Structural,Magnetic And Electric Properties Of Tellurium Based Single Crystals And BiFeO₃ Based Composites

Motivated by rich physics and potential applications of di-and tri-chalcogenides,three compounds are investigated from this family in current thesis.FeTe2,the differ-ent synthesis methods lead to different phase structure and further result in the different magnetic properties.Therefore it is essential to grow pure phase high quality single crys-tals of FeTe2 and to carefully investigate its physical properties to explore other fascinat-ing phenomena such as directional dependent magnetic properties to realize magnetic anisotropy.NbTe2,being a layered structure,hosting both ground state-CDW and su-perconductivity like NbSe2,isostructural to TaTe2(1T-MX2)has attracted great interest and has recently been proposed as a candidate of semimetal with topological protected band crossing and have shown signature of linear dispersion band at ambient pressure.Motivated by this NbTe2 is investigated under hydrostatic pressure and constitution of 2D channels accompanied by structural changes are explored.Trichalcogenide 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 ingredient in the future spintronics,therefore MnSiTe3 is an interesting material system to be stud-ied.Strongly motivated by this,we have synthesized a new half metallic ferromagnet,MnSiTe3 and investigated its physical properties.Co-based Heusler alloys are supposed to be high-grade candidates for examining itinerant electron ferromagnetism.Of these,the Co2TiSn compound is of notable impor-tance because of its similarity with the prototype half-metallic system NiMnSb,which is a potential spin-injector material for spintronics applications.Therefore,Co2TiSn is investigated for its half-metallic ferromagnetism,anomalous negative magnetoresis-tance above Tc,itinerant electron ferromagnetism.In addition to this,recently by first principles studies,Co2TiSn revealed its topological semimetallic state in FM phase and existence of intrinsic anomalous Hall conductivity.The Heusler compound Co2TiSn has been studied extensively both theoretically and experimentally.However,a detail study on its intrinsic magnetic interactions is needed to further explore its half-metallic nature and itinerant ferromagnetism.In contrast with single phase materials,the composite material system can lead to the significant ME coefficient originating from the coupling between the magnetostric-tion in the ferromagnetic particles and the electrostriction in the ferroelectric particles.The large ME effect in the composite system might accompany with colossal magne-toresistance(CMR)due to the artificial and natural grain boundaries,spin polarized tunneling between grain boundaries or spin dependent scattering at grain boundaries.CMR is very sensitive to the low external magnetic field,known as low field magne-toresistance(LFMR),and spreads over a long temperature range below Tc.Composites with both ME and LFMR are therefore definitely appealing for wide application.Moti-vated by this BiFe03 based composite are synthesized and explored for magnetoelectric effect and LFMR.Following we present the outcomes of our investigations on all these interesting materials system.We report the strong magnetoelectric effect and novel low field magnetoresistance in the composite of BaxBi1-xTixFe1-xO3((BF)1-x(BT)x)and Co1-xFe2+xO4(CFx).After optimizing the component content,both magnetoresistance and magnetoelectric coef-ficient reach maximum values of 120%at 2T and 21 mV/cm/Oe(2.6 × 1010 ps/m)for(BF0.6BT0.4)0.4—(CF0.5)0.6 composite ceramics,respectively.The temperature depen-dent resistivity(T)of the composite samples exhibited a resistivity cusp just below Tc and 40%Fe+2 doping on Co+2 sites in CoFe2O4(CF)was considered as a critical dop-ing level to observe such resistivity cusp that moved to lower temperature for increasing Fe+2 doping level.The magnetoresistance effect is observed in the vicinity of the resis-tivity cusp and spreads over wide temperature range for the composite system.The spin polarized tunneling and spin dependent scattering account for the observed low mag-netic field resistance.Our results are definitely helpful for developing new BF-based multiferroic composite ceramics with novel properties.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.The critical behavior of Heusler alloy Co2TiSn is investigated by bulk magnetiza-tion study around the paramagnetic 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 differ-ent 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 exponents 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.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 lT 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.