The Study On Electric Field Control Of Magnetism In Iron Oxides

Many researches have begun to focus on the use of electronic spin properties in information reading/writing and switching to design and develop newer devices.It is very necessary to control the magnetic properties of materials through external means.Researches based on electric control of magnetism have been carried out and achieved some success.Significant progress has been made in the study of current-control of the magnetism in materials or devices.Using the electric field instead of current to control the magnetism can significantly reduce energy consumption.Electric field control of magnetism for antiferromagnetic or ferrimagnetic materials also has made some achievements.As the representative iron oxides,?-Fe2O3 and ?-Fe2O3 are antiferromagnetic or ferrimagnetic insulated materials.They have been extensively used and the studies on the electric field control of magnetism for them are very necessary.Interface science provides more opportunities for studying the properties and potential uses of iron oxides.In this thesis,the iron oxides films with different structures were deposited on the single crystal substrates by PLD technology and studied.Recently,the potential applications of ?-Fe2O3 thin films,especially?-Fe2O3/MgO,in spintronics and information storage have attracted much attention.In this thesis,different thicknesses of ?-Fe2O3 thin films were deposited on MgO(001)single crystal substrates.The results show that the saturation magnetization at room temperature of ?-Fe2O3/MgO(001)films decreases with the increase of film thickness.This phenomenon may be related to the following two points.The first is the increased effect of disordered regions formed when the film thickness increases at higher.substrate temperatures and higher oxygen pressures.The second reason is the interface effect:the migration of some magnesium ions from the MgO substrate to the?-Fe2O3 film during the deposition may induce a large saturation magnetization.The thinner film has more significant interface effects,so larger saturation magnetization can be observed in the thinner film.The application of electric field perpendicular to the film plane through the ?-Fe2O3/MgO(001)film at room temperature can manipulate the magnetic properties of the films.The saturation magnetization of?-Fe2O3/MgO(001)films increases with the applied positive electric field(the direction of the positive electric field is from the substrate to the film),however,saturation magnetizing field decreases.The changes are non-volatile.When the negative electric field is directly applied on the fresh ?-Fe2O3/MgO(001)thin film,the change of saturation magnetization is weak.The electric field control of magnetism for the ?-Fe2O3/MgO(001)thin film may be related to the migration effect of Mg ions from MgO to ?-Fe2O3 film and O2-ions from ?-Fe2O3 film to MgO driven by the electric field.In addition,the electric field control of the magnetism for the thinner film is easier and more obvious.After the application of the positive electric field at room temperature,the resistance of the film decreases significantly while the magnetoresistance increases,possibly due to the increasing of electron hopping rate between Fe2+? and Fe3+.There exists a positive threshold electric field for enhancement of the magnetization in ?-Fe2O3/MgO(001)film.This indicates that Mg2+ cations should overcome an electric field barrier at the interface to migrate into the ?-Fe2O3 film.The Mg2+ cation is more likely to occupy the B vacancy in the inverse spinel structure of?-Fe2O3.The results of first-principle calculations using vasp software show that the Mg atom in the B site of ?-Fe2O3 may cause the nearest neighbor A-site Fe atom move towards to B-site vacancy.The inserted Mg may cause a local structural distortion and changes the charge distribution.After inserted a Mg atom in ?-Fe2O3 cell,the total magnetic moment increased by 2 ?B.The shrunk band gap and split-off state band are induced by the Mg occupation at B-site.Mg occupation of the B-site vacancy may cause the emergence of Fe2+?(0<?<1)ions.The electron hopping between Fe2+? and Fe3+ ions should be responsible for the conductivity and result in a decrease of the resistance and an increase of the magnetoresistance for ?-Fe2O3 film.When the surfaces or interfaces of the compound materials are polar,their physical properties should be different from bulk.According to classical electrostatics,ideal polar surfaces and interfaces are usually unstable,because the electrostatic potential could increase monotonically across the system and lead to the polar catastrophe problem.The investigation and design of such advanced interfaces will become a powerful route to engineer devices with novel functionality.The polar interfaces or surfaces of ?