The Study On Electric Field And Light Control Of Ferromagnetism In Oxide Films

As an important part of new materials,film materials play a significant role in modern society.Film materials have its unique advantages and are the basis for miniaturization of devices.It can be made into complex film materials with various properties,avoiding the limitations of single film material.Oxides films are important component of film materials,and are widely used in the field of advanced functional materials,memory devices,computers,environmental protection and biomedicine.Many film materials are magnetic,and the sources of magnetism are also diverse due to the complex and variable structure of films.It has been found that the information storage can be achieved by controlling the magnetic properties of film materials.Through continuous exploration,it has been found that many ways can be used to control the magnetic properties of film materials,such as electric field,light,current,stress and doping.Electric field and light are two popular methods for the control of magnetism,which have many advantages such as low power consumption,reversibility,fast speed and good compatibility.So far,the control of magnetic properties by electric field and light has been basically understood,and great progress has been made in the practical application of the deviceThe development of electronic information technology and the popularization of smart devices have led to a geometric increase in the amount of information data,and people are eager to obtain the storage devices with faster speed and higher density.Resistive Random Access Memory(RRAM)has emerged with the advantages of high density,fast operation speed and multi-value storage.More importantly,part of the mechanism of resistive switching overlaps with the source of magnetism,which allows the device to coexist the resistive switching and magnetism control,so that the device may get more states for storage.Meanwhile,the response of oxide materials to light will change the resistive switching of the device,which can improve the performance of the device.The RRAM based on oxide films combined with electric field and light can achieve the interaction between light,electricity and magnetism,which can find more combinations of materials with excellent performance and provide opportunities for miniaturized device with multi-function.This paper explores the control of magnetism by electric field and light in oxides films.The main contents are as follows:(1)We prepared the ZnO film by using RF magnetron sputtering.The ZnO film has room temperature ferromagnetism and the source of ferromagnetism is oxygen vacancies.The ITO/ZnO/Pt device has bipolar resistive switching,and the mechanism is the formation and rupture of conductive filaments of oxygen vacancies.The resistive switching of the device can be also affected by ultraviolet light,which is mainly reflected in the increase of current and the decrease of set voltage.This phenomenon is caused by the desorption of oxygen on the surface of ZnO during the illumination.The ferromagnetism of ITO/ZnO/Pt device at high and low resistance states is also different.The ferromagnetism of the device at low resistance state is larger than at high resistance state,due to the higher content of oxygen vacancies at low resistance state.Light illumination can enhance the ferromagnetism of the device,which is related to the type transition of oxygen vacancy.The resistive switching and ferromagnetism control in ZnO film may play a certain role in the field of multi-state storage.(2)Through the study of photoluminescence spectroscopy,we confirmed that the room temperature ferromagnetism of ZnO film is derived from singly ionized oxygen vacancies(Vo+).The ferromagnetism of ZnO film on Pt/Ti/SiO2/Si substrate can be enhanced by the illumination of ultraviolet light and violet light.The enhancement of the ferromagnetism is possibly correlated with the increase of singly ionized oxygen vacancies(Vo+),due to the capture of photo-generated electrons by doubly ionized oxygen vacancies(Vo++).The comparison of ZnO film on the MgO substrate shows that the built-in electric field plays an important role in this process.The recovery time is associated with the re-adsorption process of oxygen species on the outer-surface of the ZnO film.The control of ferromagnetism by non-polarized light at room temperature may have potential applications in optical-magnetic coupling devices.