Bulk Spin-orbit Torque And Field-free Switching Of The Co/Pt Layer

With the development of technology,the emergence of big data,the internet,and artificial intelligence has created new demands for memory performance.Nowadays,Spin-Orbit Torque（SOT）Magnetic Random Access Memory（MRAM）devices that use electric currents to switch the magnetization of the free layer in adjacent heavy metal layers have shown great potential for efficient and reliable sub-nanosecond switching,low write energy,high durability,and the absence of external magnetic fields.Some implementation methods for SOT-MRAM devices include three-terminal structures with either perpendicular magnetic anisotropy（PMA）or in-plane magnetic anisotropy（IMA）.Compared to in-plane magnetic anisotropy,perpendicular magnetic anisotropy can achieve higher integration density and more reliable current switching,which is why researchers are increasingly focusing on exploring perpendicular magnetic anisotropy.Compared to the traditional Co/Pt and Co Fe alloy structures with relatively thick ferromagnetic layers,our experiment utilized relatively thin Co/Pt films with compositional gradients to achieve a different type of spin-orbit torque（SOT）known as bulk SOT.Bulk SOT is a type of SOT that originates from spin-orbit coupling within the magnetic layer itself,rather than from adjacent heavy metal layers.Currently,bulk SOT can be observed in materials with broken inversion symmetry,such as Fe₃Ge Te₂ or L1₀ phase Fe Pt.Compared to more classical SOT,bulk SOT does not require heavy metal layers,which simplifies device structure and manufacturing processes.It can be strengthened by increasing the thickness or chemical order of the magnetic layer,improving switching efficiency and reliability,and tuned by external magnetic or electric fields,providing more flexibility and functionality for device applications.However,the widespread application of bulk SOT is limited by its high synthesis difficulty or low stability.To overcome this limitation,we investigated the SOT-driven magnetization reversal process of bulk SOT with Co/Pt multilayer films with vertical anisotropy and implemented high proportion,non-magnetic field-assisted magnetization reversal on a silicon substrate through a tilted substrate.The specific research content is as follows:Firstly,we explored the basic structure of Co/Pt multilayers and I achieved efficient and thermally stable Co/Pt multilayers with good flip performance.Using magneto-optical Kerr effect and photoelectric sensor,we first regulated the magnetic anisotropy of Co/Pt multilayers.Finally,we determined the basic structure of Ta（0.6）/Pt（0.8）/Co（0.3）/Pt（0.6）/Co（0.5）/Pt（0.4）/Co（0.8）/Ta（0.6）.Secondly,we conducted control experiments on the basic structure,changing the composition and observing its effect on the film properties.We prepared Hall Bar-shaped devices using micro/nano-fabrication and tested their anomalous Hall resistance.We observed that the body SOT film had a large anomalous Hall resistance of about 4Ω,which is a significant improvement compared to traditional SOT devices.Several samples also exhibited good vertical magnetic anisotropy,indicating a comprehensive balance between their internal magnetic crystal anisotropy and demagnetization field under similar structural composition.The Ta（0.6）/Pt（0.4）/Co（0.8）/Pt（0.6）/Co（0.5）/Pt（0.8）/Co（0.3）/Ta（0.6）sample could achieve complete magnetization reversal under an auxiliary magnetic field of 50 Oe in the x-axis direction and had a small reversal current density of 16.94 10⁷A/cm².We further characterized its vertical magnetic anisotropy field size using a first-harmonic signal and fitted its SOT efficiency using a second-harmonic signal,which can reach a maximum ofβ_MAX=2.91（Oe/（10⁷A/cm²））.These studies deepened our understanding of the body effect SOT of Co/Pt multilayers.Finally,after discovering the excellent properties of body effect SOT of Co/Pt multilayers on silicon substrates,we proposed a scheme to achieve field-free assisted magnetization reversal by growing Co/Pt multilayers on vicinal substrates.We grew identical films on substrates with different angles and compared their anomalous Hall resistance,SOT-driven magnetization reversal,and harmonic signals.We found that the vicinal substrate had some influence on the magnetic domains of the sample.On the 5°vicinal silicon substrate,the sample achieved a high rate of 77%field-free assisted reversal.By using the body effect spin-orbit torque of Co/Pt multilayers,we achieved a large proportion of field-free assisted magnetization reversal on silicon substrates,which has important implications for future information storage and neural network computing.