Mössbauer and Nuclear orientation study of magnetic atom behaviour in AuFe below the percolation threshold

An⁵⁷Fe Mössbauer study and⁶⁰Co and⁵⁴Mn nuclear orientation (NO) studies of the Au₈₆Fe₁₄ alloy have been performed. A transition to the spin glass state was observed below approximately 50 K. NO of the⁶⁰Co impurity showed absence of the polarization of the⁶⁰Co nuclei. For⁵⁴Mn impurities the dependence of the Mn effective magnetic field on external field andtemperature was found.     

High order Correlation polarization potential for vibrational excitation scattering of diatomic molecules by low-energy electrons

This paper introduces a correlation--polarization potential with high order terms for vibrational excitation in electron--molecule scattering. The new polarization potential generalizes the two-term approximation so that it can better reflect the dependence of correlation and polarization effects on the position coordinate of the scattering electron. It applies the new potential on the vibrational excitation scattering from N₂ in an energy range which includes the ²П_g shape resonance. The good agreement of theoretical resonant peaks with experiments shows that polarization potentials with high order terms are important and should be included in vibrational excitation scattering.     

EPR Study of the Orientation Distribution Function of HO2 • Radicals Ordered by Light Irradiation

Partial orientational alignment of HO₂ ^• radicals in the matrix of glassy hydrogen peroxide was created by light irradiation. The orientation distribution function of the radicals was found by analyzing the angular dependence of electron paramagnetic resonance spectra. The direction of the dipole transition moment of HO₂ ^• in the molecular frame of reference was determined. Authors' address: Natalia A. Chumakova, Chemistry Department, Moscow State University, Leninskiye Gory 1/3, Moscow 119899, Russian Federation     

Polarization and energy barriers in ferroelectric pyridinium tetrafluoroborate

Measurements of the ¹H and ¹⁹F nuclear magnetic resonance (NMR) second moments were performed for a polycrystalline sample of (PyH)BF₄, whereas the shape of the ²H NMR line was analysed for a polycrystalline sample of (d₅PyH)BF₄. Asymmetry parameter δ has been calculated for four models of pyridinium cation reorientation among inequivalent potential energy minima, using the experimental value of the ¹H NMR second moment as well as the ²H NMR line width. From knowledge of the potential shape and the population of its minima, the temperature dependence of polarization for all the models has been found. From the comparison of the experimentally determined polarization with the calculated polarization, the most appropriate model of the pyridinium cation reorientation has been chosen.     

Temperature dependence of the localized magnon mode in MnF2 : Co

The localized magnon mode in MnF₂ : Co was studied by far infrared resonance techniques. The halfwidth at 8 K is 1 cm^-1, about three times smaller than that obtained by other experimental methods. Therefore, accurate measurements of the temperature dependence are possible between 8 and 40 K. The temperature dependence of the effective g-factor is attributed to spin wave renormalization. The half width is magnetic field dependent at higher temperatures and can be approximated by a T² law.     

Optical nuclear polarization for sensitivity enhancement of2H NMR single-crystal studies

A gain in detection sensitivity of more than three orders of magnitude is achieved in high-resolution solid-state²H nuclear magnetic resonance of monocrystalline fluorene-d₁₀ by applying optical nuclear polarization via excited triplet states of acridine-h₉ guest molecules. The sensitivity gain is utilized to measure the angular dependence (rotation pattern) of the²H nuclear magnetic resonance lines. In this way the principal values and orientations of all²H quadrupolar tensors are determined. Except for the methylene deuterons, all tensors belonging to the same molecule have one principal axis in common, namely the axis perpendicular to the molecular plane, showing that in the crystal lattice the fluorene molecule is in a planar configuration.     

Anomalous polarization characteristics of magnetic resonance in a quasi-one-dimensional CuGeO<Subscript>3</Subscript>: Co magnet

Doping of CuGeO₃ by 2% Co was found to cause a new magnetic resonance, which has anomalous polarization characteristics. In the Faraday geometry, where a microwave field B _ω is directed along certain crystallographic directions, this mode is suppressed, which indicates that the character of magnetic oscillations in this mode differs strongly from standard spin precession. This resonance coexists with the EPR on Cu²⁺ chains and is likely to be caused by an unknown collective mode of magnetic oscillations of an antiferromagnetic quantum S = 1/2 spin chain.     