(3)We prepared the TiO2 film on Pt/Ti/SiO2/Si and Ag/Ti/SiO2/Si substrates by using RF magnetron sputtering,and the TiO2 films have room temperature ferromagnetism,which originates from the singly ionized oxygen vacancies(Vo+).The ferromagnetism of TiO2 films on Pt/Ti/SiO2/Si and Ag/Ti/SiO2/Si substrates can be controlled by violet light,while the ferromagnetism of TiO2 film on Ag/Ti/SiO2/Si substrate can be also controlled by blue light.We believe that this phenomenon is related to the effect of surface plasmon of Ag nanoparticles.Through the measurement of EDS,we found that the amount of Ag diffused into the film was larger than Pt.Meanwhile,the Ag nanoparticle can affect the light absorption of TiO2,increasing the absorption range of visible light.The effect of surface plasmon of Ag nanoparticles can lead to surface enhanced Raman scattering.The result shows that the Raman peak of TiO2 film on Ag/Ti/SiO2/Si substrate is significantly enhanced.The effect of surface plasmon of Ag nanoparticles makes the TiO2 film absorb blue light,which can change the ferromagnetism.(4)The CuxO films grown on Pt/Ti/SiO2/Si substrate have room temperature ferromagnetism,and the ferromagnetism may originate from the indirect double exchange effect(Cu+-Vo-Cu2+)and some oxygen vacancies.The ferromagnetism of CuxO film is changed under light illumination,and the light with high frequency has a larger effect on the ferromagnetism.The change of ferromagnetism for CuxO films on different substrates under light illumination shows that the built-in electric field formed at the interface plays an important role.The built-in electric field can promote the separation of photo-generated electron-hole pairs.The trapping of photo-generated electrons and holes during migration may lead to the valence transition of Cu ions,forming more Cu+-Vo-Cu2+ and enhancing ferromagnetism.Oxygen vacancies that do not participate in the indirect double exchange may also capture photo-generated electrons and have an effect on the ferromagnetism.We found that Ag/CuxO/Nb:SrTiO3/Ag device has different resistive switching,and the current-voltage curves had obvious rectification characteristic.The resistive switching of the device is accompanied with a stable change of ferromagnetism.The mechanism of resistive switching is the change of the Schottky-like barrier height at the interface caused by oxygen vacancies.The indirect double exchange effect(Cu+-Vo-Cu2+)can be used to explain the ferromagnetism of the CuxO film,and oxygen vacancies act as bridges in the indirect double exchange effect.Under different electric field,the barrier height at interface is different,which leads to different resistance states.Meanwhile,the oxygen vacancies also affect the ferromagnetism,and result in the change of ferromagnetism at different resistance states.The ferromagnetism of Ag/CuxO/Nb:SrTiO3/Ag device can also be controlled by light at room temperature.The reason of this result may be the capture of photo-generated holes or electrons,which will result in the change of the valence state of Cu ions and affect the indirect double exchange effect.(5)The Ta/La2/3Sr1/3MnO3/Nb:SrTiO3/Ag device has the coexistence of resistive switching and ferromagnetism modulation.The resistive switching of the device is affected by the migration of oxygen ions and the Schottky-like barrier at interface.Under different electric field,the migration direction of oxygen ions is different.The positive electric field will make the migration of oxygen ions and leave oxygen vacancies in the film.The increase of oxygen vacancies will penetrate the Schottky-like barrier and form the conductive filaments or conductive regions,making the device into low resistance state.When the negative electric field is applied,the oxygen ions backfill oxygen vacancies,and the conductive filaments or conductive regions are broken.The Schottky-like barrrier will be restored to the original state,and the device will return to high resistance state.The resistive switching of the device is also accompanied with the change of ferromagnetism.The ferromagnetism of the device at the high and low resistance states is also different.The reason is that the migration of oxygen ions affects the double exchange effect.The magnetic properties of La2/3Sr1/3MnO3/Nb:SrTiO3 can be controlled by light at 70K.The injection of photo-generated electrons may modify the relative occupation of the electronic orbitals on the Fermi level,leading to the change of magnetic anisotropy and coercivity.The sources of ferromagnetism in several oxide films are mainly oxygen vacancies,the indirect double exchange effect and double exchange effect.The ferromagnetism caused by oxygen vacancies and indirect double exchange effect are relatively weak,while the ferromagnetism caused by double exchange effect is relatively strong.The electric field is a strong external field,which can directly change the ferromagnetism of oxide films at room temperature.Light is a relatively weak external field that can only change weak ferromagnetism at room temperature,and needs to cooperate with low temperature to change strong ferromagnetism.Our research on the ferromagnetism madulation of oxide films may be helpful for the practical applications.