| In recent years,two-dimensional magnetic materials have received a lot of attention.On the one hand,they are ideal platforms for the study of many models of magnetic exchange interactions.On the other hand,they have promising applications in spintronics devices.As early as in 1966,it was theoretically pointed out that two-dimensional materials could suppress thermal upheaval through magnetic anisotropy,thus achieving long-range magnetic ordering and forming two-dimensional(2D)magnets.However,they have only been discovered experimentally one after another in the last decade.At first,single or few layers were successfully prepared on Cr2Ge2Te6,CrI3,Fe3Ge2Te6,which verified that the long order of magnetism can indeed exist in 2D materials.However,their ferromagnetic transition temperature(TC)is much lower than room temperature,which limits the application in spintronics devices.Thus,the possibility of finding higher temperature or even room temperature 2D magnetic materials became a key scientific question both theoretically and experimentally.Later,attention was drawn to the transition metal disulfide compound.Monolayer selenide VSe2,which was successfully grown by molecular beam epitaxy and could maintain ferromagnetism above room temperature despite its unstable chemical properties.In telluride systems,room-temperature ferromagnetism has been found one after another on the recently reported CrTe2 and self-inserted Cr2Te3 monolayers or few layers with stable properties.In this thesis,several chromium-based telluride single crystals are studied to analyze the magnetic and electrical transport properties of the materials,from which some novel physical phenomena are found to provide new ideas for the application of spintronics devices.The main scientific questions addressed in this thesis are:What are the intrinsic physical properties of these Cr-based telluride single crystals?What are the interesting physical phenomena in this system and what insights can we gain from it?The main studies in this thesis are:(1)Monoclinic phase Cr5Te8 single crystals were obtained by a mixed flux method.The magnetic field along its c axis and ab plane as well as the electrical transport properties were systematically investigated.Strong magnetic anisotropy was observed in this material,with an anisotropic isothermal magnetic entropy change of about 1.1 J kg1 K-1 at a maximum of 4.5 T.The Tc response of this material is very sensitive to pressure,with a Tc of 186 K at atmospheric pressure and a rate of change of up to-50 K GPa-1.Further studies revealed that when the current is along the ab plane,Cr5Te8 exhibits an anomalous Hall effect.And the analysis indicates that its origin is an skew scattering mechanism.Anomalous jumps were also observed from both the anomalous Hall and isothermal magnetization curves,and it is speculated that there may be a metastable state induced by both temperature and magnetic field.The critical magnetic field of this metastable state is 0.26 T.Finally,the corresponding magnetic phase diagram is plotted to give the possible evolution process.(2)Cr2Te3 single crystals were obtained by self-melting method growth.The results measured by these combined experimental means,such as specific heat,differential scanning calorimetry,variable temperature X-ray diffraction and linear thermal expansion,indicate the presence of a primary ferromagnetic-antiferromagnetic phase transition with Tc of 160 K and a secondary antiferromagnetic-paramagnetic phase transition with antiferromagnetic transition temperature(TN)of 181 K in Cr2Te3.By applying a relatively modest magnetic field,a significant enhancement of TC is observed,while TN is suppressed until it disappears.Linear thermal expansion measurements present a direct correlation between structure,applied magnetic field and temperature.Over a wide temperature range,Cr2Te3 exhibits a large negative expansion behavior in the ab plane with a negative expansion coefficient of-17.2 ppm/K.Combined with first principles calculations,we further give its possible ground-state magnetic structure and plot the magnetic phase diagram.(3)Cr3Te4 single crystals were obtained by chemical vapor transport method.Its TC is up to 329 K.Unlike other chromium-based tellurides,the easy magnetization axis is in the ab plane.This was confirmed by angular-dependent magnetization measurements.Then we measured the magnetization curves at different temperatures in both two directions and obtained the variation of magnetic anisotropy constant with temperature by fitting.Finally,measurements of the magnetocaloric effect were performed,and by exponential fitting of the data extracted from the magnetic entropy change,we obtained the critical exponents β,γ,and δ in two directions.The results indicate that the magnetic exchange model of Cr3Te4 is close to the 3D Heisenberg model in the c direction,while the ab plane direction is close to the 2D Ising model. |
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