Surface enhanced Raman scattering (SERS) of chemisorbed species on various kinds of metals and semiconductors

SERS spectra of pyridine adsorbed on various kinds of vacuum evaporated (10^?5 Torr) metals (Ag, Au, Ni, Pd, Pt, Ti and Co) and on single crystals of semiconductors (NiO and TiO₂) were obtained at room temperature. The peak frequencies as shifted from those of free pyridine are assigned to the bands of N-bonded pyridine (chemisorbed pyridine). The λ₀ dependence varied remarkably from metal to metal. The peak frequency and the λ₀ dependence for the pyridine adsorbed on NiO or TiO₂ are in good agreement with those on Ni or Ti, respectively, showing the chemical bonding between the N atom and the Ni or Ti atom. The effects of background and of polarization on the SERS spectra were examined in detail, thus revealing the orientation of the adsorbed molecules. Carbon monoxide chemisorbed on Ag was measured by infrared specular reflection as well as by SERS. The results indicate that chemisorbed species on the same substrate do not always give SERS. The SERS spectra obtained are well interpreted as being due to the mechanism of resonance Raman scattering via charge transfer excitation of the adsorbent-adsorbate interaction.     

Hyperfine Field in Ferromagnetic CoPd Alloys

The magnetic hyperfine interaction of ⁶⁰Co in the completely miscible alloy Co _x Pd_1−x was investigated for different values of x by measuring the nuclear orientation of ⁶⁰Co as function of temperature and by nuclear magnetic resonance of the oriented ⁶⁰Co nuclei. A broad resonance signal of Gaussian shape could be observed down to x=60%. The dependence of the mean magnetic hyperfine field on the Co concentration was observed to be linear very similar to that of experimental values of the magnetic moment per atom in the literature. The magnetic hyperfine field and its broad distribution are discussed in a simple model with RKKY interaction. This revised version was published online in September 2006 with corrections to the Cover Date.     

New polarization effect and collective excitation in S = 1/2 quasi-one-dimensional antiferromagnetic quantum spin chain

Anomalous polarization characteristics of magnetic resonance in CuGeO₃ doped with 2% Co impurity are reported. For the Faraday geometry, this mode is damped for the microwave field B _ω aligned along a certain crystallographic direction, showing that the character of magnetic oscillation differs from the standard spin precession. The observed resonance coexists with the ESR on Cu²⁺ chains; it is argued not to be caused by “impurity” EPR, as previously claimed, but to correspond to a previously unknown collective mode of magnetic oscillations in an S = 1/2 AF quantum spin chain. The text was submitted by the authors in English.     

Experimental study of the resonance radiation group delay in degenerate systems

The dependence of the polarization-and intensity-modulation group delay on the polarization of electromagnetic wave was studied experimentally for different transitions between the hfs components of the ⁸⁷Rb D₁ absorption line. It was found that the polarization-modulation delay strongly depends on the degeneracy structure of resonant transition and, in the general case, on the ellipticity of light-wave polarization. It is demonstrated that the polarization-modulation delay does not occur for the transitions not involving dark states. The polarization delay was studied as a function of the polarization ellipticity angle. The intensity-modulation delay was measured for the resonance radiation to show that it is observed for all ⁸⁷Rb D₁-line transitions and is independent of polarization.     

Polarization modulation spectroscopy of surface plasmon resonance

The features of surface plasmon resonance in gold nanofilms deposited on the surface of a total-internal-reflection prism have been investigated theoretically, using the Fresnel equation, and experimentally, with application of the polarization modulation technique. The angular characteristics of the polarization difference of the reflection coefficients for s-and p-polarized light, Δρ = R _s ² ? R _p ² , were measured in the wave-length range λ = 0.4–2.0 μm. It is shown that the characteristics of Δρ, in contrast to the results of standard measurements by the surface plasmon resonance method, have a resonance peak. Due to this, the characteristics of the polarization difference contain nonresonant components whose magnitudes are determined by the internal reflection coefficients for the metal and insulator; these parameters depend on the film thickness. The calculated and experimental data coincide when the model assumes exponential dependence of the refractive indices and extinction coefficients on the thickness of the metal film. It is established that the characteristic parameter of the exponential is a metal film thickness of 11.0 ± 0.5 nm, at which the film optical parameters correspond to the bulk characteristics.     

Magnetic circular dichroism in Co 2p photoemission of Co/Cu(1 1 13): Separation of the fundamental spectra

We measured high-quality Co 2p magnetic circular dichroism (MCD) spectra in photoemission for > 5 ML Co films grown on Cu(1 1 13) using a “complete” experiment, where the sample magnetization and the light helicity vector were reversed separately. We show how the four measured spectra, M^±P^±, can be used to make new linear combinations, which correspond to the circular dichroism in the angular dependence (CDAD), magnetic linear dichroism in the angular dependence (MLDAD) and MCD spectra. The integrated signals of the MLDAD and CDAD can be used to estimate the error caused by the difference in the degrees of magnetization and light polarization, respectively, in the opposite alignments. The MCD signal integrated over the entire 2p region does not average to zero, as one would have expected from the sum rule for photoemission to a non-interacting continuum state. There is a strong MCD signal in the entire region between the 2p _3/2 and 2p _1/2 main lines with pronounced satellite structure. The differences between the measured and calculated results for an independent-particle and an atomic model indicate the presence of interatomic electron correlation effects and configurational mixing. Received 26 September 2000     

