Anomalous Ferromagnetism Research Of Doped Transition Metal Dichalcogenides

Quasi-two-dimensional transition-metal dichalcogenide TaS2 is a typical "van der Waals material" with weak coupling between layers.There are many interesting anomalies in ferromagnetic FexTaS2 with the intercalation of Fe between TaS2 layers:the magnetization curve is sharply switched in a rectangular shape;the magnetocrystalline anisotropy is extremely strong and is comparable with rare-earth ferromagnets;magnetization,magnetoresistance,magnetic coercive force and effective magnetic moment increase with the increasing of Fe concentration;the ferromagnetic Curie temperature Tc changes significantly with the variation of doping concentration x.Especially,quasi-two-dimensional Fe0.25TaS2 exhibits ferromagnetic transition temperature of as high as up to 160 K.In this thesis,we focus on Fe0.25TaS2 so as to understand and explain the microscopic mechanism of the ferromagnetic properties of FexTaS2.The key point is to theoretically uncover the microscopic origin of the high ferromagnetic Curie temperature and strong magnetic anisotropy in Fe0.25TaS2.Firstly,based on the crystal structure of Fe0.25TaS2 the electronic structure properties of the system were obtained by the first-principle electronic structures calculations with the LDA+SOC method.Then the theoretical analysis and Green's function method are used to derive the spin-spin interaction in the system in detail;Finally,an effective Hamiltonian model is proposed based on the derived RKKY-type interaction,and the mean field method and the self-consistent-field cluster method with the finite-temperature Lanczos method as solver are applied to obtain the finite-temperature ferromagnetic properties and the phase transition point.Our theoretical results provide satisfied explanation on the extremely high Tc and large magnetic anisotropy of Fe0.25TaS2.This provides a theoretical scenario for many similar quasi-two-dimensional ferromagnetic materials.In the first chapter,we make a brief overview of the current status of 2D materials research and its application prospects,the classic theory of magnetic materials,basic knowledge of Dirac materials,research status and development of 2D magnetic materials;and then introduce the basic physical concepts of magnetic anisotropy,spin-orbit coupling and abnormal Hall effect.We focus on the modern magnetism and quantum magnetic theory,classification of magnetic materials and application prospects of magnetic materials.Then,we briefly introduce the background and the experimental and theoretical research status of magnetic properties of FexTaS2 system.Then we present the crystal structures and some theoretical hypothesis on the ferromagnetic origin of Fe0.25TaS2,and point out its problems.Finally,we propose our research motivations,theoretical methods and application implications.In the second chapter,we mainly study the influence of magnetocrystalline anisotropy on ferromagnetic transition temperature.According to the experimental measurement of the crystal structure and electron diffraction pattern of Fe0.25TaS2,we can find that the Fe ions form a triangular lattice,and the nearest Fe distance is twice the basic hexagonal lattice constant of the TaS2 array.Recombination experiments have confirmed that the ferromagnetism of the system is mainly derived from the spin between Fe2+,and the system is in ferromagnetic order;We can describe the spin wave of the system using the J1-J2 Heisenberg model of the ferromagnetic triangular lattice with spin S=2.In order to consider the influence of the magnetocrystalline anisotropy energy of the system on the ferromagnetic Curie temperature,we use the mean field method to approximately study the finite-temperature ferromagnetism and magnetic phase transition.We find that the magnetocrystalline anisotropy in the FexTaS2 system increases the ferromagnetic transition temperature,but its impact does not play a leading role.In the third chapter,based on the energy band of the Fe0.25TaS2 obtained by the first-principles electronic structure calculation,we know that there are traditional dispersion spectra and Dirac dispersion spectra in the Fe0.25TaS2 system.Therefore,we adopt the second-order perturbation theory to derive the spin-spin interaction in the presence of both the Dirac linear-dispersion and conventional parabolic-dispersion carriers in detail.After comparing the component of the RKKY interaction strength from the traditional parabolic-dispersion carriers with that from the Dirac linear-dispersion carriers,we find that the Dirac electrons contribute the dominant part of the RKKY interaction in Fe0.25TaS2.In the fourth chapter,we mainly study the finite-temperature ferromagnetic properties of Fe0.25TaS2.Firstly,we use the Green's functions method to derive the RKKY interaction Hamiltonian in the presence of Dirac carriers with large spin-orbit coupling,from which we propose a short-range effective model suitable for describing the magnetic properties of Fe0.25TaS2.And then we utilize the self-consistent-field cluster method with the finite-temperature Lanczos method as solver to numerically calculate the ferromagnetic transition temperature.Our numerical results show that the anisotropic effective model could not only account for the high Te,but also the large magnetic anisotropy in Fe0.25TaS2.In the end of the thesis,we summarize our results and point out possible further research directions.