Spin-orbit Torque Induced Magnetization Switching And Domain Wall Motion In Perpendicularly Magnetized Heterostructures

Recently,the explosive growth of the data puts forward the higher requirements for data storage techniques.Therefore,researchers pay more and more attentions on data storage technology with high efficiency,low energy consumption,and large density.Comparing with the conventional data storage devices,the magnetic random-access memory?MRAM?based on spin-orbit torques?SOTs?with its low power consumption,high storage density,high efficiency,excellent stability,etc,stands out among the other types of MRAM and becomes one of the best candidates for the new generation of MRAM.The writing efficiency in SOT-MRAM depends on the amplitude of the bulk spin Hall effect?SHE?or the interfacial Rashba effect in heavy metal?HM?/ferromagnetic?FM?heterostructures,in which the charge current will be transferred into the pure spin current due to the strong spin-orbit coupling effect of HM.The absorption of the injected pure spin currents by FM will generate SOTs,which would be acted on the magnetic moments or domain wall?DW?to achieve the magnetization switching or DW motion.Meanwhile,as one of the important parameters of SOT devices for application,the thermal stability has been widely studied.How to achieve the effective and stable magnetization switching and the high thermal stability at the same time,as well as the efficient and controllable DW motion have not been clearly addressed and become a hot topic.In this thesis,we mainly focus on solving the above-mentioned problems,firstly,the HM/FM heterostructures with PMA have been prepared by magnetron sputtering.Then,the SOTs induced magnetization switching and the DW motion behavior have been studied by magnetotransport measurement technique and a polar magneto-optical Kerr microscope,respectively.The main results are as follows:?1?The perpendicularly magnetized samples with the structure of Pt/Co/Cr,in which the spin Hall angles of Pt and Cr having opposite signs,were prepared by magnetron sputtering.The PMA and SOTs are systematically studied as a function of the Cr thickness.At first,an enhanced perpendicular magnetic anisotropy?PMA?field around 10k Oe is observed and ascribed to the enhanced interfacial anisotropy between Co and Cr.Next,SOT-induced magnetization switching is achieved under relatively small critical current density in the order of 10⁶ A/cm² due to the enhanced SOTs,revealing a large effective spin Hall angle up to 0.27.Finally,the observed nearly linear dependence between spin Hall angle and the square of longitudinal resistivity from their temperature dependences suggests that the spin Hall effect mainly arises from the intrinsic mechanism in Pt/Co/Cr systems.?2?DW motion by the external magnetic field and SOTs in Pt/Co/Cr trilayers with PMA were studied using a polar magneto-optical Kerr microscope working in the differential mode at room temperature.We reveal the domain wall in Pt/Co/Cr trilayers belonging to the left-hand Néel-type domain wall,due to the inherent Dzyaloshinskii-Moriya interaction.We also demonstrate that the accompanying Joule heating effect also plays a key role for the magnitudes of the switching field,the propagation field of DW motion,and the DW motion velocity.Furthermore,we achieve the reversible electric-current-driven magnetization switching through SOTs-induced DW motion without any external magnetic field.Finally,five Boolean logic gates have been designed and achieved utilizing the SOT-induced DW motion.The results indicate a great significance for the practical applications based on the SOT-driven magnetic domain wall motion.?3?PMA,SOTs,DW motion velocity,and thermal stability are studied in perpendicularly magnetized ultrathin Co film sandwiched by heavy metal Pt and Ta with an interface decoration by inserting a 3d transition metal Cr layer as thin as 2 nm between Co/Ta interface.We find that the effective perpendicular magnetic anisotropy field?H_a⁰ _n?has been enhanced from⁶000 Oe to¹1560 Oe due to Cr interfacial decoration.Meanwhile,a large effective spin Hall angle(?_SH^eff)of Pt/Co/Cr/Ta around0.41 was obtained.The enhancement of?_SH^eff could be caused by employing the dissimilar metals Pt and Ta with opposite signs of spin Hall angle and an additional interfacial spin-orbit coupling effect related to Cr insertion.Thus,a spin-orbit torque induced magnetization switching is achieved under relatively small critical current density in the order of 10⁶ A/cm² due to the improved SOTs in Pt/Co/Cr/Ta system.Moreover,we studied the influence of depinning energy barrier on DW motion in the creep regime using the polar magneto-optical Kerr microscopy in the Cr layer decorated Pt/Co/Ta structures.We find that although the SOT efficiency shows a significant enhancement in the Pt/Co/Cr/Ta system,but the current-induced DW motion efficiency degrades.We confirm that this unexpected result is caused by the enhanced pinning energy barrier.We also reveal that the energy barrier depends on the interface between the ferromagnet and heavy metal.The results are useful for understanding the underlying physical mechanism of SOT-driven DW motion.?4?The field-free magnetization switching is achieved by inserting a wedged MgO layer to induce an asymmetric oxide content at the Ta/CoFeB interface in the perpendicularly magnetized Ta/CoFeB/MgO structure.Our results demonstrate that the field-free SOT switching is determined by a current-induced perpendicular effective field(H_z^eff)originated from the interfacial Rashba effect,due to the lateral structural symmetry-breaking introduced by a wedged oxide layer.Furthermore,we show that the sign and the magnitude of H_z^eff exhibit a significant dependence on the interfacial oxygen content,which can be controlled by the inserted oxide thickness.Our findings provide a deeper insight into the field-free SOT switching by the interfacial Rashba effect.