Session F1: CM2: Condensed Matter

Ferromagnetic and non-ferromagnetic dust interactions in complex plasmas

A GEC rf reference cell is used to create groupings of 4.5 micron melamine formaldehyde dust particles and also of 4.5 micron ferromagnetic dust particles. It is shown that ferromagnetic dust particles respond to variations in chamber pressure in a similar fashion to non-ferromagnetic dust. It is also found that non-ferromagnetic dust particles exhibit more short-range ordering and structure than ferromagnetic dust particles for the range of pressures and powers tested.   

Crystal Fields and Metamagnetism in NdNiPb and Nd$_{5}$NiPb$_{3}$

We report magnetic, transport and thermodynamic measurements for recently-synthesized NdNiPb (orthorhombic TiNiSi-type structure) and Nd$_{5}$NiPb$_{3}$ (hexagonal Hf$_{5}$CuSn$_{3}$-type structure), as well as non-magnetic Y-based analogs. High-temperature Curie-Weiss fits yield effective moments of 3.59 $\mu_{B}$ for NdNiPb and 3.70 $\mu_{B}$ for Nd$_{5}$NiPb$_{3}$. These are close to the Nd$^{3+}$ ionic moment, 3.62 $\mu_{B}$, showing that Ni is nonmagnetic in both cases. For NdNiPb a peak seen in both the magnetization and specific heat at 3.5 K indicates an apparent antiferromagnetic transition at that temperature. Specific heat measurements show this transition to be formed from crystal-field-split Nd magnetic levels, and we have made preliminary estimates of the crystal field scheme. Nd$_{5}$NiPb$_{3}$ exhibits two magnetic transitions, an antiferromagnetic transition at 42 K and an apparently weak ferromagnetic canting transition at 8 K. Entropy measurements also show the ground state to be composed of crystal-field-split doublet of magnetic levels. $M$-$H$ curves show metamagnetism at temperatures between the two magnetic transitions. The materials are metallic, and we will discuss transport results providing a further probe of the magnetic behavior.   

Magnetocrystalline and Shape Anisotropy in Mn$_{12}$-acetate Micro-Crystals

We have aligned micro-crystals of Mn$_{12}$-acetate in a solvent bath by applying an external magnetic field H = 0.5 T at room temperatures. Various states ranging from randomly-oriented to well-oriented state of the same suspension sample have been prepared by applying an external magnetic field 0 T $\le $ H $\le $ 1 T at room temperature. DC magnetization has subsequently been measured for these states and alignment behavior was studied as a function of the field. For T $<$ 50 K, the well-aligned state shows a higher magnetization than the randomly-oriented state of the sample. However, for T $>$ 100 K, where the alignment occurs, no significant difference in magnetization was observed between the different states. The observed magnetization difference below 50 K comes from the magnetocrystalline anisotropy. And, shape anisotropy of the micro-crystals may be the main origin of the observed alignment.   

Studies of oxidation and thermal reduction of the Cu(100) surface using a slow positron beam

Positron probes of surfaces of oxides that play a fundamental role in modern science and technology are capable to non-destructively provide information that is both unique to the probe and complimentary to that extracted using other more standard techniques. We discuss recent progress in studies of oxidation and thermal reduction of the Cu(100) surface using positron-annihilation-induced Auger-electron spectroscopy (PAES). PAES measurements show a large increase in the intensity of the Cu M2,3VV Auger peak as the sample is subjected to a series of isochronal anneals in vacuum up to annealing temperature 300\r{ } C. The intensity then decreases monotonically as the annealing temperature is increased to 600\r{ } C. Experimental PAES results are analyzed by performing calculations of positron surface states and annihilation probabilities of surface-trapped positrons with relevant core electrons taking into account the charge redistribution at the surface and surface reconstruction. The effects of oxygen adsorption and defects on localization of the positron surface state wave function and positron annihilation characteristics are also analyzed. Possible explanations are provided for the observed behavior of the intensity of the positron annihilation induced Cu M2,3VV Auger peak with changes of the annealing temperature.   

