Interface And Antiferromagnetic Spin Transport

Spintronics is the forefront field of magnetics.It studies the effect of electron spin on the macroscopic properties of matter,which has attracted widespread attention in academia and industry.Academically,electron spin is one of the basic properties of matter.The study of electron spin is conducive to the in-depth study of the microscopic nature of matter.Industrially,spintronics can design high-speed and low-power logic and memory devices through the manipulation and detection of spin.In recent years,with the cross application of spintronics and other fields,it has shown vigorous vitality and development prospects.The conversion of spin current and charge current plays a very important role in spintronics.By the conversion of current and spin current,the manipulation and detection of spin become simple.At present,there are mainly two methods: spin Hall effect and interface Rashba effect.They are widely used in various studies of spintronics,such as spin Hall magnetoresistance,spin pumping effect,spin Seebeck effect,magnon transport,terahertz emission,and so on.These effects usually occur in the heterojunction of magnetic materials and non-magnetic metals?NM?.Non-magnetic materials are generally heavy metals used to achieve the conversion of spin current and current,and magnetic materials are used to achieve the generation,manipulation,and propagation of spin current.In this thesis,the ferromagnetic insulator?FMI?YIG(Y₃Fe₅O₁₂),the antiferromagnetic insulator NiO,and the ferromagnetic metal permalloy(Fe₂₀Ni₈₀,Py)were selected as the magnetic materials.The non-magnetic material is mainly heavy metal Pt.Insulator materials were grown using pulsed laser deposition?PLD?,and metal materials were grown using magnon sputtering.After assembled into heterojunctions,the effects of spin current and spin accumulation were investigated.The main research contents of this thesis include:1.The spin Hall magnetoresistance found in the FMI/NM bilayer is caused by the spin Hall effect of non-magnetic layer,which requires the non-magnetic layer to be a thick heavy metal.In fact,when a Rashba spin-orbit coupling exists at FMI/NM interface,a similar new type of magnetoresistance effect exists,which we call as spin-orbit magnetoresistance?SOMR?.We chose Cu/YIG system to study the magnetoresistance caused by interface.By enhancing the Rashba spin orbit coupling at the Cu/YIG interface with an ultra-thin Pt layer,SOMR was first observed in the Cu/YIG system.Through a series of comparative experiments and theoretical analysis,we prove that this magnetoresistance is indeed caused by the interface Rashba.2.Anisotropic magnetoresistance?AMR?exists in ferromagnetic metals.If the Rashba effect exists on the ferromagnetic metal interface,its magnetoresistance will also be affected,which is called anisotropic interface magnetoresistance?AIMR?.We chose Py/?Pt/Al₂O₃ sample to study the influence of the interface modification on the AIMR.At the same time,the spin torque ferromagnetic resonance?ST-FMR?measurements and analysis interface were performed on the Py/?Pt/Al₂O₃ sample.The spin transfer torque caused by Rashba effect can be got,which reflects the interface Rashba coupling strength.The results show that there is a proportional relationship between AIMR and Rashba coupling strength.In addition,the change in magnetic resistance also indicates that the AMR of ferromagnetic metals can be modulated quantitatively through the interface modification.3.Magnon is an important form of spin angular momentum propagated in insulators,and magnon transport in antiferromagnetic materials has unique properties.We selected the antiferromagnetic insulator NiO and assembled it into Pt/NiO/Pt trilayers to study the nonlocal transport of magnons on the vertical direction. Through nonlocal harmonic technology,we have obtained the characteristics of magnon propagation in NiO: controlling the Néel vector with a large magnetic field to affect the antiferromagnetic magnon transport efficiency.The first-order signal caused by the electric injection spin current is mainly related to the Ne?el vector.When spin polarization is parallel to the Néel vector,the transmission efficiency of the magnon is the highest.Because there is no net magnetic moment in NiO,there are almost no second-order signals caused by thermal spin Seebeck effect.4.Terahertz emission in ferromagnetic metal/heavy metal structures is a remarkable discovery in ultrafast spin dynamics.Neither traditional electrical excitation nor thermal excitation can effectively generate spin current in AFM without applying a large magnetic field.In this study,we report the THz emission caused by photoexcited ultrafast spin currents in an antiferromagnetic/heavy metal heterostructure NiO/Pt for the first time.Our experimental results show that THz emission is closely related to the azimuth of the sample,laser polarization,and NiO crystal orientation.This dependence reveals the close relationship between spin dynamics and optical processes in NiO.The second-order nonlinear differential frequency excitation?DFG?when the laser illuminates NiO induces instantaneous magnetic moment in the antiferromagnetic NiO,and the excited magnons transfer spin angular momentum to adjacent non-magnetic layer,that is spin current,which is converted into an instantaneous current by the inverse spin Hall effect in the Pt layer,and then THz emission is detected.