Resistive Switching And Electric-field Control Of Magnetism In Transition-metal Oxide Based Heterostructures

Recently,the exponential growth of the data with the rapid development of information technology causes an emerging need for high performance memories with higher density,lower power consumption and faster operation speed.Transition-metal oxide heterostructures have attracted a lot of attention because they can exhibit a variety of physical properties which can be used to develop high-performance memory devices,such as the resistance switching effect and electric control of magnetism related to the migration of oxygen vacancies and/or ferroelectric reversal under the electric fields.The unipolar RRAM has been widely studied due to its simple structure,high integration,low power consumption,high resistance off/on ratio.However,there is no sub-nanosecond multilevel switchings reported in unipolar RRAM,which limits its further development in ultra-fast and low-power applications.In addition,the simultaneous resistance switching and magnetism tunablity through electric operation have been observed in magnetoelectric coupled heterostructures,but the mechanism is complex and dynamic characteristics related to operating speed require in-depth study.This dissertation mainly studies the resistance switching and electric control of magnetism in Au/Y3Fe5O12(YIG)/n-Si and Co/BiFeO3/SrRuO3 transition metal oxide heterostructures,and investigated their mechanisms through the characterization of the dynamic behaviors.Ultrafast nonvolatile multi-state magnetoelectric-coupling memory prototypes are obtained.The main contents of the dissertation are divided into six chapters as follows:In Chapter 1,the mechanisms of different memory devices are introduced,including Ferroelectric Random Access Memory,Ferroelectric Tunnel Junction Random Access Memory,Phase change Random Access Memory,Magneto-resistive Random Access Memory and Resistive Random Access Memory.The latest research progresses of resistive switchings and the electric field control of magnetism in transition-metal oxide heterostructures are reviewed.In Chapter 2,we proposed an Au/YIG/n-Si based RRAM device,which shows the unipolar resistance switching with multiple non-volatile resistance states under both DC voltage sweeping and pulsed voltage operation.At room temperature,the switching speed can be as fast as sub-nanosecond(600 ps)with the resistance off/on ratio of as high as 104.Especially,the resistance switching at 600 ps survives at 85℃,with a resistance off/on ratio of approximately 103.Based on X-ray photoelectron spectroscopy studies at different resistance states,we found that its resistance switching is related to the oxygen vacancy migration controlled by the electric field.In Chapter 3,we mainly studied the electric control of magnetism in Au/YIG/n-Si heterostructures.Owing to the effect of oxygen vacancy migration in electric field on the double exchange of Fe3+and Fe2+ in YIG,125%magnetic moment control with a speed of 600 ps was obtained.The control of the magnetoresistance effect by the electric field is also achieved.A negative magnetoresistance effect of-0.8%was observed at the low resistance state.In addition,the Pt/YIG/GGG heterostructure was constructed and the spin Hall magnetoresistance effect of 0.045%was observed.In Chapter 4,we prepared Co(6 nm)/BiFeO3(120 nm)/SrRuO3 heterostructure.By changing the direction of BiFeO3 ferroelectric polarization,non-volatile and reversible control of the magnetic coercive field of Co film can be obtained at room temperature,and the operation speed can be fast to 100 ns.Through X-ray absorption spectroscopy studies,as well as dynamics studies of ferroelectric domain switching and magnetic coercive field of Co film,it was found that when the ferroelectric polarization points to SrRuO3,CoOx appears at the Co/BiFeO3 interface and magnetic coercive field of Co film increases;when the ferroelectric polarization points to Co,the CoOx at the interface is reduced to Co,and magnetic coercive field of Co thin film decreases.With continuous switching of ferroelectric domains,the magnetic coercive field of the Co thin film can be continuously tuned.In addition,the Co/BiFeO3 heterostructure exhibits the exchange bias effect which can be tuned by the reverse of ferroelectric polarization.In Chapter 5,we constructed the Pt(2 nm)/Co(1.5 nm)/BiFeO3(30 nm)/SrRuO3 heterostructures,in which perpendicular magnetic anisotropy was characterized by the anomalous Hall effect.Nonvolatile electric field controls of magnetic coercive field and remanence ratio of Co film were obtained,with changes of 120%and 38%respectively.In Chapter 6,the related results in the aspects of the the resistance switching and electric control of magnetism in the YIG-based RRAM and the Co/BiFeO3 heterostructure were summarized.And then some areas needed to be improved were pointed out and some possible methods were proposed.