The Study Of Spontaneous Exchange Bias And Room Temperature Magnetoelectric Coupling Effect In Perovskite Iron Oxides

In recent years,the transition metal oxides of perovskite structure have attracted extensive attentions of scientists due to their novel and rich physical/chemical properties.In these systems,the cross coupling between the inner electron,spin,lattice and orbit may result in a series of interesting phenomena such as exchange bias,high temperature superconductivity,metal-insulator transition,electronic phase separation,ferroelectric and multiferroic properties,which not only enrich the connotation of basic physics research,but also play a very important role in practical application.As a typical complex oxide material,perovskite iron oxides have potential applications in solid state fuel cells,photocatalysis and ferroelectric storage because that the Fe element is cheap,non-toxic and nonvolatile.SrFeO_3-xis a kind of antiferromagnetic material with very complicated magnetic structure.A variety of magnetic phases will appear with the continuous change of oxygen content.For example,SrFeO₃ belongs to spiral antiferromagnetic phase,SrFeO_2.5 belongs to G-type antiferromagnetic phase,and SrFeO_2.75 belongs to inclined antiferromagnetic phase.If this material is combined with others perovskite oxides,some novel physical phenomena may occur.In addition,both inclined antiferromagnetic Sm FeO₃（SFO）and ferromagnetic Ga_0.6Fe_1.4O₃（GFO）belong to rare improper room temperature single-phase multiferroic materials.Although some research groups at home and abroad have reported their multiferroic properties,the magnetoelectric coupling effect of the important indicator,has not been studied in depth till now.If these two single-phase multiferroic materials can realize reversible and nonvolatile room temperature electronic control of magnetic effects in nanoscale range,they will possess potential applications in the field of next generation of electric-write magnetic-read data storage devices.Based on the above reasons,this work selects above perovskite iron oxides as the research objects,and conducts detailed studies on the spontaneous positive exchange bias phenomenon of SrFeO_3-x/La_0.3Sr_0.7MnO₃(SrFeO_3-x/LSMO)bilayer.Meanwhile,the room temperature electronic control of magnetic effect of the SFO and GFO thin films is also studied.The specific contents and results are as follows:（1）Fully epitaxial SrFeO_3-x/LSMO bilayer was fabricated on（001）-oriented Sr TiO₃（STO）single crystal substrates by in-suit reflection high energy electron diffraction-assisted pulsed laser deposition instrument.The results show that the shift of the loop center always towards the initial magnetization direction when the magnetization-magnetic field（M-H）loop of the film is measured from different initial magnetization directions in a zero-field cooling condition at 5 K.So far,it has been reported that the positive exchange bias effect under field cooling or zero field cooling treatment results from the strong antiferromagnetic exchange coupling at heterogeneous interface.However,our experimental results and first principles calculations simultaneously prove that the spontaneous positive exchange bias effect is attributed to ferromagnetic coupling bilayer interface.Moreover,the direction and magnitude of the exchange bias can be effectively manipulated by using some other means（such as room temperature instantaneous magnetic field or ionic liquid gate）.These results can provide an important foundation for further promoting the application of low energy consumption spontaneous exchange bias spintronic devices.（2）By optimizing the experimental conditions,a series of（001）-oriented SFO epitaxial film with antiferromagnetic ground state were successfully prepared on Nb-Sr TiO₃（Nb-STO）single crystal substrate which could provide tensile strain and La AlO₃（LAO）single crystal substrate which could provide compressive strain.The results show that the films under different strain induce an obvious room temperature ferromagnetism and ferroelectricity.At the same time,with increasing the film thickness,the magnetic anisotropy gradually transfers from the out-of-plane to in-plane direction,which is attributed to the variations of O 2p-Fe 3d hybridization intensity and Fe 3d orbit occupancy caused by the strain-relaxed effect.Moreover,electrically driven reversible magnetic switching further proves that the SFO films exhibit the room temperature magnetoelectric coupling behavior,suggesting promising applications in multi-resistance states,low energy consumption,nonvolatile electric-write magnetic-read data storage devices.（3）In order to realize effective electronic control of magnetic analysis,we replace the study object of antiferromagnetic SFO by ferrimagnetic GFO system.During the experiment,90 nm GFO film is firstly prepared on Nb-STO（111）single crystal substrate.The magnetic measurement results indicate that the films exhibit strong ferrimagnetism at room temperature,and the magnetic domain structures are clearly visible at the remanence state.The ferroelectric measurement results show that the film is ferroelectric at room temperature.In addition,by using conductive Nb-STO substrate as a bottom electrode and Co/Cr tip of atomic force microscope as a top electrode,reversible nanoscale magnetic domains reversal under an external voltage is further switched by taking advantage of its magnetoelectric coupling effect,and the magnetic domains after manipulation possess long-time non-volatile properties.Within the given regions,the magnetic domain reversions can reach up to a rate of more than 72%.This is the first time that we have observed obvious electronic control of magnetic domain switching effect at room temperature in ferrimagnetic GFO single phase multiferroic materials.