Tunneling Rectification Magnetoresistance And Electrical Modulation Of Magnetism In Co-based Materials

With the fast development of information storage technology,electronic devices need to be more efficient with higher storage density,faster processing speed,and lower power consumption.In order to meet these requirements,researchers have pinned their hopes on spintronic devices.The ultimate goal of spintronic devices is to make full use of the spin of electrons to overcome the problems in traditional electronic devices,which are caused by quantum tunneling and high power consumption at the micro-nano scale.There are two orientations of spin,i.e.,spin-up and spin-down,which are usually manifested by the magnetic properties of a device.The orientations of spin can be used as a medium for information processing and storage.Magnetoresistance(MR)effects have always been a research hotspot in the field of spintronics.As early as the 1980s,Albert Fert and Peter Griinberg discovered the giant magnetoresistance(GMR)effect in[Fe/Cr]n and Fe/Cr/Fe multilayers.After that,the tunneling magnetoresistance(TMR)was discovered successively in both Al2O3-and MgO-based magnetic tunnel junctions(MTJs).Compared with GMR effect,TMR effect has a higher MR ration and a higher integration density.And now the TMR effect has been widely used in modern magnetic random access memory(MRAM),spin logic devices and spin transfer nano-oscillator.Moreover,electrical modulation of spin degree of freedom(magnetism)is also a hotspot of spintronics.Compared with the magnetic field control of spin,electrical control of spin has a faster speed for data writing and a higher density for information storage.Moreover,the electrical control of magnetism is compatible with the existing industrial integrated circuit technology Therefore,the manipulation of spin by electrical methods not only advances the understanding of physical mechanism,but also is the key to the realistic application of spintronic devices.Focused on the MR effect and electric control of magnetism,the main work of this dissertation includes the following aspects:(1)Electrically tunable tunneling rectification magnetoresistance in Co/CoO-ZnO/Co MTJs with asymmetric barriers.We designed and fabricated the Co/CoO-ZnO/Co MTJs with asymmetric tunneling barriers.Here,we realized a novel tunneling rectification magnetoresistance(TRMR)by integrating charge-related rectification effect and spin-related TMR in the Co/CoO-ZnO/Co magnetic tunneling junctions.The TRMR means when an alternating current(AC)is applied to the MTJs,a rectification voltage can be detected,and the magnetic field dependence of the rectification voltage is directly related to the spin-dependent tunneling effect.Moreover,by simultaneously applying direct current(DC)and AC to the devices,the TRMR was remarkably tuned in the range from-300%to 2200%by changing the DC component while keeping the AC component fixed.This proof-of-conception investigation provides an alternative way towards controlling the MR by using alternating current.Moreover,this TRMR can be adapted to other heterojunctions with asymmetric barriers,where emergent functionalities could be observed,and provides a new way towards multifunctional spintronic devices.(2)Electrical-field control of magnetization and anomalous Hall effect in Co/PMN-PT hybrid heterostructures.We prepared the Co/PMN-PT hybrid heterostructures by using magneton sputtering and metallic shadow mask techniques.By combing the electrical-field-induced strain effect and oxygen-ion migration effect,the nonvolatile electrical-field control of the magnetization and the anomalous Hall effect was realized in Co/PMN-PT hybrid heterostructures at room temperature.In this hybrid heterostructure,the strain effect and oxygen-ion migration effect exerted opposing effects on the magnetization of Co layer.Unpon applying a positive electric field,the oxygen ions migrated from CoO into ZnO,turning anti ferromagnetic insulating CoO into ferromagnetic metallic Co,which enhanced the saturation magnetization and the anomalous Hall effect(AHE).Under negative electric field,the oxygen ions moved from ZnO into Co and form antiferromgnetic CoO,which decreased the magnetization and AHE.On the contrary,the positive(negative)electric-field-induced strain decreased(enhanced)the magnetization.The competition between strain effect and oxygen-ion migration effect determined the final magnetization of Co.This proof-of-concept investigation opens an alternative way to optimize and enhance the electric-field effect on magnetism through the combination of multiple electrical manipulation mechanisms in hybrid multiferroic devices.(3)Current-induced magnetization switching in Pt/[Co/Pt/Co]magnetic heterostructures.We fabricated the Pt/[Co/Pt/Co]magnetic heterostructures with perpendicular magnetic anisotropy by magneton sputtering.The samples were patterned into Hall bar devices with the width of 5 ?m by ultra-violet lithography and Ar ion etching.Based on the spin Hall effect in the heavy metal Pt,we explored the current-induced magnetization switching under various in-plane magnetic field.The efficiency of damping-like torque was quantitatively estimated by the harmonic Hall voltage measurements,the efficiency of damping-like torque on magnetic domain was(13.52±0.28)Oe/(107A/cm2)for+Mz and(-13.28±0.38)Oe/(107A/cm2)for-Mz.And the effective spin Hall angle of Pt is 0.052.Using Magneto-Optical Kerr measurements,we observed the domain walls motion during both the out-of-plane magnetic filed induced and current-induced magnetization switching,Hence,the DWs nucleation and propagation dominated the magnetization switching.And we explained the magnetic switching by the magnetic domain-wall movement model.Our experiment has verified the previous researchers' conclusions and laid the foundation for the in-depth study of the synthetic antiferromagnetic devices based on Pt/[Co/Pt/Co]structures.