Study On Controllability Of Topological Defect Transformation And Domain Structure Evolution In Ferroic Materials

Topological defects in condensed matter are attracting enormous attention due to their fascinating properties and important roles in phase transitions.Among the various types of matter,ferroic materials,e.g.,ferroelectrics,ferromagnetics and multiferroics which possesses switchable order parameter(spontaneous polarization or magnetization)and fruitful domain structures,are ideal materials to form topological defects.Vortex domain structures and skyrmions are two kinds of typical topological defects in ferroics.They can be stable at ferroic nanostructures with the size being as small as several nanometers,and present unique properties quite different with other domain structures,which are promising for developing novel functional devices.For example,the chirality of the vortex domain structures can be employed to design high-density and multi-function functional devices.Meanwhile,topological defects can be abundant tuned by external stimuli in nanoscale ferroics.For instance,the gyration and breathe mode in the skyrmion which can be excited by applied magnetic field.However,vortex domain structures and skyrmions always exist in specific ferroic system.It is important to systematically investigate the rule of their transformation and the evolution process of domain structure,it is very important to better use of ferroic materials.In this thesis,a couple of theoretical approaches including phase field model of ferromagnetic,micromagnetic model,phase field model of ferroelectric and thermodynamic analysis have been employed to comprehensively discuss the characteristics and controllability of the topological defects(i.e.,vortex domain structure and skyrmion)by extern stimuli such as mechanical field,thermal field,electric field and magnetic field in ferroic materials.The research is very helpful to further understand the properties of nanoscale ferroics and provides a theoretical basis for future applications.The main contents and results in this thesis are summarized as follows:(1)Focusing on ferromagnetic nanodot systems,we investigated the stability of polar and vortex domain structure and analyzed their transformation controlled by mechanical field and magnetic field.Firstly,we successively calculated the phase diagrams of domain structure stability in ferromagnetic FeGa nanodot as a function of the lateral dimensions under free standing state,isotropic biaxial in-plane compressive stress and tensile stress.We found that the phase diagram of magnetic domain structure as a function of the lateral dimensions is composed by polar region,vortex region and transitional region where the domain structure is either the polar domain structure or the vortex domain structure,and isotropic biaxial in-plane compressive stress has great influence on the phase diagram.Furthermore,we discussed the mechanical transformation of the nanodot in the transitional region and found that isotropic biaxial in-plane compressive stress can control the transformation between the polar domain structure and the vortex domain structure.Lastly,we simulated the nanodot hysteresis loop in the different region of phase diagram with applying alternating magnetic field and discussed the effect on hysteresis loop by mechanical field.(2)Focusing on ferroelectric nanobelt systems,we studied the number and circulation of vortex domain structure controlled by local heat in barium titanate(BTO)nanobelt.We conducted ferroelectric phase-field simulations to analyze the controllable rules of the number of vortex domain structure in nanobelt under local heat.Results showed that the transformation from one vortex to three vortices can be induced by loading and removing local heat,and the transformation is affected by the length of nanobelt,the temperature and length of local heat.On this basis,we moved the local heat from the boundary of adjacent vortex domain structure,and it could make the middle vortex in the three vortices domain structure grow up and occupy the whole nanobelt which can switch the circulation of initial 1 vortex domain structure.(3)Focusing on multiferroic heterostructure systems,we investigated the size effect and the electrical control of stability of magnetic domain in the ferroelectric PZT film-ferromagnetic FeGa nanodot multiferroic heterostructure system.We found that the size phase diagram of ferromagnetic nanodot domain structure in the multiferroic heterostructure can be divided into five regions,and the stable of magnetic domain structure in the phase diagram is strongly sensitive to not only the size and shape of the nanodot but also the magnitude and direction of the applied electric field.Meanwhile,it is revealed that electric field in multiferroic heterostructure can induce various magnetic state transformations include those between states with different orders(i.e.,one is polar and the other is vortex),as well as those between states with the same order(i.e.,both are polar or both are vortex).Using these controllability of magnetic domain structures,we design the tunnel junction unit of magnetic random access memory(MRAM)and propose the memory mode of different high and low resistance state.(4)Focusing on magnetic skyrmion systems,we conducted the micromagnetic model to study the multi-skyrmion interaction controlled by voltage controlled magnetic anisotropy(VCMA)of electrode gate and driving current density in skyrmion bilayer racetrack.We successively analyzed the operation mode which is induced by interaction with different initial number of skyrmions,and we summarized the operation phase diagram which can be desigened the skyrmion operator.Then we discussed the effect on the motion of skyrmion by changing the width of racetrack and obtained the phase diagram of the skyrmion arrangement in the different width racetrack.On this basis,we developed the skyrmion diverter.