Investigation On Magnetism And Magnetrotransport Properties Of Non-colinear Antiferromagnetic Mn₃X（X= Sn,Pt） Thin Films

Spintronics is a science that uses the spin degree of freedom of electrons to manipulate the electronic state and transport.It mainly focusing on the interaction between charge transport and spin state and magnetodynamics,is a new and attractive subject and technology.Magnetic information storage and logic devices based on spintronics have the advantages of low power consumption,high speed,anti-radiation and non-volatile,which are expected to become the key technologies to overcome the power bottleneck of the"end of Moore’s law".In recent years,they have attracted wide attention from academia and industry.Compared with ferromagnetic spintronics materials,antiferromagnetic spintronics materials have more advantages,such as no stray field,insensitive to disturbance,and higher precession frequency.These outstanding advantages and characteristics provide an important idea for the development of a new generation of ultra-fast and ultra-high density spin storage and logic devices.Since the net magnetic moment of antiferromagnetic materials is zero and the external magnetic field is not sensitive,the manipulation and detection of antiferromagnetic moment has become a research hotspot and difficulty in the field of spintronics.Due to the kagome structure of nonco-linear antiferromagnetic Mn₃X（X=Ge,Sn,Pt,Ir）in the momentum space has a non-zero Berry curvature,showing anomalous Hall effect,anomalous Nernst effect,spin Hall magnetoresistance effect and other novel physical phenomena,which can realize the detection and reading of magnetoelectric signals in antiferromagnetism,attracting widespread attention of scientific researchers.However,there are still many problems,such as the accurate modulation of the sub-lattice spin arrangement of the nonco-linear antiferromagnetic Mn₃X film,the intrinsic correlation between the anomalous Hall effect and magnetism,the antiferromagnetic phase transformation and the current-driven magnetization switching efficiency,which need further experiments and theories to clarify.In this paper,the films of Mn₃Sn and Mn₃Pt and the corresponding heterojunctions were prepared by magnetron sputtering,and their magnetic and magnetrotransport properties were studied in detail.The main conclusions are as follows:（1）Polycrystalline antiferromagnetic Mn₃Sn films with different thicknesses were prepared by magnetron sputtering method with different sputtering time.The structural results show that there is a small amount of ferromagnetic Mn₂Sn in the hexagonal antiferromagnetic Mn₃Sn;the weak ferromagnetic signal above 250 K comes from the tilt of the spin magnetic moment of Mn atom in antiferromagnetic Mn₃Sn,and the ferromagnetism of Mn₂Sn below 250 K leads to a strong ferromagnetic signal.The comparative analysis of room temperature Hall resistance curves and magnetism shows that the anomalous Hall effect of antiferromagnetic Mn₃Sn is independent of the weak ferromagnetism,which is derived from the non-zero Berry curvature in the momentum space.（2）Based on the basic magnetic properties and anomalous Hall effect of Mn₃Sn film,single-phase polycrystalline Mn₃Sn thin film was prepared by adding the Mn content in the target.The results show that the high defect concentration in the film can produce large room temperature ferromagnetism,and the maximum saturation magnetization reaches 36 mμ_B/Mn,while the minimum value of the anomalous Hall coercivity in the high-quality Mn₃Sn film is 380 Oe,which effectively reduces the swiching magnetic field.The result of electric field modulation shows that piezoelectric stress has a great influence on the non-zero Berry curvature in antiferromagnetic Mn₃Sn,resulting into the anomalous Hall effect tuned effectively by electric field.（3）Based on the study of Mn₃Sn film,single-phase polycrystalline Mn₃Sn/Pt heterojunction was prepared.Here,the spin current provided by heavy metal Pt,was injected into the antiferromagnetic Mn₃Sn layer to realize the current-driven magnetization switching.It was verified that this magnetization switching originates from the spin-orbit torque,and has little correlation with the Oersted field and the interface Rashaba effect.Single crystal Mn₃Sn/Pt heterojunction was prepared by optimizing experimental parameters and selecting crystal orientation.The critical switching current density was reduced to 10⁶ A/cm²,and the spin process of Mn atom in current-driven magnetization switching was clarified.（4）Based on the study of hexagonal antiferromagnetic Mn₃Sn,cubic antiferromagnetic Mn₃Pt films were further prepared by magnetron sputtering.The modulation of substrate stress on its structure,magnetism and anomalous Hall effect was studied.The Mn₃Pt film with rich-Mn has obvious room temperature ferromagnetic signal and anomalous Hall effect,and the room temperature hysteresis loop and anomalous Hall resistance curve with large coercivity are obtained.The substrate stress can effectively tune the magnetism and anomalous Hall effect of Mn₃Pt films.The variation trend of ferromagnetic signal derived from the spin tilt of Mn atoms and the anomalous Hall resistance derived from non-zero Berry curvature is same.The anomalous Hall conductance of up to 102（Ωcm）^-1 is achieved in the Mn₃Pt film with a thickness of 24 nm,which is the maximum Hall conductance obtained in the antiferromagnetic Mn₃Pt.In conclusion,we systematically studied the structure,magnetic properties,anomalous Hall effect and current-driven magnetization switching of nonco-linear antiferromagnetic Mn₃Sn and Mn₃Pt films.It is clarified that the anomalous Hall effect originates from the non-zero Berry curvature in the momentum space.The modulation of electric field and stress on the magnetic properties and anomalous Hall effect is verified;and the current-driven magnetization switching is realized.These results will provide important reference for realizing high density and low power antiferromagnetic spintronic devices.