Nobuyoshi Hosoito

Successive magnetic ordering in CuFeO2 : a new type of partially disordered phase in a triangular lattice antiferromagnet

Magnetic ordering in a rhombohedrally stacked triangular lattice antiferromagnet, CuFeO 2 was investigated on powder and single crystals by measurements of neutron diffraction, magnetic susceptibility, Mossbauer effect together with Monte Carlo simulations. CuFeO 2 was found to exhibit transitions at T N1 =16±0.5 K and T N2 =11±0.5 K. The low temperature phase has an orthorhombic magnetic unit cell with collinear moments along the c -axis, whereas the intermediate temperature phase is a partially disordered phase with a monoclinic magnetic unit cell where 1/5 of moments remain paramagnetic. The Monte Carlo simulations for the Ising spin tiangular lattice antiferromagnet can reproduce the observed successive transition in the case of the exchange parameters 4 J 3 <2 J 2 ∼ J 1 <0. This combination of exchange parameters gives an account for the stability of the collinear magnetic structure in CuFeO 2 .

Magnetic Properties of Fe-V Multilayered Films with Artificial Superstructures

Multilayered films with artificial superstructures were synthesized by alternate deposition of Fe and V in ultrahigh vacuum. X-ray diffraction confirmed the establishment of artificial periodicities with very short wavelengths, e.g. Fe(4 A)-V(8 A) of the minimum case, and bcc [110] texture. Magnetic properties as a function of Fe layer thickness were studied from Mossbauer and SQUID measurements. Local magnetization at the Fe interface layer contacting with V is discussed from Mossbauer results on surface-selectively enriched samples and also from polarized neutron diffraction experiments.

Magnetic Determination of Ginzburg-Landau Coherence Lengthfor Organic Superconductor κ-(BEDT-TTF)2X (X=Cu(NCS)2,Cu[N(CN)2]Br): Effect of Isotope Substitution

The Ginzburg-Landau coherence lengths are determined from the temperature dependence of the magnetization by fitting measured results with the renormalization theory of the fluctuation developed for layered superconductors. The derived interlayer coherence lengths are 6±2 A and 3.2±0.5 A for κ-(BEDT-TTF) 2 Cu[N(CN) 2 ]Br and κ-(BEDT-TTF) 2 Cu(NCS) 2 , respectively, which are remarkably shorter than the interlayer spacing. It is also found that these coherence lengths are virtually unaffected by isotope substitution in BEDT-TTF molecules.

On the metallic states in highly conducting iodine-doped polyacetylene

Abstract The temperature dependences of the conductivity, the magnetic susceptibility and the thermoelectric power of iodine-doped highly conducting ( ca. 10 5 S/cm) polyacetylene are presented. The conductivity shows non-activated behavior down to mK region. The density of states derived from the susceptibility and the thermoelectric power is consistent with one-electron band calculation. The electron localization due to the quasi one-dimensionality inherent to polymer is considered to beviolated by mesoscopic potential disorder, resulting in metallic strands, which are connected by tunneling.

Magnetic Properties of Fe/Dy Artificial Superstructured Films

Artificial superstructured films composed of two magnetic constituents, Fe and Dy, were prepared by means of alternate deposition in ultrahigh vacuum, and magnetic properties have been studied from Mossbauer spectroscopy and SQUID magnetization measurements. When Fe layers are thinner than 20 A, the structure is amorphous and the magnetic transition temperature is about 270 K, which agrees with that of amorphous pure Fe. When Fe layers are thicker (e.g. 30 A) and Dy layers are thinner than 20 A, a perpendicular magnetic anisotropy becomes significant and the spin direction of Fe changes with temperature.

Evidence for antiferromagnetic coupling between Fe layers through Cr from neutron diffraction

Small-angle neutron diffraction measurements have been made for a multilayer, [Fe(27 A)/Cr(12 A)]×30, at room temperature. It was found that the magnetizations of adjacent Fe layers are coupled in antiparallel. The decrease of antiferromagnetic peak intensity as a function of external field was observed. It is confirmed that the giant magnetoresistance in Fe/Cr multilayers is due to the antiferromagnetic ordering of Fe layers caused by the interlayer coupling across an intervening Cr layer.

Magnetic Properties of Fe–Mg Artificial Superstructure Films

Artificial metallic superlattices were synthesized from an insoluble combination, Fe–Mg, by alternate deposition in ultrahigh vacuum. The shortest artificial periodicity confirmed by X-ray diffraction was Fe(4 A)–Mg(4 A). A sample with Fe monolayers, [Fe(1 A)–Mg(16 A)], was prepared successfully. Magnetic properties were studied from Mossbauer and SQUID measurements. Novel magnetic properties were observed in extremely thin Fe layers. Mg-diluted Fe alloys prepared by co-deposition were also investigated and have been compared with artificial superstructure films.

Mössbauer study of FeMg multilayered films with artificial superstructures

Abstract By alternate deposition in ultrahigh vacuum, multilayered films with artificial superstructures were prepared from mutually insoluble elements, Fe and Mg. X-ray diffraction confirmed the establishment of compositional modulation with very short wavelengths such as Fe(4A)Mg(4A), or Fe(2A)Mg(16A). 57 Fe Mossbauer spectra at 4.2K indicate that Fe layers sandwiched in between Mg layers are ferromagnetic even when the thickness is 2A.

Microscopic Magnetic Properties of Fe-V Metallic Superlattice Investigated from 51V NMR

A 51 V NMR investigation has been made on the magnetic properties, particularly the interface magnetism, of an Fe-V metallic superlattice. Three kinds of signals have been observed; the two kinds associated with the atomic layers having induced V magnetic moments at the interface region, and the other with the nearly nonmagnetic sites in the more distant V layers. From the distribution of the hyperfine field it is concluded that the interface atomic layer is the mixture of Fe(50%)-V(50%) atomic concentration with a positive atomic short range order, and that the compositional modulation is restricted to three atomic layers at the interface. It is also found that the hyperfine field at the nearly nonmagnetic sites has an anisotropy which can not be expected in random alloys.

FERROMAGNETIC BEHAVIOR OF A TETRAFLUOROTETRACYANOQUINODIMETHANIDE SALT AT ROOM TEMPERATURE

Abstract Recrystallization of N(CH 3 ) 4 + · TCNQF 4 −. in the presence of TCNQF 4 from acetonitrile/ether gave a crystal of { N(CH 3 ) 4 + · TCNQF 4 −. }· 1 2 ( TCNQF 4 ) , which very interestingly exhibited ferromagnetism at temperature. This organic ferromagnet has characteristics of very small saturation moment (1.03 × 10 −3 μ B /molecule) and coercive field (≈ 25 Oe). The origin of this ferromagnetism could be attributed to a unique crystal structure of the radical anion salt: the TCNQF 4 /TCNQF 4 −. and N(CH 3 ) 4 + ion layers are alternately stacked and in the TCNQF 4 /TCNQF 4 −. layer each pair of two TCNQF 4 −. molecules is arranged in a completely perpendicular manner to each of one TCNQF 4 molecule.