Hyun-Joo Koo

Spin exchange interactions and magnetic structures of extended magnetic solids with localized spins: theoretical descriptions on formal, quantitative and qualitative levels

Low-energy excitation energies of a magnetic solid with localized spins are probed by magnetic susceptibility, neutron scattering and Raman scattering measurements, and are analyzed using a spin Hamiltonian with a set of spin exchange parameters. The nature and values of the spin exchange parameters deduced from this analysis depend on what spin exchange paths one includes in the spin Hamiltonian. In this article, we review how spin exchange interactions of magnetic solids with localized spins are described on formal, quantitative and qualitative theoretical levels, investigate antisymmetric and anisotropic interactions for general spin dimers, and discuss the spin exchange interactions and magnetic structures of various extended magnetic solids on the basis of spin dimer analysis. Strongly interacting spin exchange paths of a magnetic solid are determined by the overlap between its magnetic orbitals, so that the strongly interacting spin unit of a magnetic solid does not necessarily have the same geometrical feature as does the arrangement of its magnetic ions or spin-carrying molecules. Therefore, in interpreting results of magnetic susceptibility, inelastic neutron scattering or Raman scattering measurements, it is essential to employ a set of spin exchange parameters chosen on the basis of proper electronic structure considerations. Spin dimer analyses based on extended Huckel tight binding calculations provide a reliable and expedient means to study the relative strengths of superexchange and super-superexchange spin exchange interactions.

Study of the 18-electron band gap and ferromagnetism in semi-Heusler compounds by non-spin-polarized electronic band structure calculations☆

Abstract Based on non-spin-polarized electronic band structure calculations, we examined why the electronic structure of a semi-Heusler compound ABX has the 18-electron band gap, why 17-and 19-electron ABX compounds can be weakly ferromagnetic based on the Stoner criterion, and how the ferromagnetism of the 22-electron ABX compounds differs from that of the 17- and 19-electron analogs. To a first approximation, the electronic structure of ABX with 18 or more valence electrons is described in terms of the d 10 ion for B, the s 2 p 6 ion for X, and the d n ion for A ( n =0, 1 and 4 for the case of 18, 19 and 22 valence electrons, respectively). Even for a 17-electron ABX compound the d-electron count for the electronegative transition metal B is close to d 10 . The ferromagnetism of the 17- and 19-electron ABX compounds is explained in terms of the Stoner criterion, and that the 22-electron ABX compounds by the strong tendency for the d-electrons of the d 4 ion to localize.

Effect of magnetic dipole-dipole interactions on the spin orientation and magnetic ordering of the spin-ladder compound Sr3Fe2O5.

First-principles density functional theory calculations show that the spin-lattice of Sr(3)Fe(2)O(5) is practically 2D in terms of its spin-exchange interactions. The magnetic dipole-dipole interactions are found to be essential for the 3D magnetic ordering of Sr(3)Fe(2)O(5) at a very low temperature.

A genuine two-dimensional Ising ferromagnet with magnetically driven re-entrant transition.

Inorganic compounds made up of low-dimensional ferromagnetic (FM) units display fascinating properties and provide a rich opportunity to investigate FM ground states, fieldinduced transitions, and magnetization steps. Even the spin-valve effect, realized in multilayer thin films, is found in the magnetic metal Ca3Ru2O7 [5] and its Cr-doped analogue in which FM double-perovskite layers are antiferromagnetically coupled. The inorganic compound Cr2Si2Te6, also consisting of FM layers, turns out to be a unique example of a bulk 2D FM Ising system. It is a great synthetic challenge to discover new magnetic transition-metal oxides made up of FM layers. In searching for such materials, a rational approach rather than by a blind exploration of chemical systems should be used. In general, a transition-metal cation at a coordinate site with three-fold or higher rotational symmetry can lead to uniaxial magnetism if its d electron count and spin state are such that there occurs an unevenly-filled degenerate level, as found for high-spin Fe ions at linear-coordinate sites and high-spin Co and Co ions at trigonal-prismatic sites. In principle, high-spin FeO6 octahedra can support uniaxial magnetism as long as they possess three-fold rotational symmetry. It can be imagined that isolated FM layers form from such FeO6 octahedra by edge-sharing because the Fe-O-Fe angle will be close to 908 so that the nearest-neighbor spin exchange would be FM. Our guided search for such a magnetic system led to the synthesis of BaFe2(PO4)2 that turns out to be the first oxide 2D Ising ferromagnet. It consists of FM honeycomb layers of edge-sharing FeO6 octahedra containing high-spin Fe 2+ ions. Such FeO6 octahedra showing uniaxial magnetism are expected to be susceptible to Jahn–Teller (JT) instability. Indeed, on cooling, BaFe2(PO4)2 undergoes a rare re-entrant structural transition owing to the competition between uniaxial magnetism and the JT distortion.

Investigations of the oxidation states and spin distributions in Ca3Co2O6 and Ca3CoRhO6 by spin-polarized electronic band structure calculations

Abstract Spin-polarized electronic band structure calculations were carried out for the magnetic solids Ca 3 CoMO 6 (M=Co, Rh), and the local electronic structures of their transition metal atoms at the octahedral and trigonal prism sites were examined. Our calculations show that Ca 3 CoMO 6 (M=Co, Rh) has four unpaired spins per formula unit, the magnetic moment comes predominantly from the trigonal prism site, and the metal atoms of both the octahedral and trigonal prism sites have the oxidation state +3.

