Study On The Critical Phenomenon And Transport Of 2D Ferromagnets And Magnetic Semimetal Materials

Since the reduction of dimensionality in electronic states,two-dimensional(2D)magnetic materials exhibit fascinating quantum phenomena and physical properties,such as layer-number-dependent physics,anomalous Hall effect,and stripe domain structure.In addition,due to the weak interaction between layers,these layered materi-als can be exfoliated easily down to few layers or even monolayer,which can still retain ferromagnetic order at finite temperature.Therefore,the exploration of van der Waals systems with intrinsic ferromagnetism is extremely significant not only for studying fundamental low-dimensional magnetism but also for developing the next-generation spintronic devices.As we know,the quantum electronic states of magnetic topologi-cal semimetals can be modulated by external magnetic field,electric field,strain and temperature,which may result in new topological phase transitions and novel trans-port behavior.On the other hand,critical analysis and magnetic entropy change(MEC)method are effective means to reveal the magnetic mechanism of phase transition,which can also uncover the magnetic interaction,the decay distance of spin interaction,type of the magnetic coupling,spatial and spin dimensionality,long-or short-range mag-netic interaction,etc.In this work,we choose two kinds of 2D magnetic materials and two kinds of magnetic semimetals to perform a systematic investigation on the intrinsic magnetism and phase transition by critical analysis and MCE method.Moreover,the transport of 2D magnetic semiconductor Cr2Ge2Te6 under high magnetic field has also been studied.The dissertation is divided into four chapters as follow:In the first chapter,we briefly introduce the classification of magnetism and their characteristics.Then,we mainly introduce the magnetic phase transition and critical exponents,and give two methods to obtain the critical exponents.At last,we focus on the universality and construction of H-T phase diagram by critical exponents.In the second chapter,we perform a systematic investigation on critical behavior and magnetism of two kinds of 2D magnetic materials,and concentrate on the trans-port of 2D magnetic semiconductor Cr2Ge2Te6 under high magnetic field.Firstly,the critical behavior of Cr2Ge2Te6 is investigated by MEC method.The magnetic prop-erties,including M(T),M(φ),and M(H)with the external field applied along different orientations,reveal weak out-of-plane magnetic anisotropy(along the c axis)and no magnetic anisotropy in the ab plane.The critical behavior indicates that magnetic cou-pling in layered Cr2Ge2Te6 is of a 2D-Ising-like type with a long-range ferromagnetic order.These findings are well consistent with the renormalized spinwave theory.Sec-ondly,with the help of external field,we study the transport behaviors of Cr2Ge2Te6 in detail.The angle-dependent resistance shows that single crystal Cr2Ge2Te6 exhibits absolute isotropy in the ab-plane.However,it displays anisotropy when the magnetic field is rotated from the ab-plane to the c-axis.The resistance behavior is consistent with its magnetic behavior.The resistance reaches the maximum when H//c while it is the minimum when H//ab.In addition,R(H)curves of Cr2Ge2Te6 exhibit a positive MR effect when H//c while a negative one when H//ab.Based on the ESR spectra,we construct a microscopic Hamiltonian involving the exchange interaction between the electrons and ferromagnetic moments to explain our experimental results,which gives a consistent theoretical interpretation for the experimental results.Lastly,utilizing mag-netic measurement and MEC method,We systematically study the magnetic anisotropy and critical behavior of Fe3-x GeTe2(x ≈ 0.28)with H//ab and H//c,respectively.The M(φ)curves show absolute isotropic characteristics in the ab plane while strong mag-netic anisotropy along c axis.The critical exponents with H//c,in agreement with the previous reports belong to the theoretical prediction of 3D Heisenberg model,which suggests a short-range magnetic coupling.However,the critical exponents with H//ab are close to those of mean-field model,which indicates a long-range magnetic coupling.The anisotropic critical exponents suggest that the magnetic coupling in Fe3-xGeTe2 is dependent on orientations of applied magnetic field.In the third chapter,the phase transition and critical behavior of two kinds of mag-netic semimetals is investigated in detail.Firstly,we focus on the complex magnetic phase transitions in DySb,and obtain the critical exponents when H//[001]by means of critical analysis.The critcial exponents suggest that the critical behavior of DySb when H//[001]obey tricritical mean-field model.Then,We construct the detailed H-T phase diagram around the phase transition for H//[001].A field-induced tricritical point is determined at the temperature and field of(7.5 K,51.4 kOe),which locates among the antiferromagnetic(HoP-type AFM),forced ferromagnetic(FFM),and paramagnetic phases(PM).In addition to the tricritical point,a triple point is also found at(9.2 K,19.7 kOe),which locates at the intersection point among NiO-type AFM,HoP-type AFM,and PM phases.Lastly,the critical behavior of potential Weyl semimetal PrAlGe is also studied by MEC method.The results indicate that PrAlGe is a system with Ising-type magnetic coupling belonging to 2D-Ising model,which is of great significance for understanding the intrinsic magnetic properties of Weyl magnetic semimetal.In fourth chapter,there are the summary and prospect.