Casting a Wider Net: Rational Synthesis Design of Low-Dimensional Bulk Materials.

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 SmSb2 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. LnSb2 (Ln = La-Nd, Sm) have been shown to be highly anisotropic with SmSb2 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 SmSb2 structure type to produce ternary materials. We describe compounds adopting the HfCuSi2 structure type as an insertion of a tetrahedral transition metal-antimony subunit into the LnSb2 host structure. We studied LnNi1-xSb2 (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(Cu1-xNix)ySb2 (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 LnSb2 (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 La2Fe4Sb5 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 Ln2Fe4Sb5 (Ln = La-Nd, Sm) and evidence that the transition metal sublattice contributes to the magnetic interactions of Ln2Fe4Sb5. We investigated the magnetic properties of Pr2Fe4-xCoxSb5 (x < 2.3) and found that as the Co concentration increases, a second magnetic transition leads from a localized to an itinerant system. The La2Fe4Sb5 structure type is quite robust and allows for the incorporation of other transition metals, thereby making it an excellent candidate to study competing magnetic interactions in lanthanide-containing intermetallic compounds. In this manuscript, we aim to share our experiences of bulk intermetallic compounds to inspire the development of new low-dimensional materials.

The Role of Crystal Growth Conditions on the Magnetic Properties of Ln2Fe4- xCo xSb5 (Ln = La and Ce).

Single crystals of Ln2Fe4- xCo xSb5- yBi y (Ln = La, Ce; 0 ≤ x < 0.5; 0 ≤ y ≤ 0.2) were grown using Bi flux and self-flux methods. The compounds adopt the La2Fe4Sb5 structure type with tetragonal space group I4/ mmm. The La2Fe4Sb5 structure type is comprised of rare earth atoms capping square Sb nets in a square antiprismatic fashion and two transition-metal networks forming a PbO-type layer with Sb and transition-metal isosceles triangles. Substituting Co into the transition-metal sublattice results in a decrease in the transition temperature and reduced frustration, indicative of a transition from localized to itinerant behavior. In this manuscript, we demonstrated that Bi can be used as an alternate flux to grow single crystals of antimonides. Even with the incorporation of Bi into the Sb square net, the magnetic properties are not significantly affected. In addition, we have shown that the incorporation of Co into the Fe triangular sublattice leads to an itinerant magnetic system.

Emergence of Magnetic States in Pr2Fe(4-x)Co(x)Sb5 (1 < x < 2.5).

Single crystals of Pr2Fe(4-x)Co(x)Sb5 (1 < x < 2.5) were grown from a Bi flux and characterized by X-ray diffraction. The compounds adopt the La2Fe4Sb5 structure type (I4/mmm). The structure of Pr2Fe(4-x)Co(x)Sb5 (1 < x < 2.5) contains a network of transition metals forming isosceles triangles. The x ∼ 1 analogue is metallic and exhibits a magnetic transition at T1 ≈ 25 K. The magnetic moment obtained from the Curie-Weiss fit is 11.49(4) µ(B), which is larger than the spin-only Pr(3+) moment. The x ∼ 2 analogue orders magnetically at T1 ≈ 80 and T2 ≈ 45 K. This is the first case of the substitution of Co into the La2Fe4Sb5 structure type, evidenced by the increased concentration of dopant with decreased lattice parameters coupled with a change in the transition temperature with a change in the cobalt concentration. The added complexity in the magnetic behavior of the x ∼ 1 and 2 analogues indicates that the increased concentration of Co invokes an additional magnetic contribution of the transition metal in the sublattice. Furthermore, X-ray photoelectron spectroscopy measurements support the change in the oxidation states of transition metals with increasing cobalt concentration.

Spin density wave instability in a ferromagnet.

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.