In Situ Observation And Multi-field Regulation Of Magnetic Skyrmions In Nd₂Fe₁₄B Magnets

In recent years,skyrmions in the field of magnetism has caused a crazy wave of research,mainly due to its rich physical properties and application value.The skyrmions concept was first proposed by Tony Skyrme in 1962,a British particle physicist,and it is a solitary particle with topological protection.Until 2009,scientists at the Technical University of Munich,Germany,used small-angle neutron scattering to find a magnetic vortex structure in the helical magnet MnSi,which was confirmed to be magnetic skyrmions.Its magnetic structure is different from the traditional ferromagnet,antiferromagnet and trivial magnetic bubble,but a kind of magnetic bubble with special configuration and topological protection.At the same time,the magnetic skyrmions is also granular,which can be used as the basic storage unit of the storage,providing the possibility of racetrack memory.Its minimum size can be up to 3 nm,and in the same effective storage volume,the storage density will be improved by at least one to two orders of magnitude compared with the current traditional magnetic domain storage technology.Due to the special spin magnetic structure,it is easy to interact with the conduction electrons.And the driving current density required is five orders of magnitude smaller than that of the conventional magnetic domain.Therefore,the skyrmions-based memory has the advantages of small driving current,high density,and non-volatile.This paper focuses on the third generation rare-earth permanent magnet Nd₂Fe₁₄B.When the temperature decreases below the spin reorientation temperature,T_SR,the easy axis of Nd₂Fe₁₄B will deviate from the c-axis.In this work,the in-plane magnetic domain evolution process of Nd₂Fe₁₄B with temperature was observed in situ by Lorentz transmission electron microscope.Under the temperature of T_SR,as the temperature decreases,the magnetic domain expands and the shape of the magnetic domain also changes.And this change process is also fitted by micromagnetic simulation.At the same time,using the spin reorientation effect of Nd₂Fe₁₄B to regulate the temperature and magnetic field in samples with vertical anisotropy.During in-situ observation of magnetization,most of the reverse domains gradually shrink into stripe domains and suddenly disappear,eventually reaching saturation.Due to the increase of magnetocrystalline anisotropy,the saturation field increases with the decrease of temperature.At the temperature higher than T_SR,a magnetic bubble with a topological number of zero is formed.As a result of the spin reorientation,magnetic skyrmions are found at the temperature much lower than T_SR.When the temperature is below T_SR,adjustable anisotropy and saturation magnetization are the main reasons for the formation of magnetic skyrmions.On this basis,the intrinsic parameters of the magnet were adjusted by the elemental impurity method,and skyrmions with relatively high density was observed in the hybrid magnet.In addition,the chirality of domain wall can be changed by the direction of the magnetic field.And it has been proved theoretically that the ordinary magnetic bubbles can be regulated into the skyrmions if the shape anisotropy is combined.These results demonstrate the feasibility of generating skyrmions in ordinary rare earth permanent magnet materials.