Study Of Spin Memory Magnetoresistance And Rectification Magnetoresistance

With the rapid development of information technology,new concepts such as "big data" and "cloud storage" are on the rise,which require chips to scarify the demand for high speed,low power consumption,high integration density.In the past 50 years,by improving the precision of lithography,adopting new technology,chip integration processed rapidly following Moore's law.However,with the reduction of the device size and the increase of integration degree,Moore's law will soon meet the insuperable obstacle because of rapidly increase of the energy consumption per unit area and quantum confinement effect.Spintronics was regarded as the key step to break the bottleneck of Moore's law.It is well known that electron has two intrinsic properties,charge and spin.The traditional microelectronics only utilizes the charge degree of freedom of electrons,spintronics utilizes charge and spin degree of freedom of electrons simultaneously and introduces new mode of information storage and processing,which makes the spintronics device has the advantage of high speed,low power consumption,high integration density compared with traditional microelectronics device.In the developing progress of spintronics,magnetoresistance(MR)effect was always the research hotspot due to its irreplaceable role in magnetic sensor and magnetic storage.MR represents the change of the electrical resistance of a material or artificial structures under the application of external magnetic field.The study of MR has a long history,starting from the first observation of anisotropic MR(AMR)in iron and nickel by Thomson in 1857,which results from the different scattering cross-section due to spin orbital coupling.In 1988,Griinberg and Fert observed giant MR effect(GMR)in Fe\Cr multilayer respectively,and attribute it to spin-dependent scattering.Afterwards,GMR was widely used as a read head of hard disk.In 1994,Colossal MR(CMR)was observed by Jin et al.in perovskite-type manganese oxides,which is much higher than that of GMR.CMR effect originates from the magnetism dependent metal-insulator transition.In 1975,Tunneling MR(TMR)was found in Fe/Ge/Fe magnetic tunneling junctions(MTJs)by Julliere,which was attributed to spin dependent tunneling.At that time,TMR attracted less attention due to the bad experimental reproducibility limited to sample preparation technology.Later,in the Al2O3 and MgO based MTJs,large TMR ratio was observed.Moreover,due to spin filter effect of MgO barrier,TMR as high as 600%was realized at roomtemperature.Compared with GMR device,TMR device has the advantage of higher MR ratio,lower power consumption and higher integration density,so it is widely used in magnetic sensor,magnetic read head,magnetic random access memory(MRAM)and reconfigurable spin logic device.Extraordinary MR(EMR)was found in nonmagnetic semiconductors,such as Si,Ge,InSb,Ag2Se,GaAs and so on.Compared with the magnetic materials,this kind of MR effect presents two distinguished features.One is that the MR ratio is very large,comparable to,or even larger than that of magnetic materials.The other is that the resistivity increases approximately linearly with the external magnetic field.Generally speaking,EMR effect is closely related to the inhomogeneity of materials.Recently,Zhang et al.utilizes the non-linear transport property of diode and the Hall Effect or Anomalous Hall Effect to implement huge MR ratio,which could be named as diode enhanced MR effect(DEMR).Based on EMR and DEMR,researchers propose the reconfigurable magnetoresistance logic device.In brief,the study of MR effect is not only beneficial to fundamental physics,but also promotes the development of magnetic sensor,magnetic read head,MRAM and reconfigurable magnetic logic device.Therefore,the discovery of novel MR effect and the combination of MR effect with other effects will provide alternative ways towards multifunctional device.This paper includes the following parts,we fabricate the Co/CoO-ZnO/Co magnetic tunneling junction(MTJ).By electric field controlled oxygen ion(O2-)migration,we successfully combine the resistance switching(RS)and MR effect to realize spin memory magnetoresistance.We firstly observe Rectification MR(RMR)in Al/Ge/Al Schottky heterojunction and found that the RMR effect requires simultaneous implementation of the rectification effect and MR effect in the same devices.We observed spin RMR in Co/CoO-ZnO/Co MTJ,which originates from the spin polarized electron tunneling through asymmetric CoO-ZnO barrier.Based on the RMR effect,we realize the electrical control of RMR by applying direct current(DC)and alternating current(AC)simultaneously to the Al/Ge/In Schottky heterojunction.And,we realize RMR by combining the rectification device and MR device in parallel.There are five parts for this dissertation.?.Spin memory MR in Co/CoO-ZnO/Co MTJs with CoO-ZnO composite barrierWe prepared the Co/CoO-ZnO/Co MTJs with junction area of 100 ?m×100 ?m by magneton sputtering and metallic shadow mask.RS effect was realized due to the metal-insulator transition of CoO controlled by