The Study On The Generation And Manipulation Of Skyrmion In Pt/Co/Ta Multilayers

Skyrmion is non-trivial topologically protected spin textures.Their stabilization and dynamic behavior strongly depend on their topological properties.The size of skyrmions can be smaller to several nanometer,with diameters in the wide ranges in dfferent materials.Skyrmion can be regarded as a quasi-particle with topological charge,and can be created,annihilated and driven by applied external fields.Skyrmions also can be manipulated by electric currents with current density being five to six orders of magnitude smaller compared to traditional magnetic domain.These features make it possible as an information storage unit suitable for ‘abacus’-type applications in information storage and logic devices,such as hard disks,racetrack memory.It is generally believed that chiral skyrmions are induced by DMI in non-centrosymmetric system or in multilayers with broken inversion symmetry.The predomint skyrmions in the non-centrosymmetric magnetic alloys or epitaxial thin films can only be stable below room temperature with a narrow temperature range and under applied magnetic fields,which is not conducive to application.Recently,intense research studies have found that the observed skyrmions can be stable at room temperature in the multilayers with interfacial broken symmetry and can be driven by electric current.At present,room-temperature skyrmions mainly focuse on the multilayers with perpendicular magnetic anisotropy.The easily-tuned parameters of the multilayers,the good compatibility with the device and the advantages of the low-current driven behavior prompt the skyrmion application.But the density of generated skyrmion is low under the applied magnetic fields in the multilayers with PMA.How to obtain stable and tunable room temperature magnetic skyrmions are still the current research challenges.For designing and developing a suitable material system,the systematic research on the generation and manipulation of skyrmions are the key to solve the current problems.For the multilayers with perpendicular magnetic anisotropy,the DMI resulting from the lack of symmetry in the interface is critical to generate stable chiral skyrmions.In fact,the competition of other energies in multilayers system such as Heisenberg exchange interaction energy,perpendicular magnetic anisotropy energy and external field energy(such as Zeeman energy introduced by external magnetic field)is also very important for the generation and manipulation of skyrmions.Therefore,systematic studying the influence of material parameters on skyrmions will guide significance for achieving tunable room temperature skyrmions.The room temperature skyrmions in multilayers have potentials to become "revolutionary" magnetic storage units.It’s expected to achieve high density,low energy consumption for information storage.In this paper,the relationship between perpendicular magnetic anisotropy,external fields(electric and magnetic field)and the generation of skyrmions in Pt/Co/Ta multilayers are systematically studied via Lorentz transmission electron microscopy with the combination of magnetic properties and magnetic transport measurement techniques.High-density skyrmions are observed for the first time in an in-plane magnetic anisotropic multilayers;The relationship between the evolution of skyrmions and topological resistivity is established from the microscopic magnetic structure view;Through the synergistic manipulation of electric current and magnetic fields,the density of skyrmion could be adjusted in the perpendicular anisotropic multilayers and the non-volatile high-density skyrmions at zero field is obtained under the optimaized manipulation conditions.Main achievements are summarized as follows:1、The Pt/Co/Ta multilayer system is designed and grown.The relationship between the perpendicular magnetic anisotropy(PMA)and the formation of skyrmions(~ 50 nm)is systematically investigated.Multilayers design ideas: a、Choosing the strong interface DMI Pt/Co interface and the asymmetric Pt/Co/Ta sandwich structure to enhance the net interface DMI to prompt the chiral skyrmion generation.b、The magnetic moment of multilayer system can be continuously engineered from " perpendicular film " that magnetic moment perpendicular to the film surface to "in-plane film" that magnetic moment parallel to the film surface by tuneing Co layer thickness.Therefore,the multilayers with different PMA can be easily obtained only by growing Co layers with different thicknesses,and the growing technique for Pt/Co/Ta multilayer is easy to implement and reliable.The quantitative relationship between the PMA and the Co layer thickness is determined by the hysteresis loop measurement,corresponding with the microscopic magnetic domain evolution observed by Lorentz transmission electron microscopy.It is found that the skyrmions exhibit smooth transition with density increase and spin configuration evolution when crossing to the “in-plane film” with in-plane anisotropy from "vertical film" by simply increasing the Co layer thickness.For the first time,high-density skyrmions are found in the multilayers with in-plane anisotropy,which might be due to the smaller skyrmions generation barrier in the multilayers with in-plane anisotropy,more conducive to the production of skyrmions.2、 The relationship between the topological Hall resistivity and the skyrmion evolution under external magnetic fields is established by using the Lorentz TEM and magnetic transport measurements.The significant topological Hall resistivity peaks are observed from the topological Hall resistivity curves in both perpendicular anisotropy and in-plane anisotropic films,further confirming the non-trivial topological properties of skyrmions in this multilayers system.With the increase of the thickness of Co layer,the topological Hall resistivity peak broadens with applied magnetic field and the peak is asymmetry during increasing/decreasing magnetic fields,corresponding well with the microscopic magnetic skyrmion evolution observed by Lorentz transmission electron microscopy.Then the corresponding relationship between the topological Hall resistivity and the microstructure evolution of skyrmions under external magnetic field is established.The discovery of high density skyrmions,the apparent topological Hall resistivity(110-130 nΩ?cm)and the wide temperature stability(12-300K)in the in-plane anisotropic multilayers provides new possibilities for exploring new magnetic roomtemperature skyrmion multilayer materials,and provides important research data for further experimental and theoretical studies to promote the application of skyrmions devices.3、On the basis of the above work,the generation and manipulation of skyrmions in Pt/Co(1.85)/Ta multilayers with PMA is studied by means of electric and magnetic coordinated control via in-situ TEM electrical TEM holder in Lorentz TEM.Experiments show that the skyrmion density is tunable and can be significantly enhanced.With a suitable pre-applied fixed magnetic field,the electric current can generate high-density skyrmions.These generated skyrmions after the optimized manipulation sustain at zero field with both the in-plane current and perpendicular magnetic field switched off.The generation of high-density,zero-field stable skyrmions involves the dynamic effect of spinHall moment on the stripe domain dynamics due to the electromagnetic interaction,pinning effect of structural defects and energy barrier changes of magnetic phase transition process.This result not only opens up an alternative way to generate skyrmions but also provides key data for further theoretical simulations to reveal the nature of the interaction between current and different spin configurations.