| Over the past 50 years,thanks to the tremendous technological progress achieved in techniques for preparation of thin films and micro-nano processing,semiconductor microelectronics have consistently reduced the size of devices to ensure that their chip capacity continues to evolve as predicted by Moore's law.However,the reduction in device size is not infinite.Especially when the device's characteristic size is reduced to less than 10 nm,the problem of power consumption caused by quantum tunneling effect has become a physical bottleneck for the continuous development of semiconductor devices.Meanwhile,novel magnetoresistance effects such as the anisotropic magnetoresistance,giant magnetoresistance and tunnel magnetoresistance have been observed and been applied in the area of hard disk read head,magnetic random access memory and high sensitivity sensor devices.Moreover,the fast development of superlattice preparation and micro-nano processing technology and the in-depth understanding of the strongly correlated electronic systems have greatly accelerated the development of spintronics.Different from the traditional semiconductor microelectronics,spintronics takes both charge and spin degree of freedom into account.By virtue of the correlation effect between spin conduction and magnetism,spintronics introduces a new way towards high-density storage and low-power consumption devices.As an easily detectable variable in materials/devices,magnetoresistance(MR)can be used in the design of information storage and magnetic sensors.However,until 1988,the MR effect represented by the Hall effect and the anisotropic MR was not widely used because of their weak MR signal.In 1988,Fert found a MR ratio of nearly 17%at room temperature in Fe/Cr/Fe multilayer structure,which was far beyond other types of magnetoresistance,and thus it was named giant MR(GMR).The spin valve based on the GMR effect has been developed and used in the commercial design of the read-head of a hard disk,where the market was already worth millions of dollars a year.After that,the magnetic tunnel junction(MTJ)with AlOx and MgO as the insulating layer were found.Compared with GMR,the tunneling MR(TMR)has a higher MR ratio(604%at room temperature)and has been widely used in modern magnetic random access memory,magnetic sensor and magnetic reading head devices.In addition,research in magnetotransport of semiconductors is a field of ongoing interest.For example,an extraordinary MR effect(10000%)can be observed in those semiconductors with small enough impurity entrations owing to the quantization of the carrier motion by the magnetic field.Since the MR of non-magnetic materials has no hysteresis and the MR ratio increases approximately linearly with the magnetic field,it holds promising application aspect in the next-generation magnetic sensors.In addition,researchers are seeking for effective methods to control the MR effect.For example,voltage control of TMR can be achieved by combining the MTJ with the piezoelectric substrate.In summary,looking for new MR effects in both magnetic and non-magnetic systems,and exploring novel MR control methods are highly required to the development of multifunctional spintronic devices.In addition to the MR effect the control of magnetism and spin phenomena,which corresponds to switching between the basic "0" and "1" signals in information technology,has been intensely pursued during the past few decades.Recently,the spin-transfer torque(STT)and spin-orbit torque(SOT)based on spin angular momentum transfer instead of magnetic-field-based control have attracted increasing attention.However,the charge-spin conversion efficiency is small.To realize magnetization switching by STT or SOT,a high current density in the order of 106?107A/cm2 is needed.An alternative method to reduce the current density is by using the voltage assisted magnetization switching.For example,it is reported that the write current of MTJ is reduced significantly by combining the voltage control of magnetic anisotropy.So far,based on the electrical field induced variation of strain,carrier density,exchange coupling,orbital reconstruction,and electrochemical effects,it is shown that electric field is able to control the magnetic anisotropy,magnetic transformation temperature,magnetization switching,magnetoresistance,and exchange coupling in various materials,including ferromagnetic semiconductors,ferromagnetic metals,perovskite oxides,and two-dimensional(2D)materials.Therefore,voltage control of magnetism(VCM)not only advance the understanding of fundamental physics,but also has a very important application prospect in the preparation of miniaturized,high speed and low powe multifunctional devices.We have focused our research on the interfacial magnetoresistance effect and VCM.Main achievements of our work are given in the following:?.Electrically