The Study Of Magnetoelectric Properties In Transition Metal Sulfides And Halides

Magnetism is a fascinating physical phenomenon in lots of materials,which has a great impact on the development of the human civilization.The earliest application of magnetism is the Sinan in China that is considered to be the origin of the compass.Nowadays,the usage of magnetism is applied to plenty field of our daily life.After the development of physics,now we know the magnetism is related to the spin of electron.Thus it is so important to study the micro-mechanism of magnetic materials.As a familiar property in materials,ferroelectricity is also gained extensive attention since it has been found in 1920.Due to the important application value of this two kinds of materials,it is necessary to study the magnetism and ferroelectricity from microscopic view.As we known,the transition metal oxides were studied most in the past,while the transition metal sulfide and halides were paid less attention.In this thesis,we mainly studied the low temperature phase diagram of ferroelectric spinels Ga Mo₄S₈ and the magnetic transition in a two-dimensional bilayer FeBr₂.Trough studying the magnetism and ferroelectricity in non-oxide transition metal compound,we tried to find a way to realize the controlling of magnetism through electric field.In the first chapter,a brief introduction was given about the class of magnetism with putting the emphasis on introducing the non-collinear magnetism.In this chapter,two important parts was arranged as for our main study topics.In the first part,we introduced a quiet special non-collinear magnetic structure which called skyrmion which is a stable spin texture due to its topological property.The development about the skyrmion was given through explaining several representative materials that can hold skyrmion in the recent ten years.As for the spinel materials Ga X₄M₈(X=V,Mo;M=S,Se),we gave a more detailed introduction.Because in this kind of structure,the magnetism and ferroelectricity can be coexisting,which brings out the possibility to control skyrmion by external electric filed.Inspired by the above studies,a method trying to enhcace the stability of skyrmion was put forword.In the second part,we introduced the two-dimensional magnetic materials that were reported in the nearly 2-3 years and the manipulation methods of magnetism,which gives a train of thoughts for controlling the magnetism.In the second chapter,a fundamental theory method about the Density Functional Theory calculations is presented.By explaining the Schr?dinger equation and calculation of variations method,and Hartree-Fock approximation,Hohenberg-Kohn theory,Kohn-Shame equations set,local density approximation,the formulas were derived in the end which is being used in the present Density Functional Theory calculations.In the third chapter,the magnetism in low temperature area in Ga Mo₄S₈ was studied both using Density Functional Theory method and Monte Carlo method.Through using density functional theory method,we study the magnetism,electronic structure,ferroelectricity,exchange interactions,DM interactions and magnetic anisotropy.After the parameters related to the magnetism were obtained,a Heisenberg Hamiltonian can be drawn out.Then by performing the Monte Carlo simulations,the low temperature phase diagram was simulated showing the appearance of a stable skyrmion phase in large area.In the fourth chapter,we studied the properties of the FeBr₂bulk and confirmed its A-type antiferromagnetism and insulating state.Then the antiferromagnetism and insulating state were proved also to be preserved in the FeBr₂bilayer.Considering the weak coupling between layers linked by van der Waals interactions,the external electric field was applied trying to control its magnetism.When increasing the electric field,there was a magnetic phase transition from A-type antiferromagnetic to ferromagnetic.Then the structure and phonon spectrum were calculated carefully to explain the phase transition.In the fifth chapter,a summary of our studies is presented and the future work is proposed.