| In recent years,with the rapid development of emerging information technology fields like Internet of Things(IoT),Artificial Intelligence(AI),Big Data,and Cloud Computing,the amount of data generated globally each year has shown explosive growth,and massive data puts forward higher and higher requirements for storage capacity and working speed.Among various new types of nonvolatile memory,magnetic random access memory(MRAM)has a very broad application prospect due to its advantages of high-speed reading and writing,low power consumption,high durability,and radiation resistance.Currently,spin-orbit torque(SOT)has been proven to be a very efficient MRAM writing method.Compared with the previous generation of spin-transfer torque MRAM(STT-MRAM),SOT-MRAM can realize the separation of reading and writing paths,and has lower power consumption,faster writing speed,higher storage density and longer storage life.Therefore,research related to SOT has aroused the interest of scientists all over the world.In the past decade or so,SOT has been widely used to induce magnetization switching,drive the motion of chiral Neel wall and magnetic Skyrmion,and excite high-frequency magnetic oscillation,etc.,opening the way for designing new memory and logic devices.In this thesis,we have conducted a series of explorations around the SOT-induced magnetization switching in the single-layer films with vertical composition gradient,synthetic antiferromagnets(SAF)and ferromagnet/nonmagnetic metal/ferrimagnet(FM/NM/FIM)heterostructures.The main contents are as follows:1.Study on SOT switching in ferromagnetic single-layer films with vertical composition gradientIn CoPt single-layer films with perpendicular magnetic anisotropy,we broke the central inversion symmetry of the crystal structure by constructing a composition gradient in the thickness direction of the CoPt alloy,and then achieved SOT-induced magnetization switching utilizing spin polarization generated by the bulk Rashba effect.The line scan results along the thickness direction using Energy Dispersive X-ray Spectroscopy(EDS)clearly show the existence of vertical composition gradient in our sample.We also found that the polarity(clockwise or counterclockwise)of the SOT switching curve will reverse with the reversal of the composition gradient,which directly proves the contribution of the composition gradient to the SOT switching.In addition,magnetic domain imaging during the SOT switching process and domain wall(DW)velocity measurements using the magneto-optical Kerr effect(MOKE)microscope indicate that the DW displacement switching mechanism plays a major role and there exists chiral Neel walls and DMI effective fields(|HDMI|≈155 Oe)in our samples.Furthermore,in Ru/IrMn/Co/Ru/CoPt/MgO heterostructure,we achieved field-free SOT switching of CoPt composition gradient film through the synergy of RKKY interlayer exchange coupling(IEC)and exchange bias effect,and realized reversible manipulation of the SOT switching curve polarity.This work shows that SOT switching can be achieved in ferromagnetic single-layer films without the need for additional heavy-metal spin injection layers,providing a new idea for the flexible design of spintronic devices.2.Study on field-free SOT switching of synthetic antiferromagnets(SAF)In magnetization compensated Ru/Pt/[Co/Pt/Co]/Ru/[Co/Pt]n/Co/[capping layer]/Ru SAF heterostructures,we achieved stable field-free SOT switching using the oblique sputtering method.Our research shows that under the same oblique sputtering conditions,the structure of the capping layer has a significant influence on SOT switching,and the intermediate polarity reversal field in our samples with different capping layers ranges from-200 Oe to+460 Oe.After comparison,we found that the SOT switching performance of Ta/Pt SAF is significantly better than that of Pt/Ta SAF,and compared to Pt/Ta SAF,the SOT switching curve of Ta/Pt SAF is more prone to appear three polarity reversals,and oblique sputtering can also cause significant asymmetry in the additional two polarity reversals.At the same time,there is a certain correlation between the number of polarity reversals(one or three times)of the SOT switching curve and the sign of the intermediate polarity reversal field,that is,one reversal corresponds to a positive sign,and three reversals correspond to a negative sign.We infer that the influence of the capping layer on SOT switching is mainly related to the manipulation of interface DMI and spin current,and a significant difference in the DMI field between the upper and lower interfaces in the SAF structure helps to achieve field-free SOT switching.This work is of great significance for the low-power and high integration design of SAF devices.3.Study on SOT switching in ferromagnet/nonmagnetic metal/ferrimagnet(FM/NM/FIM)heterostructuresIn the Ru/Pt/[Co/Pt/Co]/Ru(tRu)/CoxTb100-x/Ru perpendicular magnetic heterostructures,we successfully constructed four typical spin configurations by controlling long-range RKKY interlayer coupling and near-neighbor ferrimagnetic coupling of rare-earth transition-metal(RE-TM),and achieved SOT-induced magnetization switching in a wide temperature range.In particular,we simultaneously achieved the magnetization compensation and the anomalous Hall resistance enhancement in the[Co/Pt/Co]/Ru(1.3 nm)/Co65Tb35 heterostructure with macroscopic antiferromagnetic IEC,where the total magnetic moment of the Co65Tb35 layer is dominated by the Tb element.Further research shows that the IEC between the[Co/Pt/Co]layer and the CoxTb100-x layer is essentially determined by the Co3d-Ru5s-Co3 d IEC,while the Co3d-Ru5s-Tb4f IEC is almost negligible.Utilizing the above characteristics of FM/NM/FIM heterostructures,we achieved a complete fitting of the entire RKKY oscillation curve.In addition,we observed that the polarity reversal number(three or one)of the SOT switching curve is only related to the intrinsic IEC,but not to the macroscopic IEC.Finally,our temperature dependence measurements on the Co75Tb75 heterostructures show that the samples can exhibit stable SOT switching in the temperature range of 10 K-300 K,and especially the polarity of the SOT switching curve does not change as the temperature passes through the magnetization compensation point of Co75Tb75.The FM/NM/FIM heterostructure covers the disadvantage of the anomalous Hall effect cancellation in traditional SAF with magnetization compensation,and has important reference value for the design of new spintronic devices. |
PDF Full Text Request