Mechanism,Manipulation And Application Of Advanced Spin-Orbit Torque Devices

With the rapid development of information technology,higher performance of information processing and memory devices are highly pursued.However,as the devices size continue to decrease,traditional semiconductor devices based on charge property of electrons are confronted with the challenges from thermal dissipation and quantum size effects.Thus semiconductor technology has entered the Post-Moore’s Law era.Since the discovery of the giant magnetoresistance effect,people have gradually recognized the spin properties of electrons and tried to deploy it to develop new generation of electronic devices,leading to the establishment and rapid development of a new discipline-spintronics.Different from the traditional charge-based electronic devices,spin-based electronic devices are non-volatile and effectively address the heat and energy challenges and therefore shed great prospects for the future information storage and logic computing.The spin current converted by the charge current through the spin-orbit coupling mechanism injects into the ferromagnetic layer and subsequently generates torque onto the magnetization,which is called the spin-orbit torque effect（SOT）.Spin-orbit torque can induce magnetization switching and implement information writing with the advantages of high write speed,high endurance,high integration density and low energy consumption,and therefore quickly becomes a hot topic of spintronics and a mainstream technology for the new generation of spintronic devices.However,the efficiency of the spin-orbit torque based on the conventional heavy metal/magnetic metal heterojunction still requires large improvement and the perpendicular magnetization switching process still needs an assist magnetic field to break the spatial symmetry of the magnetization.Hence,elevating the SOT efficiency and reducing the device energy consumption with new strategy and mechanism,eliminating the need of the external magnetic field to achieve the magnetization switching by purely electrical methods,and exploring the application of spintronic devices are the present important research goals of spin-orbit torque.This thesis studies the mechanism,manipulation and application of spin-orbit torque by using electric field,wedge structure,metallic Ru interlayer and interfacial two-dimensional electron gas.The main contents are introduced as follows.（1）We achieved the non-volatile modulation of spin transport with electric field in Li Fe₅O₈/Pt heterojunctions.Firstly,we fabricated Li Fe₅O₈/Pt heterojunctions on the PMN-PT single crystal substrate by pulsed laser deposition and magnetron sputtering,and measured the spin Hall angleθ_SH,spin diffusion lengthλ_sd and the real part of spin mixing conductivity G_r at various temperature by the spin Hall magnetoresistance.The experimental results demonstrate that the spin diffusion lengthλ_sd is independent of the temperature,but spin Hall angleθ_SHdecreases monotonically by decreasing temperature,which indicates the contribution of D’yakonov–Perel’mechanism in the spin relaxation process in Li Fe₅O₈/Pt heterostructures.Meanwhile,the spin Hall angle linearly depends on the resistivity of Pt layer,which confirms that the generation of spin current in heavy metal Pt layer is uniquely contributed by the side jump scattering and intrinsic mechanism.Finally,the spin Hall angle of heavy metal Pt layer can be enhanced by almost 28%by applying a 5 k V/cm electric field on the PMN-PT substrate,which can be attributed to the modulation of strain mechanism induced by ferroelastic domain switching of the PMN-PT substrate.The above results are helpful for people to gain comprehensive insights into the spin transport mechanism in heavy metal/ferromagnetic oxide heterojunctions and promote the development of spintronics devices with non-volatile electric field modulation.（2）We studied the SOT field-free switching and synaptic plasticity induced by the wedge-shaped Pt/Co/Pt heterojunctions.Based on the Pt/Co/Pt heterojunctions with perpendicular magnetic anisotropy,an additional spatial symmetry breaking along the thickness direction was introduced by a wedge-shaped top Pt layer,which has induced a novel out-of-plane polarized spin current and achieved SOT field-free switching.Meanwhile,the current induced nonvolatile multi-state magnetization switching characteristic in Pt/Co/Pt（wedge）heterojunction was exploited to successfully simulate the plasticity behavior of biological synapses,including excitatory and inhibitory postsynaptic potentials,long-term potentiation and depression by varying the number,amplitude,and width of pulsed current,as well as to realize a simple biological learning rule,i.e.,spiking-time-dependent plasticity.The above results indicate that wedge-shaped structure is also a feasible alternative to generate out-of-plane polarized spin current,in addition to interfacial asymmetry,spin processing at non-magnetic/ferromagnetic interfacial and magnetic structure,which also provides a reference for the practical development of prototype device of neuromorphic computing based on spin properties.（3）We studied the modulation of spin-orbit torque with metallic Ru interlayer in Pt/Co/Pt/Ru/Pt heterojunction.Based on the Pt/Co/Pt heterojunction with perpendicular magnetic anisotropy,we replaced the top Pt layer with a Pt/Ru/Pt trilayer.We found the damping-like and field-like effective fields and the magnetization switching efficiency oscillate with the thickness of Ru layer,and the maximum values of damping-like and field-like effective fields and the magnetization switching efficiency appear at the Ru layer thickness of 0.4 nm and 1.0 nm,where the SOT magnetization switching efficiency is enhanced by 47%.By changing the spin polarization direction accumulated on the sides of metal Ru layer and replacing the Ru layer with metallic Ag without strong interlayer coupling effect,as well as combining with the relevant theoretical calculations,it is found that the modulation of spin-orbit torque originates from a new physical mechanism of interlayer coupling,i.e.,the interlayer coupling effect between spin current.Not only provide a new strategy to modulate the SOT efficiency,these experimental results are also of great significance for further expansion of the research scope of the interlayer coupling effect.（4）We studied the spin-orbit torque in two-dimensional electron gas/ferromagnetic heterojunction.After bombarding the surface（100）-oriented single-crystal Sr Ti O₃ substrate with Ar ions,we have confirmed that a conducting layer of about 4.4 nm was formed on the surface of Sr Ti O₃ by XPS measurement,and it exhibits transport characteristics of two-dimensional electron gas,such as the metallic conductivity,weak antilocalization and anisotropic magnetoresistance and in-plane negative magnetoresistance etc.Furthermore,we have fabricated the perpendicular magnetized ferromagnet on the etched Sr Ti O₃ substrate,and quantitatively measured its SOT effective field and the SOT-induced magnetization switching.The experimental results demonstrate that the heterojunctions based on the two-dimensional electron gas exhibits giant SOT effective field at room temperature which is larger than the conventional heavy metals.More remarkably,the SOT effect in the heterojunctions with two-dimensional electron gas can be significantly modulated by the gate voltage.An extremely low critical switching current density of 4.3×10⁵ A/cm² with-100 V gate voltage has been demonstrated,which is lower by one or two order of magnitude than the conventional 5d heavy metal-based heterojunctions.The voltage modulation can be explained by the shift of Fermi surface of the two-dimensional electron gas under gate voltage,which induces a non-monotonic modulation of the Rashba spin splitting strength.The above results show great prospects of spin orbit torque effect by deploying two-dimensional electron gas,and promote the development and spintronics devices with ultra-low power consumption.