Half-metallic ferromagnetism and large negative magnetoresistance in the new lacunar spinel GaTi3VS8.

The lacunar spinel compounds GaTi(4-x)V(x)S(8) (0 < x < 4), consisting of Ti(4-x)V(x) tetrahedral clusters, were prepared and their structures were determined by powder X-ray diffraction. The electronic structures of GaTi(4-x)V(x)S(8) (x = 0, 1, 2, 3) were examined by density functional calculations, and the electrical resistivity and magnetic susceptibility of these compounds were measured. Our calculations predict that GaTi(3)VS(8) is a ferromagnetic half-metal, and this prediction was confirmed by magnetotransport experiments performed on polycrystalline samples of GaTi(3)VS(8). The latter reveal a large negative magnetoresistance (up to 22% at 2 K), which is consistent with the intergrain tunnelling magnetoresistance expected for powder samples of a ferromagnetic half-metal and indicates the presence of high spin polarization greater than 53% in GaTi(3)VS(8).

Determination of the Spin-Lattice Relevant for the Quaternary Magnetic Oxide Bi4Cu3V2O14 on the Basis of Tight-Binding and Density Functional Calculations

The quaternary magnetic oxide Bi4Cu3V2O14 consists of Cu4O8 triple chains made up of corner-sharing CuO4 square planes. To determine its spin-lattice, the spin exchange interactions of Bi4Cu3V2O14 were evaluated by performing a spin dimer analysis based on tight-binding calculations and a mapping analysis based on first principles density functional theory calculations. Both calculations show that the spin-lattice of Bi4Cu3V2O14 is not an antiferromagnetically coupled diamond chain, which results from an idealized view of the structure of the Cu4O8 triple chain and a neglect of super-superexchange interactions. The correct spin-lattice is an antiferromagnetic chain made up of antiferromagnetic linear trimers coupled through their midpoints via super-superexchange interaction, which predicts that Bi4Cu3V2O14 has an antiferromagnetic spin ground state and has no spin frustration, both in agreement with experiment.

Crystal structure, physical properties and electronic structure of a new organic conductor β″-(BEDT-TTF)2SF5CHFCF2SO3

John A. Schlueter,* Brian H. Ward, Urs Geiser, Hau H. Wang, Aravinda M. Kini, James Parakka, Emilio Morales, H.-J. Koo, M.-H. Whangbo, R. W. Winter, J. Mohtasham and Gary L. Gard Materials Science Division, Argonne National Laboratory, Argonne IL 60439. E-mail: JASchlueter@anl.gov Department of Chemistry, North Carolina State University, Raleigh, USA NC 27695-8204 Department of Chemistry, Portland State University, Portland, USA OR 97207-0751

Novel soft-chemistry route of Ag2Mo3O10·2H2O nanowires and in situ photogeneration of a Ag@Ag2Mo3O10·2H2O plasmonic heterostructure.

Ultrathin Ag2Mo3O10·2H2O nanowires (NWs) were synthesized by soft chemistry under atmospheric pressure from a hybrid organic-inorganic polyoxometalate (CH3NH3)2[Mo7O22] and characterized by powder X-ray diffraction, DSC/TGA analyses, FT-IR and FT-Raman spectroscopies, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Their diameters are a few tens of nanometers and hence much thinner than that found for silver molybdates commonly obtained under hydrothermal conditions. The optical properties of Ag2Mo3O10·2H2O NWs before and after UV irradiation were investigated by UV-vis-NIR diffuse reflectance spectroscopy revealing, in addition to photoreduction of Mo(6+) to Mo(5+) cations, in situ photogeneration of well-dispersed silver Ag(0) nanoparticles on the surface of the NWs. The resulting Ag@Ag2Mo3O10·2H2O heterostructure was confirmed by electron energy-loss spectroscopy (EELS), X-ray photoelectron spectroscopy (XPS), and Auger spectroscopy. Concomitant reduction of Mo(6+) and Ag(+) cations under UV excitation was discussed on the basis of electronic band structure calculations. The Ag@Ag2Mo3O10·2H2O nanocomposite is an efficient visible-light-driven plasmonic photocatalyst for degradation of Rhodamine B dye in aqueous solution.

Trends in the structure and bonding in the layered platinum dioxide and dichalcogenides PtQ2 (Q=O, S, Se, Te)

Abstract The structure and bonding of the layered platinum dioxide and dichalcogenides PtQ2 (Q=O, S, Se, Te) were analyzed on the basis of electronic band structure calculations using the full potential linearized augmented plane wave method. We examined why the c/a ratio in PtQ2 is considerably small compared with the value expected from the consideration of closely packed Q atoms (i.e., ∼1.40 vs. 1.67), and identified the electronic factor that causes the semiconducting properties in PtO2 and PtS2, the semimetallic property in PtSe2, and the metallic property in PtTe2. To a first approximation, the oxidation states of oxygen and platinum in PtO2 can be regarded as –2 and +4, respectively, but this picture is not applicable to PtTe2. As the ligand Q is changed from O to S to Se to Te, the energy gap between the Pt 5d and the ligand p levels gradually decreases, so that the ionic character of the Pt–Q bonding in PtQ2 is gradually diminished.