Study Of The Novel Topological Domains In Rare-earth Based Magnets

With the first experimental discovery of magnetic skyrmions in Mn Si,various types of topological magnetic domains,such as meron,biskyrmions,bobber,etc.,have been found in experiments and are expected to be the information bits in the new generation of memory devices due to their advantages including small size,low energy consumption and high stability.The magnetic skyrmions are firstly found in crystals with broken symmetry and only in a narrow region below room temperature with the requirement of magnetic field.In recent years,many breakthroughs have been made in the exploration of topological domains in thin film and centrosymmetric materials.The topological domains with wide temperature range near room temperature have been found,which provides a new version for the future practical application.Based on the former systematic study in our group,it has been found that the canted magnetic order with rotated tilt angle due to magnetic interactions competition is the direct reason for the generation of topological domains,where the spin reorientation transition（SRT）between in-plane and out-of-plane plays an important role.Therefore,this thesis focuses on the key role of SRT in the rare-earth magnetic materials.The formation,transformation and manipulation of topological domain structures in amorphous ferromagnetic thin film Gd Fe Co and Kagome magnet Tb Mn₆Sn₆ are systematically investigated under Focused Ion Beam and Lorentz transmission electron microscope（L-TEM）.The magnetic simulation further reveals the intrinsic physical mechanism.The dominant results are as follows:Formation and transformation of domain wall meron pairs and skyrmions have been designed and realized in amorphous Gd Fe Co film with different SRT temperature by tuning the anisotropy transition between in-plane and out-of-plane through temperature.Amorphous Gd Fe Co films with different SRT temperature regions were prepared by adjusting the ratio proportion of rare-earth Gd and transitional metal Fe Co.The domain wall meron pairs were found in all these samples with different components under real-space observation by L-TEM and the reversible transformation between meron pairs and skyrmions in the domain wall has been achieved firstly in experiment during SRT without the external magnetic field.Based on the parameters measured in experiment,OOMMF was used to simulate the transition of topological domains with temperature,revealing the important role of SRT in the whole process.The micromagnetic simulation demonstrated the response of topological domains along the domain walls under electrical current but only creep motion of skyrmions along the domain walls was observed due to the significant pining effects in experiment,which is expected to overcome the deflection induced by the topological Hall effect.We also found the SRT process could be conveniently induced by higher amplitude of current.The RKKY-mediated Topological domains have been found in Kagome magnet Tb Mn₆Sn₆ and correlation between real-space topology and electronic topology are established via Hall resistance measurement.The ac-plane TEM samples of Tb Mn₆Sn₆were prepared by focused ion beam（FIB）.The temperature-induced SRT domain change was observed under L-TEM with magnetic anisotropy from c-axis to the ab-plane during heating,which is consistent with the magnetic measurement results.The long-period helix with modulation vector along the c-axis occurred during SRT,whose period is about 100 nm within the temperature range from 310 K to 400 K.Then a biskyrmion lattice was realized while applying the perpendicular magnetic field.The skyrmion lattice gradually collapses and disappear into saturated state with increasing the magnetic field.By performing the field-cooling manipulation,the temperature range could be extended to near room temperature 300 K in contrast to the equilibrium range of 310 K to 330 K,which broadens the application possibility of topological domains near room temperature.The mechanism where RKKY stabilizes skyrmions in frustrated rare-earth magnet Tb Mn₆Sn₆ was proposed and the generation of biskyrmions was successfully simulated by constructing competing exchange model in OOMMF.The field-induced phase transition of the domain structures was further simulated and the results agreed well with the experimental ones.Finally,ac-plane samples for Hall resistivity measurements were prepared by FIB and corresponding topological Hall signals were obtained in the phase region where biskyrmions exists,suggesting the possible coupling and interaction between magnetic topology and electrical topology.The generation and manipulation of topological domains induced by SRT domain boundary in the ab-plane of Kagome magnet Tb Mn₆Sn₆.The SRT in the ab-plane of Tb Mn₆Sn₆ was achieved by conveniently converging and diverging the electron beam in L-TEM due to the and typeⅠand typeⅡbubble lattices were obtained after further applying the perpendicular field,revealing the critical role of domain boundary transition in SRT.Furthermore,the field-free bubble chains and individual bubbles at room temperature were obtained in the geometry-constrained samples due to the extrusion of the boundary.The straight motion of bubble chain and isolated bubble was realized by utilizing the shift of domain boundary across the SRT expansion.Then above processes were successfully simulated in OOMMF.It was revealed that the generation and motion of topological domains were contributed from the switching boundary between in-plane and out-of-plane anisotropy,which proves the dominant role of SRT.