We present the crystal structure, electronic structure, and transport properties of the material YbMnSb2, a candidate system for the investigation of Dirac physics in the presence of magnetic order. Our measurements reveal that this system is a low-carrier-density semimetal with a 2D Fermi surface arising from a Dirac dispersion, consistent with the predictions of density functional theory calculations of the antiferromagnetic system. The low temperature resistivity is very large, suggesting scattering in this system is highly efficient at dissipating momentum despite its Dirac-like nature.arXiv:1708.03308v1 [cond-mat.mtrl-sci]

The discovery of novel magnetic and electronic properties in low-dimensional materials has led to the pursuit of hierarchical materials with specific substructures. Low-dimensional solids are highly anisotropic by nature and show promise in new quantum materials leading to exotic physical properties not realized in three-dimensional materials. We have the opportunity to extend our synthetic strategy of the flux-growth method to designing single crystalline low-dimensional materials in bulk. The goal of this Account is to highlight the synthesis and physical properties of several low-dimensional intermetallic compounds containing specific structural motifs that are linked to desirable magnetic and electrical properties. We turned our efforts toward intermetallic compounds consisting of antimony nets because they are closely linked to properties such as high carrier mobility (the velocity of an electron moving through a material under a magnetic field) and large magnetoresistance (the change in resistivity with an applied magnetic field), both of which are desirable properties for technological applications. The SmSb structure type is of particular interest because it is comprised of rectangular antimony nets and rare earth ions stacked between the antimony nets in a square antiprismatic environment. LnSb (Ln = La-Nd, Sm) have been shown to be highly anisotropic with SmSb exhibiting magnetoresistance of over 50000% for H∥c axis and ∼2400% for H∥ab. Using this structure type as an initial building block, we envision the insertion of transition metal substructures into the SmSb structure type to produce ternary materials. We describe compounds adopting the HfCuSi structure type as an insertion of a tetrahedral transition metal-antimony subunit into the LnSb host structure. We studied LnNiSb (Ln = Y, Gd-Er), where positive magnetoresistance reaching above 100% was found for the Y, Gd, and Ho analogues. We investigated the influence of the transition metal sublattice by substituting Ni into Ce(CuNi)Sb (y < 0.8) and found that the material is highly anisotropic and metamagnetic transitions appear at ∼0.5 and 1 T in compounds with higher Ni concentration. Metamagnetism is characterized by a sharp increase in the magnetic response of a material with increasing applied magnetic field, which was also observed in LnSb (Ln = Ce-Nd). We also endeavored to study materials that possess a transition metal sublattice with the potential for geometric frustration. An example is the LaFeSb structure type, which consists of antimony square nets and an iron-based network arranged in nearly equilateral triangles, a feature found in magnetically frustrated systems. We discovered spin glass behavior in LnFeSb (Ln = La-Nd, Sm) and evidence that the transition metal sublattice contributes to the magnetic interactions of LnFeSb. We investigated the magnetic properties of PrFeCoSb (x < 2.3) and found that as the Co concentration increases, a second magnetic transition leads from a localized to an itinerant system. The LaFeSb structure type is...

Single crystals of LnM x Ga 3 (Ln = Ho, Er; M = Fe, Co; x < 0.2) were grown from a Ga self-flux. The compounds crystallize in a "stuffed" variant of the AuCu 3 -structure type where Fe or Co partially occupies the body-centered position of the unit cell surrounded by six Ga atoms. The insertion of Fe or Co guest atoms into the AuCu 3 -structure type causes a structural change in the host material where chemical pressure is exerted on Ga-Ga contacts surrounding the filled octahedra. This in turn affects the stability of the AuCu 3 -structure type and the intrinsic properties of the materials. With increasing Ho−Ho distances of 4.2355(5), 4.2377(5), and 4.2441(3) Å, HoCo 0.16(2) Ga 3 , HoFe 0.11(1) Ga 3 , and HoFe 0.16(2) Ga 3 order antiferromagnetically with decreasing Néel temperatures of T N = 10.2, 7.2, and 6.5 K, respectively. ErCo 0.06(2) Ga 3 , ErFe 0.06(1) Ga 3 , and ErFe 0.11(2) Ga 3 order antiferromagnetically with Néel temperatures of T N = 3.3, 5.2, and 6.2 K. The magnetic properties are attributed to changes in the coupling strength of local magnetic moment with conduction electrons as a function of transition metal and rare earth.

Due to its cooperative nature, magnetic ordering involves a complex interplay between spin, charge, and lattice degrees of freedom, which can lead to strong competition between magnetic states. Binary Fe3Ga4 is one such material that exhibits competing orders having a ferromagnetic (FM) ground state, an antiferromagnetic (AFM) behavior at intermediate temperatures, and a conspicuous re-entrance of the FM state at high temperature. Through a combination of neutron diffraction experiments and simulations, we have discovered that the AFM state is an incommensurate spin-density wave (ISDW) ordering generated by nesting in the spin polarized Fermi surface. These two magnetic states, FM and ISDW, are seldom observed in the same material without application of a polarizing magnetic field. To date, this unusual mechanism has never been observed and its elemental origins could have far reaching implications in many other magnetic systems that contain strong competition between these types of magnetic order. Furthermore, the competition between magnetic states results in a susceptibility to external perturbations allowing the magnetic transitions in Fe3Ga4 to be controlled via temperature, magnetic field, disorder, and pressure. Thus, Fe3Ga4 has potential for application in novel magnetic memory devices, such as the magnetic components of tunneling magnetoresistance spintronics devices.

Single crystals of the ternary Pr3Co2+x Ge7 and Ln3Co2+x ­Ge7–y ­Sn y (Ln = Pr, Nd, Sm) adopting a disordered version of the La3­Co2Sn7 structure type have been prepared via flux-growth methods and characterized by single crystal X-ray diffraction. The structure consists of two lanthanide crystallographic sites with one occupying a cuboctahedral coordination environment and a second in a trigonal prismatic environment. The structure can also be described as an intergrowth of AuCu3 and CeNiSi2 structure types, and the stability of the germanide analogues requires Sn incorporation and Co site preferences. Magnetic properties of the Pr3­Co2+x ­Ge7–y ­Sn y series are highlighted with the Sn-substituted Pr3­Co2.514(5)­Ge6.66(7)­Sn0.360(2) orders magnetically near 5.8 K, while the germanide Pr3­Co2.3376(5)­Ge7.056(7) exhibits magnetic transitions at 5.3 and 9.3 K, arising from the magnetic sublattices with field-dependent magnetization revealing three metamagnetic transitions at 0.46, 0.80, and 2.1 T.