Study On The Spin Transport Properties Of Synthetic Antiferromagnets

Synthetic antiferromagnet(SyAF),which contains two ferromagnetic layers separated by one nonmagnetic spacer layer,has been widely researched in spintronics due to its great advantages,such as stable magnetic structure,adjustable exchange coupling strength and good thermal stability.In recent years,spin-orbit torque(SOT)has drawn much attention since it can induce the magnetization switching with lower critical current density and realize nonvolatile memory.So,the SOT-based spintronic devices are expected to be the next generation data storage and processor.Therefore,efforts have been made to investigate the mechanism,improve the SOT efficiency and design new spintronic devices.The SOT-induced magnetization switching behaviors have also been reported in SyAF.However,these studies ignored the SOT contribution from the upper magnetic layers(UMLs).Meanwhile,it is still limited about the study on the SOT efficiency and thermoelectric effect in SyAF.Obviously,it’s significant to further investigate the SOT-related spin transport behaviors in SyAF to reveal the physical mechanism and develop new spintronics devices.This thesis focuses on the SyAF structure and studies the SOT effect from UMLs on the magnetization switching behaviors of SyAF.We separated the contributions of SOT and thermoelectric effect and clarified the impact of antiferromagnetic exchange coupling on the SOT efficiency.Basing on the SOT in SyAF,we promoted a new device to realize multi-state storage and Boolean logic functions in a single unit.This method brings a possibility to break the von Neumann bottleneck,physically simulate the neurons and promote the development of artificial intelligence.The main achievements of this thesis are as follows:(1)Study on the SOT-induced magnetization switching behaviors in SyAF.Firstly,it is found that the Ta buffer layer can greatly enhance the antiferromagnetic exchange coupling strength of SyAF.Then,the thicknesses of Ru layer and UML are demonstrated to have great influence on both the perpendicular magnetic anisotropy of SyAF and the chirality of the magnetization switching curves.This indicates that the SOT effect in UML has important impact on the magnetization switching behaviors of SyAF due to the opposite Dzyaloshinskii-Moriya interactions originated from the bottom and upper magnetic layers.Moreover,we observed the field-free SOT-induced magnetization switching behaviors in SyAF by optimizing the stacking structure.This can be attributed to the in-plane effective field which is derived from the magnetic moment canting induced by the interlayer exchange coupling.(2)Study on the physical mechanism of SyAF and the separation between the Hall resistance components of SOT and thermoelectric effect.By applying the harmonic Hall resistance measurements,an obvious enhanced second harmonic Hall resistance was observed under a large external magnetic field(Hext).This is because the exchange coupling field(Hexc)can partially counteract Hext.Considering the magnetization state in SyAF is determined by the competition between Hexc and Hext,we proposed the equations to well fit the second harmonic Hall resistance results.Accordingly,we can separate the Hall resistance components of SOT and thermoelectric effect,and calculated the effective spin Hall angle(θSH).The value of θSH of SyAF with antiferromagnetic exchange coupling is much larger than that of conventional nonmagnetic/ferromagnetic bilayers,especially is 7 times larger than that in the structure with ferromagnetic coupling.It is clarified that the thermoelectric effect in SyAF is also affected by the magnetization state.In SyAF,the SOT effective field and thermoelectric effect at the antiparallel state are both larger than that at the parallel state.(3)Investigation on the SyAF-based logic-in-memory in a single unit.The multi-resistance states were observed in both the anomalous Hall effect and SOT measurements by adjusting the thicknesses of the Co and Pt layers in multi-Pt/Co bilayers separated by Al2O3 spacer layer.Moreover,we clarified the interfacial and bulk contributions to the anomalous Hall voltage.Basing on the multi-resistance states,we proposed a concept logic-in-memory to realize the multi-state storage and brain-like-computation in a single unit.Moreover,five Boolean logic operations including NOR,OR,AND,NAND and NOT gates can be obtained by modulating the initializations through the preset current and magnetic field.Finally,we proposed an encrypted database in a SyAF-based single unit by controlling the external magnetic field.These results imply that the SyAF have broad prospects in the future logic-in-memory and spintronic devices.