tunable rectification magnetoresistance in Al/Ge/In Schottky heterojunctionThe rectification magnetoresistance(RMR)has been observed in the Al/Ge/In Schottky heterojunctions with asymmetric barrier.In this work,we simultaneously applied DC and AC currents to the Al/Ge/In Schottky heterojunctions,and realized a remarkable control of RMR in the range of-530%to 32500%at room temperature by controlling the relative strength of the applied DC and AC components.This remarkable electrical manipulation of RMR is a result of the joint action from both magnetic field and rectification effect.On one hand,the magnetic field causes the shrinkage of the electron wave functions and thus narrows the energy band of impurity or interfacial state and increases it to a higher energy,causing the carriers to be more localized and resulting in a high resistivity.On the other hand,the total current waveform(AC+DC)can be moved up/down by changing the DC component,resulting the change of the detected rectification voltage.In addition,the anisotropy of the rectification magnetoresistance was also measured,larger RMR was always found when the applied magnetic field was perpendicular to the current flow.This unique manipulation method relying on the simultaneous application of AC and DC could be adapt to other similar heterojunctions with asymmetric barrier,thus providing an alternative method for obtaining extraordinary MR effect in experiment.?.Distinguishing interface MR and bulk MR through rectification of Schottky heterojunctionsThe scattering mechanism of interfacial electrons in magnetic Schottky heterojunction is difficult to be measured by conventional DC magnetoresistance.Here we utilize the interface-sensitive RMR measurement to separate the interfacial magnetoresistance of the magnetic Schottky heterojunction.First of all,we fabricated the Schottky heterojunction of In/GeOx/n-Ge and briefly introduced the method how to distinguish the interface MR and bulk MR through the rectification of Schottky heterojunction.By comparing the RMR effect in the case of Schottky contact and Ohmic contact,we proved the RMR is interface-sensitive.Then we adapt this method to the ferromagnetic heterojunctions with spin polarized transport behaviouss.At 100 K,the low field "butterfly" hysteresis loop is observed in the conventional DC magnetoresistance curve,sugesting that there exists the spin-dependent scattering signal.However,the RMR curve shows no sign of any hysteresis behavior,demonstrating that the spin scattering signal does not come from the Schottky interface.We further measured the anisotropic magnetoresistance of ferromagnetic layer Co,clear "butterfly" behavior is observed and the coercive fields are consistent with the DC magnetoresistance curve.Therefore,we conclude that the spin scattering characteristics in the magnetic Schottky heterojunction actually come from the ferromagnetic layer,rather than from the Co/GeOx/n-Ge interface.The application of interface-sensitive RMR in magnetic heterojunction such as MTJ not only helps to understand the interfacial transport properties,but also advances the development of interface-related spintronic devices.?.Electric-field control of perpendicular magnetic anisotropyand spin orbit torque switching in Pt/Co/CoO/Oxide structureWe have fabricated Pt/Co/CoO/Oxide/Pt heterojunction by using the photolithography and the Ar ion-beam etching techniques.In this structure,interfacial exchange coupling between ferromagnetic metal and antiferromagnetic oxides is used as an extra source to stabilize the PMA.The gate voltage(VG)was applied between the top and bottom Pt electrode,while the spin current generated by the bottom Pt can provide SOT on the Co layer.Under the application of electric field,both the exchange coupling and magnetic anisotropy could be tuned reversibility.When a negative VG is applied,O2-from oxide is forced into the metallic Co and leads to an increase of the CoO effective thickness and a reduction of the Co effective thickness.Above effective thickness variation enhances the exchange coupling between Co and CoO,which enhances the perpendicular magnetic anisotropy of Co.On the contrary,when a positive VG is applied,the decrease of CoO and the increase of Co effective thickness weaken both the perpendicular exchange coupling across the Co/CoO interface and the magnetic anisotropy of Co layer.In addition,a controllable SOT switching and the memristive behavior that arises from SOT-induced multilevel intermediate states are realized by using the electric field to manipulate the out-of-plane component of the Co magnetization.Tunable PMA and SOT switching makes heavy metal/ferromagnetic metal/antiferromagnetic oxide heterojunctions a promising candidate to future low-power and high-density spintronics devices. |
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