Micro-Characterization And Magnetic-Field Regulation Of Topological Magnetic Domain Structure In Rare-Earth Based Magnetic Materials

Localized nanoscopic topological magnetic domains exhibit topological stability against neighbor magnetization and external field disturbance.Due to their rich forms and easy to be motivated by low current,topological magnetic domains are expected to be a new-generation information unit.The long-period spin helical magnetic ordering plays a key role in the stabilization of topological magnetic domain.Exploring the existence of topological magnetic domains in rare-earth based magnets has great advantages:on one hand,most heavy rare-earth elements exhibit the intrinsic helical magnetic order within specific temperature range;on the other hand,the magnetocrystalline anisotropy energy competition between sublattices may induce spin reorientation transition,which contributes to stabilize the long-period spin helical magnetic order described above.Based on Lorentz transmission electron microscope and external field regulation such as temperature and magnetic,the dissertation does research on the magnetic domain structure ańd its evolution and explores the topological magnetic domain in the rare-earth based ferrimagnetic materials HoMn₆Sn₆,DyMn₆Sn₆ and Ho_0.95Tb_0.05Co₃,respectively.The main results as follows:1.Centrosymmetric RMn₆Sn₆（R=Ho,Dy）:（1）By in-situ observation of the magnetic domain structure and its evolution during cooling,the dissertation systematically investigates the spin reorientation transition in RMn₆Sn₆（R=Ho,Dy）,where HoMn₆Sn₆ and DyMn₆Sn₆ undergo a magnetization transition from in-plane helical magnetization to easy conical magnetization near T_SR=195 K and 277 K,respectively.In the ac plane of RMn₆Sn₆（R=Ho,Dy）,dense stripe domains with the propagation direction along the c-axis are observed above the T_SR corresponding to inplane helical magnetization,and new c-axis domains occur accompanied by the preexisting stripe domains broken during cooling to below the T_SR ascribed to the deviation of easy magnetization direction from the ab-plane into cone state.（2）By the external magnetic field regulation,the dissertation achieves nonvolatile biskyrmions at room temperature in DyMn₆Sn₆.Stripe domains with the period length 260 nm are observed at T=283 K and transform into biskyrmions by applying the perpendicular magnetic field B⊥=0.21 T to the stripe domains in the ac plane of DyMn₆Sn₆.2.Centrosymmetric Ho_0.95Tb_0.05Co₃:（1）The dissertation systematically investigates the magnetic domain structure and its evolution during cooling and warming along the[210]zone-axis.A series of domain evolution occurs during cooling,where the trivial bubbles are observed at T=265 K and transform into dense stripe domains at T=262 K with annihilating as cooling.The magnetic domain evolution during warming is the reverse process corresponding to cooling,whereas its whole evolution temperature range is increased about 20 K and the trivial bubbles sustain from T=279 to 285 K.（2）By field-cooling regulation,high-density biskyrmions are observed at T=184 K along the[210]zone-axis.However,the energy barrier formed by the field-cooling parameters is not enough to maintain the high-density biskyrmions stably exist in the wide temperature region and the existence temperature region is only 14 K.（3）After field-cooling regulation,the magnetic domains observed along the[210]zone-axis have more out-of-plane magnetization component.I-type magnetic bubbles occur spontaneously within the temperature range from 268 to 278 K.