Synthesis and characterization of highly textured Pt–Bi thin films

Pt–Bi films were synthesized on glass and thermally oxidized silicon substrates by e-beam evaporation and annealing. The structures were characterized using X-ray diffraction (XRD) and transmission electron microscopy/selected area electron diffraction (TEM/SAED) techniques. Single-phase PtBi was obtained at an annealing temperature of 300°C, whereas a higher annealing temperature of 400°C was required to obtain the highly textured γ-PtBi₂ phase. TEM/SAED analysis showed that the films annealed at 400°C contain a dominant γ-PtBi₂ phase with a small amount of β-PtBi₂ and α-PtBi₂ phases. Both the PtBi and γ-PtBi₂ phases are highly textured in these two kinds of film: the c-axis of the hexagonal PtBi phase is mostly in the film plane, whereas the c-axis of the trigonal γ-PtBi₂ phase is perpendicular to the film plane. The electrical resistivity of the film with the γ-PtBi₂ phase was smaller by one order of magnitude than that of the film with the PtBi phase.     

Structure and magnetism of Co:CoO core–shell nanoclusters

Transmission electron microscopy (TEM) and electron diffraction (ED) are used to investigate the nanostructures of two ensembles of Co:CoO core–shell particles. TEM images show that particles of size about 12 nm are almost fully oxidized, while particles with size about 18 nm have a core–shell structure where a Co core is surrounded by a shell of CoO. ED simulation confirms that the larger particles have an fcc-structured Co core and a rock-salt CoO shell structure, while the smaller particles mostly have the rock-salt CoO structure. The core–shell structure is responsible for the unusual magnetic properties of the Co:CoO nanoclusters, especially the occurrence of inverted hysteresis loops (proteresis), but previous research has been indirect, largely based on magnetic measurements and on a cross-comparison with granular materials. Our measurements show that the structures have ferromagnetic fcc Co cores of varying sizes down to 1 nm which are surrounded by antiferromagnetic rock-salt CoO shells. The core radii obtained from the TEM pictures are used to estimate the exchange interactions responsible for proteresis and to pinpoint the core-size window in which proteresis occurs.     

Metal-iodide cyclodextrins as model quasi-one-dimensional compounds

The synthesis and electrical and magnetic properties of two metal-iodide cyclodextrin compounds are described. Both Ba and Fe ions were added to the α-cyclodextrins to form needle-shaped single crystals. The compounds have electrical conductivities of about 10^?9 (ω cm)^?1 at room temperature. The Ba compound is diamagnetic but the Fe compound shows Curie-Weiss behavior in its susceptibility between 1.5 and 108 K, with an effective moment of 4.5 μ_B and a Weiss temperature of ?5.6 K. The results indicate antiferromagnetic interactions between Fe(II) ions. A discussion is given of some problems associated with synthesizing quasi-one-dimensional conductors or magnetic compounds, using the cyclodextrins as a basis.     

Magnetic properties of uranium — Transition-metal glasses

Results of resistivity, magnetization and susceptibility measurements are reported on U₆₆T₃₄ where T denotes Fe, Co, or Ni. Magnetic susceptibility data on each of the alloys displayed thermomagnetic-history effects indicating some spin-glass like character. The high-field magnetization data showed a significant non-linearity indicating a contribution to the magnetization from a cluster term. The evidence suggests that these alloys form cluster glasses in which only those T atoms whose local environment is sufficiently dense in T atoms bear a stable magnetic moment.     

Anomalous Hall effect and electron transport in ferromagnetic MnBi films

The electron transport properties of highly c-axis oriented MnBi thin films of various thicknesses have been investigated. Samples are metallic but the low temperature resistivity shows an unusual T(3) dependence. Transverse Hall effect measurements show that both the ordinary and anomalous Hall coefficients decrease with decreasing temperature below 300 K, but the ordinary Hall coefficient (R(0)) undergoes a sign reversal around 105 K, where the magnetic anisotropy also changes sign. Analysis of the Hall data for various samples shows that the anomalous Hall coefficient (R(s)) exhibits a strong ρ(2) dependence, where ρ is the longitudinal resistivity.