Fabrication And Magnetic Properties Of Helimagnetic Nanostructures

Chiral helimagnets,such as MnSi,Cu2OSeO3 and FeGe,which exhibit diverse physical properties,have always been the frontier and hotspot in magnetism field.Particularly,various novel magnetic structures have been found in helimagnetsin recent years,such as magnetic soliton,magnetic skyrmions.These novel magnetic structures possess excellent properties,such as small size,high stability,manipulability by different stimulus and so on,which makes these novel magnetic structures as a candidate to be data carrier to construct high-performance spintronic devices.The fabrication,characterization and physical properties manipulations of helimagnet nanostructure are prerequisites and foundations for spintronic devices construction.Focusing on the issues,a micro-nano structure processing method based on focused ion beam milling has been developed;the merging of high temperature skyrmion phase and low temperature skyrmion phase in Cu2OSeO3 nanoplate has been achieved through thickness control;an effective electrical detection configuration of current and magnetic field for skyrmion detection has been proposed;a unified theory concerning magnetoelastic effects for B20 compounds has been built and verified;a new chiral soliton material MnNb3S6 has been found in experiment.These results provide the physical foundation and principle support for spintronic device construction.The main results of this thesis are as follows:(1)A micro-nano fabrication method for micro-nano structure preparation is developed.Nanostructures with desired size and shape can be prepared directly from bulk samples.Using this method,the transport test device,thin plates with large area and wedged plate with uniformly reduced thickness are successfully fabricated,which lay the crucial foundation for the following research.(2)A Cu2OSeO3 thin plate used for Resonant Elastic soft X-ray Scattering(REXS)experiment was prepared,and the detection of High Temperature Skyrmion(HTS)phase and Low Temperature Skyrmion(LTS)in Cu2OSeO3 thin plate with magnetic field parallel to[100]crystal orientation has been achieved.The HTS phase appears near TC,and the LTS phase appears in a mixed state,which is the mixing of LTS phase and helimagnetic state,at low temperature.The existing area of mixed state enlarges with decreasing temperature.At around 32K,HTS phase continuously transforms into LTS phase accompanied by diffusion of diffraction spot and increase of magnetic period.This experiment illustrates that reduced thickness cause the merging of HTS phase and LTS phase in Cu2OSeO3 thin plate.(3)The magnetostriction of skyrmion phase in MnSi has been detected using the capacitance method.When magnetic field B along[001]crystal orientation and magnetostriction measurement △L/L along[001],the existing area of skyrmion phase is the same as B‖[001]and ΔL/L‖[110]crystal orientation.The existing area of skyrmion phase is smaller than the first two configuration when B‖ΔL/L‖[110]crystal orientation.Meanwhile,a theory concerning magnetostriction effect for B20 compound has been improved and verified,and the calculated results successfully explained experimental results.This experiment illustrates that the measurement of magnetostriction effect is an effective way for skyrmions detection.(4)MnSi thin plate suitable for magnetoresistance measurement was prepared,and the detection of skyrmion phasein MnSi thin plate by magnetoresistance measurement was achieved.When magnetic field B‖[001]crystal orientation and current direction I‖[110]crystal orientation,there is no signal hint of the existence of skyrmion phase.When B‖I‖[110]or B‖[001]and I‖[110],the magnetore-sistance signal of skyrmion phase appears.Our work give the conductive geometry for skyrmion phases detection.(5)MnNb3S6 thin plate with large area suitable for REXS test are successfully prepared.Using REXS measurement and extremely low temperature Lorentz microscopy(LTEM),MnNb3S6 is firstly confirmed to be a new chiral soliton material in experiment.The period of helimagnetic in bulk MnNb3S6 is 105nm,and is increasing as the magnetic field increasing.The chiral soliton is observed in real space by LTEM experiment.At the same time,the phase transition from chiral soliton to ferromagnetic structure modulated by thickness of the thin plate is found,and the mechanism of the phase transition is the change of anisotropy induced by thickness variation.