Ferromagnetism in Mn-implanted Ge and epitaxial GeC

20 keV energy Mn ions were implanted in two samples: 1) bulk Ge (100) and 2) a 250 nm thick epitaxial GeC film, grown on a Si (100) wafer. The GeC thin film was grown by UHV chemical vapor deposition using a mixture of germane (GeH$_4$) and methylgermane (CH$_3$GeH$_3$) gases and contains less than 1\% carbon. X-ray diffraction data shows a single crystal phase for the GeC film, and the surface rms roughness is about 0.3 nm, measured with AFM. The Mn implant dose was $1.1\times10^{16}$/cm$^2$ at a temperature of 300$^{\circ}$ C for both samples. For this relatively low energy Mn ion implant, the range is about 17 nm and the straggle is about 9 nm. A SQUID magnetometer study shows ferromagnetism in both samples. While the Curie temperature for both samples is about 180 K, the in-plane saturated magnetic moment per unit area for the first sample is about $2.2\times10^{-5}$emu/cm$^2$ and that for the second sample is about $3.0\times10^{-5}$emu/cm$^2$. These results show clear enhancement of magnetic properties of the Mn-implanted GeC thin film over the identically implanted Ge layer due to the presence of a small amount of carbon.   

Geometric and Electronic Structure of Dodecanethiol SAMs Grown on Au, Ag, Cu, and Pt Crystals

The geometric and electronic structure of dodecanethiol (C$_{12}$H$_{25}$SH) SAMs on Au(111), Ag(111), Cu(111), and Pt(111)) substrates has been studied using low energy electron diffraction and angle-resolved ultra-violet photoelectron spectroscopy. The SAMs were grown both in solution and by vapor deposition in UHV. The electronic structure of the fully saturated SAM is similar on all of these substrates, with peaks observed at binding energies of 6.5, 10, 14, and 20 eV. The geometric structure of the molecular films at intermediate coverages is different for each substrate. Growth on Au proceeds through a well-ordered lying-down phase followed by a disordered phase and a well-ordered $\surd $3 standing-up phase at saturation. Initial growth on Pt(111) shows first a p(2x2)symmetry followed by a $\surd $3 symmetry, which indicates that there is dissociative adsorption on Pt. This is followed by a disordered phase at saturation. Films on Ag and Cu show a great deal of disorder at all stages of growth.   

X-Ray Absorption Spectroscopy Study of Iron Silicon Germanide and Osmium Silicide Epitaxial Films

Some of the iron- and osmium-based metal silicide and germinide phases have been predicted to be direct band gap semiconductors. Therefore, they show promise for use as optoelectronic materials. We have used synchrotron-based x-ray absorption spectroscopy to study the structure of iron silicon germanide and osmium silicide films grown by molecular beam epitaxy. Osmium silicide films which are primarily in the Os$_{2}$Si$_{3}$ phase and a series of Fe(Si$_{1-x}$Ge$_{x})_{2}$ films with a nominal Ge concentration of up to x = 0.04 have been grown. X-ray absorption near edge structure (XANES) measurements on both the iron silicon germinide and osmium silicide films has been performed. An absorption edge shift of 0.9 eV is observed for the osmium silicide films; however, no shift was observed for the iron silicon germinide films. Extended x-ray absorption fine structure (EXAFS) measurements have also been performed on the iron silicon germinide films. The nearest neighbor coordination corresponding to the $\beta $-FeSi phase of iron silicide provides the best fit with the EXAFS data.   

Interaction between silicon and thin films of hafnium oxide

Thin films (20 {\AA}) of hafnium were deposited on silicon substrates at base pressure of high 10$^{-9}$ Torr. The substrate temperature was kept at 100, 200, 300, 400, 500, and 600\r{ }C during deposition. The interfaces thus formed were analyzed \textit{in situ} by the technique of x-ray photoelectron spectroscopy using Mg anode as the source of excitation. The hafnium 4f, silicon 2p and oxygen 1s core level regions were investigated. The spectral data were obtained at various take-off angles to investigate the reactivity at various depths. The spectral data show that hafnium gets deposited as HfO$_{2}$. As the substrate temperature is increased, changes in the hafnium and oxygen core regions were observed. The data show that HfO$_{2}$ gets reduced either to elemental hafnium or to hafnium-suboxide as the substrate temperature is increased. No spectral changes were observed in the silicon core region indicating no chemical reactivity between HfO$_{2}$ and silicon till at least 600\r{ }C.