Anomalous Hall Effect In Several Magnetic Materials And Annealing Effect On Transport Properties Of Single-Crystalline 1T-TaS₂

In condensed matter physics,anomalous Hall effect(AHE)is one of the funda-mental transport propterties.Three conventional mechanisms have been proposed till now,which could give proper explanation to AHE in most of materials.However,as the development of investigations,researchers found that some AHE behaviors cannot be properly fitted by conventional mechanisms.These kinds of AHE are closely related to the Berry phase in real space or momentum space,and are thus named by topological Hall effect(THE).To futher understand and verify the THE mechanism,we conducted experiments on two uniaxial ferromagnet.Anisotropic magnetic properties and THE derived from non-coplanar spin structures were observed by electrical transport and magnetic measurements.In addition,we studied the effect of the content of interstitial iron atomes on physical properties including THE in Fe,?,Sb system.Transition metal disulfide 1T-TaS2 exhibit various physical properties,including multiple charge density wave(CDW),Mott state at low temperature,etc.2H-TaS2 is an allotrope of 1T-TaS2,CDW and superconductivity coexist in this material.What's more interesting is that the physical properties of the above two materials can be tuned by several methods.Hence,study of mechanisms hind in the intriguing phenomena is necessary.Theoretically,hydrogenation and annealing could trigger the phase transition from 1T to 2H or vice versa,because the energy difference between 1T-TaS2 and 2H-TaS2 is small.Inspired by the properties mentioned above,we systematically studied the annealing effect on single-crystalline 1T-TaS2 by measuring their transport properties and X-ray photoelectron spectroscopy.The main body of this doctoral thesis is consist of six chapters as follows.Chapter 1 IntroductionIn chapter one,we reviewed the research background of this doctoral thesis.Firstly,we gave an introduction to the anomalous Hall effect(AHE),which includes the dis-covery and the research progress of AHE.Then the topological Hall effect that closely related to Berry phase in real space or momentum space was summerized.Finally,we introduced the various and tunable physical properties of 1T-TaS2 and 2H-TaS2.At the end of this chapter,we briefly summarized the research significance and research content of this doctoral thesis.Chapter 2 Experimental principles and methodsThis chapter briefly introduce the experimental principles and methods used in this doctoral thesis,including sample preparation,structure and chemical composition char-acterization and the methods to measure electrical transport and magnetic properties.Chapter 3 Anisotropic anomalous Hall effect in triangular itinerant ferro-magnet Fe3GeTe2We successfully synthesized high quality single-crystalline Fe3GeTe2 samples by using chemical vapor transport method.Electronic transport and magnetic measure-ments indicate an obvious anisotropy in this material.When the current flows along the hard plane and magnetic along easy axis,Fe3GeTe2 exhibits conventional anomalous Hall effect below Curie temperature T,,which can be depicted by Karplus-Luttinger the-ory.On the other hand,topological Hall effect shows up below Tc with current along easy axis and magnetic field parallel to hard plane.The enhancement of Hall resistivity can be attributed to the chiral effect during the spin-flop process.Chapter 4 The electrical transport and magnetic properties of Fe1.08Sb sin-gle crystalThe content of interstitial iron(Fei)in Fe1+x Sb is 0.08?x?0.38.We synthe-sized Fe1.08 Sb single crystals with Fei content close to the lower limit of solution area and investigated its transport and magnetic properties.The ac susceptibility and field dependence of magnetization measurements demonstrate that the transition at?9 K is a spin glass transition.By analyzing temperature dependence of susceptibility and re-sistivity,we found a change of scattering in transport properties caused by the ordering of Fei moments in this compound.In addition,the Hall resistivity results reveal that the hole-type carrier is dominant and the topological Hall effect induced by spin chirality is negligible in Fe1.08Sb.Chapter 5 Magnetic anisotropy and topological Hall effect in trigonal Cr5Te8We investigated the magnetic anisotropy and topological Hall effect in trigonal Cr5Te8 single crystals.By analyzing the temperature dependence of dc and ac sus-ceptibility,we proposed a possible magnetic structure as discussed below.Below the magnetic transition temperature,the spins are canted ferromagnetic aligned along c axis with antiferromagnetic components in ab plane.This peculiar magnetic structure gives rise to the anisotropic anomalous Hall effect in Cr5Te8.When the current flows along the ab plane,Cr5Te,exhibits a conventional anomalous Hall effect depicted by skew scattering mechanism.On the other hand,the topological Hall effect appears with cur-rent along c axis,which stems from the noncoplanar spin configuration with scalar spin chirality during the spin flop process.Chapter 6 The annealing effect on single-crystalline 1T-TaS2We studied the annealing effect on transport properties of 1T-TaS2 single crystals,and characterized the samples annealed at different temperatures by using X-ray pho-toelectron spectroscopy.As the increasement of annealing temperature,the transition from NCCDW to CCDW is supressed gradually,and superconductivity appears at low temperature.When the annealing temperature is higher than 500 ?,the superconduct-ing temperature was enhanced.The Ta 4f and S 2p photoemission spectra were chosen to characterize samples.The results indicate that the 1T to 2H phase transition occurs with increasing annealing temperature.The defects increase obviously with annealing temperature higher than 500 ?,resulting in the enhancement of superconducting tem-perature and upper critical field.