or defects in the amorphous structures are considered to be responsible for the magnetism.This study sheds light on the origin of room temperature ferromagnetism for the amorphous structure and provides an effcient and extendable method to achieve large magnetization by tuning the crystallinity of various material systems.