-Fe2O3,especially the important interface between ?-Fe2O3 and MgO in spintronics,have not yet been fully understood.We studied the polar interface structure and growth mode of ?-Fe2O3/MgO(001).The results show that the stoichiometric ?-Fe2O3 with tetragonal space group of P41212 should grow on MgO(001)in the[100]or[010]direction.In this model,the first B-layer at the interface has more octahedral Fe3+ ions than the other B-layers,and the extra three Fe3+ ions modify the stoichiometry of the interfacial layer composition and provide the compensation charge to stabilize polar surface.The extra Fe3+ ions in this model provide an additional magnetic moment of 15?B,which give a reasonable interpretation for why the saturation magnetization value of the very thin?-Fe2O3/MgO films are larger than that of ?-Fe2O3 bulk in experimental studies.As is known,?-Fe2O3 layers can be used as a pinning layer of spin valves,which has large magnetoresistance due to the scattering effect of conduction electrons at the metal/insulator interfaces.It is attractive to investigate whether the electric field could directly influence the magnetic properties of the ?-Fe2O3 film.Considering that both?-Al2O3 and ?-Fe2O3 are of rhombohedral corundum crystal structure,it is better to choose ?-Al2O3 as the substrate for ?-Fe2O3 film.?-Fe2O3 is antiferromagnetic insulator due to the super-exchange interaction.However,above the Morin temperature,?-Fe2O3 shows weak ferromagnetism due to slight canting of the two sublattice magnetization caused by a Dzyaloshinsky-Moriya(DM)interaction.In this thesis,we found that the weak ferromagnetic properties of ?-Fe2O3/?-Al2O3 at room temperature might not only come from the intrinsic spin-canting of ?-Fe2O3,but also from the interface effects.We investigated the electric field control of the magnetism for ?-Fe2O3/?-Al2O3 films.It is found that the positive electric field can improve the room temperature magnetism of ?-Fe2O3/?-Al2O3 films,while the negative electric field can reduce it.And the manipulation is also non-volatile.The changes induced by electric field may be connected with the migration effects of Al3+ ions.We also find that there are some steps in the curve of saturation magnetization dependent on the applied positive electric field.The steps of the curve may indicate the ions migration effect driven by the electric field is a complicated process.The magnetization of the film deposited at a higher temperature can be changed by electric field more easily.The first-principle calculations show that the ?-Fe2O3 magnetic moment can be increased whatever Al occupies the tetrahedral or octahedral vacancy.This can be attributed to the charge redistribution and orbital hybridization changes induced by local structure distortion.Additionally,the control of the magnetic properties of ?-Fe2O3/LiTa3 films by electric fields is also investigated.The results are similar to that of ?-Fe2O3/?-Al2O3films,which may be related to the ions migration driven by electric field.The first-principles calculations show that Li occupying either the oxygen tetrahedral or the octahedral vacancy can increase the magnetic moment of ?-Fe2O3.Both ?-Fe2O3 and ?-Fe2O3 are crystalline materials.Amorphous Fe2O3 has different properties from them.Amorphous Fe2O3 is considered as an advanced material for the advanced applications.It is also very important to study the electric field control of the magnetism for amorphous Fe2O3.So an investigation on electric-field control of magnetism for amorphous-Fe2O3/LiTaO3 film was performed in this work.The hysteresis loops can be experimentally found for amorphous-Fe2O3/LiTaO3 film at room temperature,which may be related to some ferromagnetic or ferrimagnetic ordered cluster and the interface effects.A reversible manipulation of magnetism over ten times of magnitude was achieved by the positive electric field upon amorphous Fe2O3/LiTaO3 at room temperature,while the negative electric field reduce the magnetism.The manipulation is also non-volatile.The changes induced by electric field may be connected with the migration effects of the lithium ions insertion(lithiation)into or extraction(delithiation)from amorphous FeaO3.The saturation magnetization does not monotonically increase with the applied positive electric field.There are some peaks and valleys of the curve for the saturation magnetization dependent on the electric field,which may reflect these more complicated Li ions migration processes.