The Study Of Spin Orbit Torque-related Effects In Ta/CoFeB/MgO Structures

Spin transfer torque-based Magnetoresistance random access memory?STT-MRAM?with nonvolatile,fast write speed and unlimited endurance is expected to be the next generation of low power general memory.Compared to STT-MRAM,spin orbit torque-based MRAM?SOT-MRAM?will replace STT-MRAM as the third generation of memory for its lower writing current density,faster writing speed,and better compatibility.In recent years,the advent of STT-MRAM commercial chips greatly promoted the research and application of this kind of magnetic storage devices,research on SOT related effects is of great significance to the future development of magnetic storage-related spintronic devices,and breaking the status in our country that magnetic memory chips are pure imported and related technologies are monopolized.The thesis focuses on SOT related effects in heavy mental/ferromagnetic/oxide insulator structures,and the research work includes following parts:?1?Magnetization switching is studied in Ta/CoFeB/MgO structure.The Ta/CoFeB/MgO films with perpendicular magnetic anisotropy are prepared by magnetron sputtering,and annealing is performed under different temperature conditions later.The anomalous hall effect of the sample Hall devices is measured by pulse current,and the related magnetization switching images are obtained.After the sample annealing,the magnetic field needed for complete magnetization switching is reduced.In particular,280 ? annealing samples can achieve complete magnetization switching without magnetic field.The annealing treatment reduces the DMI effect of the sample,so that the chirality of the domain wall is disappearing and the magnetization switch more easily.In addition,the film thickness nonuniformity can break the space inversion symmetry of samples and equivalent form an additional magnetic field,these two mechanisms may be the possible cause of these phenomenon.?2?The relationship between the longitudinal and transverse SOT effective field and the thickness of the Ta layer in the Ta/CoFeB/MgO structure is quantitatively studied.We prepare Ta/CoFeB/MgO samples with different Ta layer thickness and anneal some samples at 240 ?,then the first-order and second-order Hall voltages of the sample devices are measured by harmonic measurement method to extract HSL and HFL,finally we obtain the relationship between the SOT effective field and the thickness of the Ta layer.When the Ta layer is thin,HSL and HFL increase with the growing of the thickness of the Ta layer but decrease gradually when the Ta layer is thick.Compared with the as-deposited samples,the HSL and the HFL in the annealed samples both increase significantly.By XRD measurements and Ta layer resistivity measurements,we know that the Ta layer with different thicknesses and annealing conditions has different proportions of the a-Ta and ?-Ta,while the contribution of HSL and HFL mainly comes from the ?-Ta.?3?The relationship between the spin Hall magnetoresistance and the thickness of Pt layer in Ta/Pt/CoFeB/MgO structure is studied.The angular dependence of the sample resistance with the CoFeB magnetic moment indicates that the magnetoresistance of the structure satisfies the Spin Hall magnetoresistance mechanism;the relationship between the Spin Hall magnetoresistance of the Ta/Pt/CoFeB structure and the thickness of the Pt layer is obtained through theoretical derivation,and by fitting with the experimental data,we get the following parameters:?_Ta=-0.05??_Pt= 0.19??_Ta= 1.8 nm??_Pt= 4.5 nm.?4?The relationship between the unidirectional spin Hall magnetoresistance?USMR?and the thickness of the Pt layer in the Ta/Pt/CoFeB/MgO structure is investigated.The second-order longitudinal magnetoresistance signal(R_2?)of the sample is measured by the harmonic measurement method;by the angle-dependent symmetry analysis and the second-order Hall voltage measurement,we eliminate the contributions of the SOT effect and the anomalous Nernst effect and obtain the pure unidirectional spin Hall magnetoresistance single(R_2?USMR);by calculating the electrochemical potential distribution of electron spins in each layer of Ta/Pt/CoFeB,combined the relationship between R_2?USMR with the thickness of Pt layer,we know that the spin accumulation at the interface of heavy metal/ferromagnetic is the main source of USMR.