The Study Of Electrically Controlled Magnetism In Oxides And Films

Magnetic materials is broadly used in social activities and national economy.With the progress in the science and technology,more and more new kinds of magnetic materials has been made and applied in many fields.Nanomaterials have special and excellent prorperties in optics,mechanics,electricity,magnetism,sensitivity,catalysis,etc,which opened up a new research and application area for the development of new materials.When the grain size of the magnetic material comes to the nanometer range,the small size effect,quantum size effect and surface effect of the nanomaterials bring it about some unique magnetic characteristic compared to the conventional polycrystalline or microcrystalline materials.With the rapid research development and excellent properties of magnetic nano materials,the widespread use of magnetic nanomaterials is gradually being explored.Magnetic nanomaterials are widely used in data storage,magnetic resonance imaging,magnetic fluid and biomedical and many other aspects,achieving remarkable results.With the rapid development of modern information technology,the demand for miniaturization and multi-functionalization of electronic components is becoming more and more urgent,which makes people pay more attention and interest in the research of materials with multiple physical effects.The coupling of electric polarization and magnetic polarization in materials provide an additional degree of freedom for the design of next-generation multifunctional electronic information recording devices,which has become a hot and new international area of research.At present,the coupling of ferroelectricity and ferromagnetism has been achieved in single-phase multifferoics materials and composite of ferroelectric and magnetic materials,which makes it possible to induce magnetic polarization following the electric polarization by external electric field or electric polarization following the magnetic polarization by the external magnetic field.This kind of property can be used in multistate storage and greatly improve the storage density.The magnetoelectric coupling effect involves a wealth of physical content,as the coupling between spin and lattice or phonon,strong correlation electron system,multiple element excitation,the correlation between electrons and phonons,and the correlation between electron and electrons,etc.The magnetoelectric coupling effect has a wide range of research and application value.Researchers have made a lot of theoretical works and experimental exploration in the magnetoelectric effect,which made a lot of progerss in the application of magnetoelectric effect.The use of magnetoelectric effect design in the memory device is expected to achieve the ideal access of magnetic read and electric write.Through the independent use of electronic charge or spin properties of materials,the traditional information device gain excellent performance.In order to obtain a more excellent memory devices,which is nonvolatile,high density,low power,reliable data transfer ratio and also fast,it is desirable to use both charge and spin on one device for mutual regulation of electrical and magnetic properties.The regulation of magnetism by electric fields is a hot topic in spintronics of semiconductors and has been studied extensively in a variety of materials.Materials with magnetic properties controlled by the electric field will provide a broad opportunity for miniaturization,versatility,and ultra low power consumption of the device.How to realize the coupling between the electrical properties and the magnetic properties,prepare the material with strong magnetic coupling effect,and understand the physical mechanism behind the magnetoelectric coupling is the key to study the magnetoelectric coupling material.The main contents and conclusions of this paper are as follows:(1)The GdFeO3 polycrystalline samples were synthesized by solid state method,and the effect of the applied electric field on the residual magnetic polarization was studied.We find that the magnetic properties of GdFeO3 have a compensating temperature near room temperature,and the remanent magnetization strongly depends on the magnetization history.When the external electric field is applied to the GdFeO3 sample,different remanent magnetization state will exhibit different modulation effects.We suppose that the electric field not only affects the magnetic properties of the sample by Joule heat effect,but also modifies the single ion anisotropy of Fe3+ ions and the DM exchange interaction between Fe3+ ions,which changes the weak ferromagnetism of GdFeO3 samples.(2)We prepare the Ag/TiO2/Nb:SrTiO3/Ag device and study the co-modulation of resistive switching and magnetism.The Ag/TiO2/Nb:SrTiO3/Ag device exhibits the pronounced electric field-induced magnetization modulation,accompanied with the bipolar resistive switching during the set and reset processes.The bipolar resistive switching in Ag/TiO2/Nb:SrTiO3/Ag device may be dominated by the modulation of Schottky-like barrier with the electron injection-trapped/detrapped process at the interface of TiO2/Nb:SrTiO3.We propose that the electric field-induced magnetization modulation results mainly from the creation/annihilation of lots of oxygen vacancies in TiO2.It is possible to achieve four logic states by encoding information via both magnetic manipulation and resistive switching.(3)We prepared Ag/HfO2/Nb:SrTiO3/Ag RRAM device and find out that electric field can induced switching of resistance and room temperature ferromagnetism simultaneously.The bipolar resistive switching may be controlled by the modification of Schottky-like barrier with the electron injection-trapped/detrapped process at the interface of HfO2/Nb:SrTiO3.The multilevel RS transition in the Ag/HfO2/Nb:SrTiO3/Ag device can be observed in the reset process with the larger negative voltage sweepings.The observed multilevel RS transition in the Ag/HfO2/Nb:SrTiO3/Ag device is connected with the different degree of electron detrapping in the interfacial depletion region of the HfO2 layer during reset process.We also demonstrate that the electrically controlled ferromagnetism in Ag/HfO2/Nb:SrTiO3/Ag device is connected with the manipulation of the density of oxygen vacancies in HfO2 film.The saturation magnetization decreases with an increasing of negative maximum sweeping voltage,due to the annihilation of parts oxygen vacancies.The multilevel resistance states and the electric field controlled ferromagnetism are potential for applications in ultrahigh-density storage and magnetic logic device.(4)We prepare the Ag/Nb2Os/Pt device and study the co-modulation of resistive switching and magnetism.In Ag/Nb2O5/Pt device we obtained the pronounced electric field-induced magnetization modulation,accompanied with the bipolar resistive switching during the set and reset processes.The bipolar resistive switching in Ag/Nb2O5/Pt device may be dominated by the forming and rupture of conductive filament of migrating Ag,while the magnetism controlled by electric field is connected with the manipulation of the density of oxygen vacancies in Nb2Os film accompany with the process of resistive switching.(5)A uniform electric field provided by a charged capacitor is applied to HfO2/NbSrTiO3 and HfO2/MgO samples,and electrical manipulation of room temperature ferromagnetism has been found in undoped HfO2 film.The manipulation of oxygen vacancy density at the interface of HfO2/Nb:SrTiO3 after a long-lasting voltage bias is believed to be the main contribution to the changeable magnetic saturation.The XPS spectrum shows that electric field can also influence the density of oxygen vacancy on the surface of HfO2.The observed tunable ferromagnetism by electric field in undoped HfO2 at room temperature may be of potential candidate in future spintronics and magnetic information storage.