Temperature behavior of the order parameter in Pb5Ge3O11

The temperature dependence of the spontaneous polarization in the Pb₅Ge₃O₁₁ lead germanate (PGO) is experimentally investigated using optical, magnetic resonance, and conventional electrical measurements. The deviations from the temperature dependence typical of second-order phase transitions at temperatures below 420 K are explained in terms of incomplete polarization switching and polarization induced by a residual depolarization field. The low-temperature anomalies are interpreted without consideration of additional structural transformations. The internal bias field is determined from the experimental temperature dependence of the perfect polarization of PGO single crystals in an electric field.     

The magnetic dipole moment of 55Co

The ground state g-factor for ⁵⁵Co has been measured as $\text{¦g¦= 1.378±0.001}$ by the technique of nuclear magnetic resonance on oriented nuclei. The temperature dependence of γ-ray anisotropy in the ⁵⁵Fe daughter decay determines both the 1408 keV level spin and the ⁵⁵Co ground state spin to be $\text{7}\text{2}$, and yields values of mixing ratios in the 1037 keV β-transition and the 477 keV γ-transition. The configuration mixing model is used to discuss 1f_{$\text{7}\text{2}$} moment systematics.     

Co-silicide layers studied by Mössbauer spectroscopy and Rutherford backscattering spectroscopy

Co-silicides were prepared with several techniques, such as annealing of evaporated Co-layers on a Si-substrate (silicide surface layers) and annealing of Co-implanted Si (buried silicide layers). By adding some⁵⁷Co to the stable⁵⁹Co, the formation of the various Co-silicides could clearly be followed as a function of annealing temperature by means of Mössbauer spectroscopy. In the case of surface silicide layers, Co₂Si, CoSi and CoSi₂ were formed subsequently. In the case of buried layers however, CoSi₂ was the only crystalline phase that could be observed. In both cases, the CoSi₂ spectra showed an anomalous side resonance. Moreover, it was found that when⁵⁷Fe was implanted (instead of⁵⁷Co), a drastic increase in the intensity of this side resonance could be detected by CEMS.     

Bias voltage dependence of tunneling magnetoresistance in granular C60–Co films with current-perpendicular-to-plane geometry

Voltage-dependence of the tunneling magnetoresistance effect in the granular C₆₀–Co films has been investigated for the samples with the current-perpendicular-to-plane geometry. The transport measurements under this geometry demonstrate that the granular C₆₀–Co films show an unusual exponential bias voltage dependence of the magnetoresistance ratio down to zero voltage. Small characteristic energies of less than 10's meV are derived from the temperature dependences of the characteristic voltage in the exponential relationship. Considering the magnitudes of the voltage drop between Co nanoparticles and also the effect of cotunneling on the energy values, the characteristic energies for the voltage-induced degradation of the spin polarization are found to show a satisfactory agreement with that for the thermally-induced one. It can be reasonably expected that the onset of magnetic disorder to the localized d-electron spins at the interface region of the C₆₀-based matrix (C₆₀–Co compound) with Co nanoparticles leading to the unusual voltage and temperature dependence of the magnetoresistance ratio and the spin polarization at low temperatures.     

Nuclear magnetic resonance (NMR) study of 73Ge in Ge[Co2]O4

Nuclear magnetic resonance of ⁷³Ge in Ge[Co₂]O₄ was investigated between 77°K and 290°K. Analysis on resonance shifts showed a presence of considerable everlap between the oxygen and germanium ions, suggesting the long-range superexchange interaction via CoOGeOCo paths.     

Untersuchung der Polarisationsebene von Photonen nach Compton-Streuung an polarisierten Elektronen

Two polarization phenomena in Compton scattering by polarized electrons were investigated. In the first experiment, the rotation of the polarization plane of photons passing through magnetized iron and gadolinium was measured. This effect arises from a spin dependence of the Compton forward scattering amplitude. For 228 and 333 keV photons and iron absorbers, the observed rotation angles areφ ₀=(3.90±0.57) ×10^?3 rad · cm^?2 and (4.75±0.58)×10^?3 rad · cm^?2, respectively. Secondly, the orientation of the photon polarization plane after scattering of unpolarized photons by polarized electrons was measured. This experiment tests time reversal invariance in quantum electrodynamics. No dependence of the polarization plane on the direction of the electron spin was found within 2×10^?3.