The Magnetoelectric Transport Properties At YIG/Cu Interface And In Ferromagnets

As the development of computer technique,the chips of demond are on the rise.In the past many years,with the reduction of the device size,Moore's law is going to meet the insuperable obstacle because of the quantum confinement effect.We all know that electron has both of charge and spin.In the microelectronics,only the charge degree of freedom of electrons was utilized.However,in spintronics,both of the charge and spin degrees of freedom of electrons are utilited,which introduces a new mode for information technology.The found of giant magnetoresistive effect in 1988 is remakable.Since then,the tunnel magnetoresistance effect,spin-transfer torque effect and spin Hall effect have been found.Compared to microelectronic device,the spintronic device has many advantages.Nowadays,a great many spintronic devices have been developed,such as magnetic sensors and magnetic head in hard disk based on GMR or TMR..There has been a lot of interest in the generation,transport,regulation and detection of spin current,especially in the potential applications of spin current.In spintronic devices,spin information transport plays an important role,such as magnetic memory and spin logic.Traditional spin information transport depends on the conduction electrons in metals,which is always accompaniwed by a charge current.In this case,the Joule heating will be induced.As quasiparticles of low excited states of ferromagnets(FM),each magnon carries a spin angular momentum.There is no net charge current during the pure magnon spin information propagation in ferromagnetic insulators(FI),which is a desirable properties for devices with dramatically reduced power consumption.The generation and detection of spin current are also important for function implementation of spintronic devices.In recent years,many methods of generating pure spin current have been found,such as SHE,circularly polarized light injection,spin Seebeck effect and spin pumping effect.The spin detection is most often achieved via the inverse spin Hall effect(ISHE).Exploration of a material with large spin Hall angle is a vital problem.The normal metal(NM)with strong spin-orbit coupling,such as Ta,Pt and W,is widely used to generation and detection of pure spin current on the research of spintronics.The interconversion between charge and spin can also be realized via anomalous Hall effect(AHE)or ISHE in FMs.In a FM,the electrons are polarized with an unequal number of electrons with opposite spin,the scattering strength to an electron is related to the relative orientation between electron spin and FM moment.Hence the situation in FM is more complicated,and the effect based on pure spin current is worth of further exploration.In addition,Rashba effect due to the structural inversion asymmetry at interfaces can also be used for efficient charge-spin interconversion.Large Rashba spin-orbit couplings(SOC)are generally found with heavy elements of large bulk SOC,such as Bi,Pb,W and so on.Recently,it was theoretically predicted that the Rashba effect also exist for a NM without bulk SOC covered by an oxide.Exploration on the other types of interfaces with large Rashba SOC is of great significance in promoting the spintronics development and its commercial applications.Based on the current research of spin current,the main work of this thesis includes the following aspects:1.The study of Rashba spin-orbit coupling at YIG/Cu interface in Pt/YIG/Cu multilayer sampleIn order to study Rashba SOC at YIG/Cu interface,we prepared Pt/YIG/Cu multilayer sample by magneton sputtering and metallic shadow mask.With the directions of charge current injection and nonlocal voltage measurement perpendicular to each other,the nonlocal voltage induced by the Rashba SOC has distinctly different magnetization direction dependence with that induced by SSE,thus allowing us to unambiously separate them.When the charge current is injected in the Cu(Pt)layer,we observed the voltage signal in the Pt(Cu)via the magnon-mediated electric current drag.The magnon-drag voltage vanishes with an ultrathin Al thin film inserted between YIG and Cu layer,which indicates that the magnon-drag voltage may originate from the inverse spin galvanic effect(spin galvanic effect)due to the Rashba SOC at the YIG/Cu interface.Besides,for the Pt/YIG/Cu samples with different thicknesses of Cu,we find that the magnon-drag voltages measured in Pt layer are unrelated with the thicknesses of Cu when we inject charge currents with costant density in the Cu layer.Therefore,we can unambiguously ascribe magnon-drag voltage to the Rashba SOC at the YIG/Cu interface.This is the first observation of voltage signal induced by the Rashba SOC at the YIG/Cu interface without any decoration.2.Detection of spin accumulation induced by anomalous Hall effect in Pt/YIG/Cu/FM multilayer samplesIn order to detect the spin accumulation induced by AHE,we prepared the Pt/YIG/Cu/FM multilayer sample by magneton sputtering and metallic shadow mask.In this sample,we inject the charge current in the FM layer and detect the magnon-drag voltage in the Pt layer.Compared with that induced by the AHE in FM layer,the magnon-drag voltage induced by Rashba SOC at YIG/Cu interface is so weak that can be neglected.Field-dependent accumulation induced by the AHE has been observed by comparison of magnon-drag voltages between Pt/YIG/Cu/Ni and Pt/YIG/Pt samples.With both magnon-drag voltage measurements in the Pt/YIG/Cu/FM(Ni?Fe and NiFe)multilayer samples and anomalous Hall voltage measurements in the monolayer FM(Ni?Fe and NiFe)samples,we demonstrate that the anomalous Hall voltage strongly depends on the spin polarization of conductivity in FMs.Our results show a prospect for FMs to be field-control spin generators via AHE,and provide a new viewpoint to realize the AHE.3.Abnormal anomalous Hall effect in SiO2/NiFe/SiO2 sampleIt is found that sign of anomalous Hall resistivity changes from negative to positive with increment in NiFe thicknesses in SiO2/NiFe/SiO2 samples.Through the AHE measurements,we demonstrate that this abnormal behavior originates from the interface of NiFe/SiO2 or SiO2/NiFe rather from the structural inversion asymmetry.Through the nonlocal measurements in Pt/YIG/Cu/NiFe sanples,it is found,however,the electrons with same spin for interfacial NiFe near the SiO2 layer and bulk NiFe are scattered to the same direction.Hence we can deduce that the signs of spin polarization of the conductivity for interfacial NiFe and bulk NiFe should be opposite to each other,which is responsible for the abnormal AHE observed.Our works demonstrate that oxide layer can drive the change of sign of AHE of NiFe,which provide an alternative way of manipulating the spin-related transport.4.The spin valve effect based on pure spin current in YIG/Cu/NiFe/IrMn multilayer sampleWe prepared the Pt/YIG/FM multilayer sample by magneton sputtering and metallic shadow mask.By using IrMn/NiFe exchange bias structure,the relative orientations of the YIG magnetization and NiFe magnetization can be switched between parallel and antiparallel by an applied magnetic field.In this structure,the detected amplitude of spin current emitted by spin pumping depends on the NiFe magnetic states(saturation or unsaturation).Compared to that when NiFe is magnetically saturated,a distinctly smaller voltage amplitude is observed when NiFe is magnetically unsaturated,which indicates a sizable spin valve effect based on pure spin current.The behavior may originate from the difference of spin absorption when the NiFe layer is magnetically saturated or unsaturated.Moreover,the reliability of the spin valve is demonstrated by a reliably different and reproducible voltage levels.