The Study Of Phase Transition And Critical Behavior Of Magnetic Rare Earth Compounds

In magnetic rare-earth compounds,both the spin-orbit coupling and crystal field effects can lead to magnetic anisotropy.In addition,the Ruderman-Kittel-KasuyaYosida(RKKY)interaction between the localized 4f electrons and the competition between localized and the itinerant electrons conspire to induce complex magnetic structures,and thus bring about rich and tunable physical properties.Recently,there is extensive research in weyl semimetal compounds owing to its topological band structures and skyrmion materials with topological magnetic structures,in which the rare-earth compounds present rich magnetic structure phase transitions and magnetoelectric coupling properties,thus promoting abundant applications in electronic spin devices,prospectively.In fact,it is of great importance to study complex magnetic structures and their phase transitions for understanding the couplings between magnetism and topology in the rare-earth compounds.On the one hand,the basic analysis helps to clarify the types of interactions and physical mechanisms from the couplings between magnetism and topology.On the other hand,this may provide a reference to investigate the mechanism of the regulations on the magnetic structures and the topological energy band structures such as elemental substitution,doping,pressurization and other regulation methods.Therefore,the study of phase transitions and critical behavior analysis of topological rare-earth compounds is an important and meaningful topic.In this thesis,magnetic weyl semimetal materials represented by ternary compound NdAlGe and CeAlSi,binary compound DyBi,and NdMn2Ge2 compounds with magnetic topological structures,are selected as the research objects to reveal the phase transitions and critical behaviors in these systems.In chapter 1,firstly,we mainly introduce the common types of magnetic structures in materials,including ordered magnetic structures categorized as colinear and noncolinear type,and disordered magnetic structures such as paramagnetic and antimagnetic.Then,we described the research status and significance of the selected rare-earth compounds,including the basic structure,magnetoelectric coupling properties,and the analysis of phase transition and critical behavior.Finally,we introduce the methods to study phase transition and critical behavior such as the acquisitions and properties of critical exponents,etc.In Chapter 2,we reveal the novel magnetic structures and abundant phase transitions in the rare-earth compound NdAIGe by using the magnetic measurements and critical analysis methods.The angle-dependent magnetization M(φ)reveal the isotropic magnetization behavior in the ab-plane and the anisotropic behavior appears out of plane,and displaying an easy magnetization behavior along the c-axis.The M(H)measurements with the magnetic field up to 32 T and the Curie-Weiss fitting of M(T)indicate a stable antiferromagnetic structure in the ab-plane.In addition,the complex magnetic structures and phase transitions are revealed along the c axis,including up-up-down spin configuration and corresponding orderd state as well as polarized ferromagnetic state(FFM).In particular,the analysis of the magnetic iterative method reveal a tricritical behavior,according to the complex magnetic interactions and multiple structures in this system.The complex magnetic phase transitions and tricirtical point TCP(5.27 K,30.1 kOe)are displayed in the constructed H-T phase diagrams of NdAIGe.In Chapter 3,the complex magnetic structure and field-induced phase transitions are present in CeAlSi compounds.The angle-dependent magnetization M(φ)reveal a rare behavior with a hard magnetization axis out of plane and easy axis along the[110]direction,the fundamental magnetization measurements indicate that the ground state of CeAlSi is a spin-glass-like metastable structure.In addition,the magnetic critical analysis reveal a tricritical mean field model and a tricritical point TCP(8.6 K,200 Oe),suggesting a complex magnetic coupling mechanism in this non-collinear magnetic structure.Furthermore,the mechanism and regulations of this metamagnetic transitions deserve further investigation.In Chapter 4,we focus on the system with a variety of magnetic structures and phase transitions induced by magnetic field in the binary rare-earth compound DyBi,including NiO-type antiferromagnetic,HoP-type antiferromagnetic,and polarized ferromagnetic FFM structures.We use a steady high magnetic field up to 32 T to investigate the magnetization behavior with the field applied along three directions,including H//[001],H//[011]and H//[111]respectively,revealing the different critical field and saturation behaviors among them.When H//[001],the Tricritical mean field behavior is obtained by magnetic critical analysis.In addition,the saturation behavior of the magnetization is observed along the H//[001]and H//[011]directions,accompanied by the appearance of the tricritical point TCP1(8.6 K,85 kOe)and TCP2(16.8 K,243 kOe),which are revealed by their H-T phase diagram,respectively.In Chapter 5,we focus on the basic magnetic properties and interaction types of the magnetic skyrmion material NdMn2Ge2,the phase transition and critical behavior of which are analyzed by the magnetic entropy method and iteration method respectively,both of which reveal an Ising type magnetic interaction.In Chapter 6,we summarize the properties of these magnetic rare-earth compounds,including the basic magnetic,complex magnetic structures and interaction types of the magnetic rare-earth compounds.Finally,we present more discussions and perspectives based on the current works.