the migration of O2-between CoO and ZnO under applied electrical field.In high resistance state(HRS),TMR effect was observed due to spin polarized electron tunneling through the CoO-ZnO barrier.In low resistance state(LRS),GMR effect was observed.Hence we finally realize four stable resistance states,which have promising applications in multibit nonvolatile data storage and artificial neuronal computing.We also observed the exchange bias effect due to the existence of antiferromagnetic CoO.During the RS process,the oxygen vacancy concentration in CoO changes,which results in a reversible electrical manipulation of the exchange bias effect.In HRS,we also observed enhanced TMR of 68%and we derived the spin polarization of Co/CoO interface is high up to 72%according to Julliere Model.The ab initio calculations of CoO and Co/CoO interface were made.The calculation results indicate that ideal CoO without oxygen vacancy is insulator and it becomes metallic with 25%oxygen vacancy concentration,which further confirms that the RS effect results from the metal-insulator transition of CoO.The calculation results also show that the spin polarization of Co/CoO interface is high up to73.2%,which is consistent with the experimental results.?.The RMR effect of AI/Ge/AI Schottky heterojunctionsWe fabricated the Al/Ge/Al Schottky heterojunctions by electron beam evaporation and photolithography technology in the intrinsic Ge(i-Ge)substrate.Rectification Magnetoresistance was observed in the Al/Ge/Al heterojunctions,.e.,the application of a pure alternating current to the device can generate a significant direct-current voltage and the voltage varies with the external magnetic field.We found that the RMR is as large as 250%at room temperature,while its conventional MR is 70%.The observation of RMR not only adds a new member to the MR family,but also provides an alternative approach to realize functional devices by using alternating current.Through a series of comparative experiments,we found that the device with only MR effect or rectification effect cannot induce any RMR effect.The RMR effect requires simultaneous implementation of rectification and MR effect in the same device.According to theoretical analyzation,we found that only a finite overlap exists between the electron wave function for small enough carrier concentration in i-Ge substrate.The application of magnetic field causes the shrinkage of the carrier wave function.In this way,magnetic field gradually narrows the band width and increases it to a slightly high energy,which further modifies the energy bending at the Al/Ge Schottky interface.Furthermore,the symmetry of I-V curves is modified and results in RMR effect.?.Spin RMR in the Co/CoO-ZnO/Co MTJs.The Co/CoO-ZnO/Co MTJs were fabricated in the same way as describe in part one.Its RMR is as high as 116%,while its TMR ratio is only about 20%.Because this RMR depends on the spin dependent tunneling in MTJs,we define it as spin rectification magnetoresistance(SRMR).I-V curves under different magnetic field indicate the existence of rectification behavior and MR effect.Differential conductance spectrum is roughly parabolic and the minimum conductance locates at-11 mV,which proves that the rectification effect results from tunneling through asymmetric CoO-ZnO barrier according to BDR model.By numerical fitting,we could get the barrier thickness and barrier height on both sides.SRMR combines the rectification effect based on charge with the MR effect based on spin successfully,which will provide new methods to develop spintronics devices.?.Electrically tunable RMRin AI/Ge/In Schottky heterojunctionWe fabricated Al/Ge/In Schottky heterojunctions by magneton sputtering and RMR was observed.We applied AC and DC current simultaneous to the device,by changing the value of DC component,the RMR is tuned from-530%up to 32.500%.This new kind of electrical modulation technique could be adapt to other devices with RMR effect,and will promote the development of multi-functional devices.Conventional MR and RMR simultaneously exist in the Al/Ge/In Schottky heterojunction.When AC current and DC current are applied to the device,the value and sign of measured total voltage could be tuned by the AC component and DC component,which results in the high tunability of RMR.?.Realization of RMR by discrete rectification component and MR component.CoZnO magnetic semiconductor film is usded as the MR component.The rectification component is a commercial Schottky diode(1N5817).The RMR device was fabricated by combining the Schottky diode and CoZnO film in parallel.Morevoer,the observed RMR could be greatly tuned from-11300%to 13500%by applying DC current and AC current simultaneously to the device.This technique could be widely adapted to conventional MR effects and expand the application range of RMR.A quantitative theoretical model has been established,which well explained both the observed RMR and the electrical manipulation behavior.According to the theoretical model,we could design the RMR and optimize its performance by tuning the physical parameters of the MR component and rectification component.