Structural,Magnetic And Electric Properties Of Tellurium Based Single Crystals

Tellurium based single crystals and Heusler alloys are the strong candidates for the next generation multifunctional devices.This thesis mainly investigated the structural,magnetic and transport properties for some of them,including tellurium based single crystals such as FeTe2,NbTe2 and MnSiTe3,and Hesuler alloy,Co2TiSn.This thesis consists of 7 chapters.Chapter 1 introduces the background and investigation moti-vation.Chapter 2 includes the synthesis method and characterization methods such as Raman,magnetization measurement and transport measurement etc.Chapter 3 in-vestigates the negative volume expansion and magneto-lattice coupling in FeTe2 single crystals.Chapter 4 explores the transport properties in NbTe2 single crystals under high pressure up to 50 GPa.Chapter 5 studies ferromagnetic properties in Transition metal tri chalcogenide MnSiTe3.Chapter 6 focuses on critical behavior in the half-metallic Heusler alloy Co2TiSn.Last chapter is summary and future prospective of the current work.All these above materials selected on the basis of their rich physics and interesting physical characteristics.For example,FeTe2,the different synthesis methods lead to dif-ferent phase structure and further result in the different magnetic properties.Therefore,it is essential to grow pure phase high quality single crystals of FeTe2 and to carefully investigate its physical properties to explore other fascinating phenomena such as di-rectional dependent magnetic properties to realize magnetic anisotropy.NbTe2,being a layered structure,hosting both ground state-CDW and superconductivity like NbSe2,isostructural to TaTe2(1T-MX2)has attracted great interest and has recently been pro-posed as a candidate of semimetal with topological protected band crossing and have shown signature of linear dispersion band at ambient pressure.Tri-chalcogenide mono-layer MnSiTe3 was previously discovered via HSE06 functional calculation,to be a half-metallic ferromagnetic.As the half metallic ferromagnetic is seen to be a key in-gredient in the future spintronics,therefore MnSiTe3 is an interesting material system to be studied.Co-based Heusler alloys are supposed to be high-grade candidates for exam-ining itinerant electron ferromagnetism.Of these,the Co2TiSn compound is of notable importance because of its similarity with the prototype half-metallic system NiMnSb,which is a potential spin-injector material for spintronics applications.Single-crystalline FeTe2 in marcasite phase with orthorhombic structure was pre-pared via chemical vapor transport.Cooling FeTe2 single crystals from room temper-ature down to 2 K,multiple magnetic phase transitions were observed.Paramagnetic(PM)to antiferromagnetic(AFM)and then to ferromagnetic(FM)occurred at 79 K and 35 K for in-plane,73 K and 29 K for out-of-plane,respectively.A strong uniaxial mag-netic anisotropy was found due to FeTe6 octahedron distortion and structural modulation in FM region.The novel negative volume expansion(NVE)initiated in the vicinity of AFM to FM transition.An abrupt frequency shift of the most intense mode at 118 cm-1 and evolution of the Te-Te stretching mode near 40 K,corresponding to the phase tran-sition from AFM to FM were observed.The temperature-dependent resistance revealed an anomaly(semiconductor to metallic transition)around AFM-FM transition,which can easily be suppressed and move to high temperature by the applied magnetic field.The results from XRD,Raman and resistivity indicated that the structural parameters,vibration frequency and transport are sensitive to the phase transition from AFM to FM.The nature of direct band gap with 0.67 eV was identified through UV-Vis NIR spectrum of FeTe2 single crystals at room temperature.Transition metal dichalcogenides,with their layered structures,have attracted huge interest as a platform for exciting physical phenomena and a broad range of potential ap-plications.Among them special attention has been paid to WTe2 and MoTe2,which ex-hibited non-trivial topology and pressure induced or enhanced superconductivity.Here we report high pressure structural and transport study on another distorted 1T material NbTe2 up to 50 GPa.From the field dependence magneto conductance measurement,a two dimensional weak antilocalization(2D-WAL)effect is experimentally observed as well as the universal conductance fluctuation in the pressure range of 15-34 GPa in NbTe2.Systematic analysis of XRD data revealed an anomaly in the similar pres-sure range,indicating the interplay between the formation of 2D channels and structural changes under high pressure.Motivated by the feature-rich characteristics and potential applications of half metal-lic ferromagnetic materials,here we report,a trichalcogenide half-metallic ferromagnet MnSiTe3,synthesized in hexagonal structure in R3 space group.Temperature depen-dent magnetization M(T)measurement revealed a ferromagnetic(FM)phase transition around 72K.Electronic transport measurement p(T)exhibited a semiconducting-like behavior at high temperature above Tc and a half-metallic character in FM region.p(T)data revealed the exponential suppression of electron-magnon scattering with tempera-ture,providing a strong evidence that MnSiTe3 indeed possess a perfect spin polarization at low temperature.Co2-based Heusler alloys show diverse magnetic behavior,extending from the lo-calized Heisenberg to the delocalized itinerant and thus cannot be generalized with one specific model.It is therefore,we carried out a detail study to reveal the origin of long-range ferromagnetism in Co2-based Heusler alloy,Co2TiSn.The critical behavior of Heusler alloy Co2TiSn is investigated by bulk magnetization study around the param-agnetic to ferromagnetic(PM-FM)transition.The precise value of Curie temperature(Tc=358K)as well as the critical exponents(?=0.527±0.003,?=1.229±0.002 and?=3.33±0.002)were determined by means of different analytical methods such as modified Arrott plot analysis,the Kouvel-Fisher method and critical isotherm analysis.With these critical exponents the isotherms M(H)curves below and above Tc collapse into two universal branches,fulfilling the single scaling equation m=f±h,where m and h are normalized magnetization and field,respectively.The reliability of the critical ex-ponents were confirmed by Widom scaling hypothesis ?=??-1 Apart from the slight increase in ? and ?,the deduced critical exponents were consistent with the theoretical prediction of the mean field model,indicating the long range magnetic interaction in Co2TiSn.Additionally,it is obtained that spin interaction decays as J(r)?r-4.7.We suggest that the competition between localized majority spins and itinerant minority spins magnetic interaction could be responsible for critical behavior in Co2TiSn.