| In condensed matter physics,due to the competition of degenerate states,the magnetic frustrated materials often show the novel physical properties,such as spin liquid,spin ice,topological Hall effect,large electron-electron interactionsand quantum criticality.The magnetic frustrated materials have received increasing attentions due to the above novel physical properties.Here,the magnetic and transport properties of magnetic frustrated materials with triangular lattice,such as transition metal compounds ?-Co5Ge3,CoSb,Ni1.8Ge and EuCd2Sb2,have been studied.According to the above content,this thesis includes six chapters as follows:In chapter one,we firstly make a brief survey of frustration,and introduce some novel physical properties and their mechanisms in magnetic frustrated materials.After that,the typical transport and magnetic properties in magnetic frustrated materials,such as anomalous Hall effect,topological Hall effect and short-range magnetic order,have been presented.At last,we talk about the discovery and some observed novel properties of heavy fermion materials.In chapter two,the experimental methods,including the preparation of the samples,characterization of lattice structure and chemical composition of the samples,and measurements of the physical properties(magnetic,transport and specific heat measurements),have been introduced.In chapter three,we synthesized ?-Co5Ge3 single crystals by using chemical vapor transport method and characterized their magnetic properties and anomalous Hall effect.The magnetic measurements revealed that a-Co5Ge3 is a ferromagnetic material and the ferromagnetic transition is observed at Tc=45 K and Tc=50 K for H?a and H//bc,respectively.The data of magnetoresistance showed that ?-Co5Ge3 has negative magnetoresistance and the value of magnetoresistance reaches the maximum value near the ferromagnetic transition.The Hall measurements indicated the intrinsic contribution plays a dominant role in the anomalous Hall effect of ?-Co5Ge3 at the temperature range of 30?50 K.But the anomalous Hall resistivity ?xyA has a 6.4 index relation with longitudinal resistivity ?xx(?xyA,??xx6.4)below 30 K,which cannot be explained by conventional mechanism at this temperature range.In chapter four,we successfully synthesized Co1.05b single crystals by using chemicalvapor transport method and characterized their magnetic and transport properties.Our results indicated that the stoichiometric CoSb is a nonmagnetic metal.And the interstitial Co in this compound has no effect on transport properties.A very low magnetoresistance was observed under the magnetic field up to 14 T,which suggests that Co1+?Sb could be a potential material of electronic devices for the application at high magnetic field.In chapter five,we synthesized Ni1.8Ge single crystals by using chemical vapor transport method and characterized their magnetic and transport properties.According to the results of magnetic measurements,Ni1.8Ge exhibits ferrimagnetic correlations and has an obvious magnetic anisotropy.The transport properties at low temperature revealed that Ni1.8Ge has strong electron-magnon scattering contribution coming from short-range magnetic order.Our results showed that the long-range magnetic order was not observed due to the geometrical frustration arising from the triangular nickel lattice and the short-range magnetic order produces an important influence to the transport properties.In chapter six,we reported the magnetic and transport measurements of EuCd2Sb2 single crystals.The magnetic measurements revealed that EuCd2Sb2 presents an antiferromagnetic order at 7.3 K.The further analysis indicated that the antiferromagnetic structure of the Eu spins in EuCd2Sb2 is both noncollinear and noncoplanar.The resistivity measurement showed that EuCd2Sb2 has a kondo-like behavior at low temperature.The anisotropic and complicated magnetoresistance indicated that EuCd2Sb2 has a complicated magneitic structure.And the topological Hall effect was observed,which also indicates that EuCd2Sb2 has a noncollinear and noncoplanar magnetic structure.The specific heat measurements revealed that the Sommerfeld coefficient of electron heat capacity are 310 and 522 mJ/mol·K2 for H=0 and 2 T,respectively,which proves that EuCd2Sb2 is a member of the family of heavy fermion materials.According to the Kadowaki-Woods(KW)relation(rKW=A/?2),the value of rKW is 51.3 ?? cm K2 mol2/J2,nearly an order of magnitude larger than the typical heavy fermion materials and 100 times larger than other Eu-based heavy fermion materials.For the origin of large rKW value,we consider there are two possible explanations.One is caused by the magnetic frustration,another is due to the influence of quantum critical behavior. |
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