H. Shechter

Thermal and magnetic studies of the nearly one-dimensional antiferromagnetic system CsNiBr3

Abstract Magnetic susceptibility, specific heat and 133Cs magnetic resonance measurements in a single crystal of CsNiBr3 are reported. The data reveal two magnetic transitions separating the paramagnetic phase from the antiferromagnetic ground state. At the higher transition temperature TN2 = (14.25 ± 0.05)K a net magnetic moment is observed only along the hexagonal c-axis, while only below the lower transition temperature TN1 = (11.75 ± 0.05)K a perpendicular component of the magnetic moment appears also. Above TN2 CsNiBr3 can be described as a one-dimensional antiferromagnet with intrachain exchange interaction J k B = −(17.0 ± 0.2) K and single-ion anisotropy constant D k B ≊ −1.5 K . Below TN1, the data are consistent with the non-colinear triangular structure of the Ni2+ moments proposed previously for the isomorphic crystal CsNiCl3. A reduced value of the zero-temperature susceptibility over the classical value is found and atrributed to the zero point deviations.

Mössbauer spectrometer calibration using 57Fe enriched metallic iron

Abstract The use of 57Fe enriched metallic iron foils as absorbers to calibrate Mossbauer spectrometers can lead to serious discrepancies in the calculated constants. These discrepancies arise from the fact that small amounts of carbon in the metallic foils give rise to apparent shifts in line positions due to a change in the magnitude of the hyperfine fields acting on the iron nuclei. After proper preparation, enriched iron foils can be used for calibration purposes. A velocity calibration based on data for a low carbon (0.02 wt%) iron foil is internally consistent with published NMR data. An accurate value for the quadrupole splitting in sodium nitroprusside and the isomer shift of this absorber with reference to metallic iron are presented.

Edge Melting of Monolayers below the 2D Melting Point

Edge melting of submonolayer films (analogous to surface melting in bulk solids) of Fe(CO)5 adsorbed on graphite causes smooth deviation of the Mossbauer spectral intensity I(T) from exponential temperature dependence for observation in direction parallel to the film-surface. This anomaly starts at T 0.9Tm (2D) and lasts until Tm (2D) is reached. The gradual I(T) changes are associated with gradual reduction of the average solid island-size at temperatures close to Tm (2D) when peripheral island-molecules become mobile.

Thermodynamic study of tetramethyltin, Sn(CH3)4, adsorbed on (0001) graphite planes

Vapour adsorption isotherms of Sn(CH3)4 on graphite have been determined between 230 and 296 K. All the data are interpreted in terms of two-dimensional mobile adsorption. Cross-sectional adsorption areas of different Sn(CH3)4 arrangements are discussed and comparison is made with X-ray diffraction results. Various thermodynamic quantities relating to adsorption are given as a function of surface coverage. Calculated isosteric heats of adsorption are compatible with those obtained from low-temperature Mossbauer binding-energy data at the same coverage. The two-dimensional critical temperature of the first monolayer of Sn(CH3)4 has been determined.

Comments on the dynamical properties of iron pentacarbonyl, Fe(CO)5, adsorbed on (0001) graphite

Abstract A fraction of a monolayer of Fe(CO) 5 was deposited on a clean Papyex stack following an adsorption vapor pressure isotherm. Mossbauer spectra for k γ parallel and perpendicular to the film surface yield evidence of a first-order phase transition at T ⋍ 120 K. The asymmetry of the spectrum suggests a possible average list of the molecular axis of ≈50° to the surface normal.

Mössbauer spectroscopy of 119mSn employing a solid state detector

Abstract The maximum resonance cross-section σ0 of 119mSn and the ƒ factors of several absorbers of tin were calculated by means of the Mossbauer effect. A high-resolution silicon-lithium drifted detector was employed to detect the 23.9 keV Mossbauer γ-rays and resolve them from the 25.8 keV X-rays of 119mSn. Thus, a correct background subtraction was possible and accurate values of 1.328±0.003 × 10−18 cm2, for σ0 and 0.63±0.02, 0.45±0.02, 0.050±0.005 for the >ƒ factors of BaSnO3, SnO2 and white tin metal, respectively, were obtained.

Nanoscopic fluctuational dynamic model of anomalous temperature and concentration dependences in the Sn Mössbauer isomer shift in Ag-Sn alloys

A nanoscopic fluctuational dynamic model is suggested for observed anomalous reversible changes of the M?ssbauer isomer shift (ARCOMIS) found recently in Ag-Sn alloys not affected by long ageing. The model considers fluctuation-induced nanoscale large transient atomic displacements (LTADs) and material disordering. They cause electron localization, taking place simultaneously and permanently in the nanometre vicinities of M?ssbauer impurity atoms in the entire alloy. This causes an additional enhancement in the electron charge density |?f(0)|2 at the M?ssbauer nuclei, leading to the ARCOMIS at temperature Ts. These phenomena are generated by persistent sequences of nanoscale short-lived (picosecond) large energy fluctuations (SLEFs) of atomic particles of the peak thermal energy 0p>?E>>kT, which occur simultaneously and constantly in the material. The model leads to the following results. (i)?A SLEF-mediated dynamic mechanism for the ARCOMIS occurring at relatively low temperatures Ts is proposed. (ii)?The temperature Ts(Ag-Sn)?500?K is calculated, in good agreement with observations. (iii)?The narrow temperature interval ?T 3% is suggested. (vi)?An explanation is offered for the absence of the ARCOMIS in SnO2 material. (vii)?Similar results are obtained for the Au-Sn alloys not affected by long ageing, in good agreement with tentative experimental data.

Dynamical properties of physisorbed monolayers: Mössbauer studies

Applications of Mossbauer spectroscopy to study the dynamical properties of condensed physisorbed films are discussed. Mossbauer spectroscopy has been proven to be a sensitive probe for observing changes in the neighbourhood of phase transitions. The angular and temperature dependence of Mossbauer spectral intensity makes it possible to distinguish the dynamical behavior of adsorbed molecules such as diffusion in the film plane from that in the direction perpendicular to it. Examples of 2D melting and edge melting which occur in submonolayers of Sn(CH 3 ) 4 and Fe(CO) 5 physisorbed on graphite and MgO are presented.

Anomalous concentration dependence of the Mössbauer isomer shift of Ag-Sn alloys

The position of the Mossbauer resonant absorption line of was measured at various temperatures and concentrations of substitutional Sn impurities in the -phase of the Ag-Sn alloy. Whereas 1.0 and 2.0 at.% Sn alloys showed the expected temperature dependence of the line shift (LS), 4.0 and 8.0 at.% Sn alloys showed several anomalous features. After aging at room temperature (RT) for several months, these alloys showed hysteresis of the LS from RT up to about 500 K. At about 520 K, the unusual slope transforms to its expected value but continues with an increased isomer shift (IS). Further thermal cycles resulted in the expected slope of the LS with no measurable hysteresis, except for the transition region around 520 K above which the IS increase persisted. The experiments indicate that after thermal cycling the high-concentration alloys are in an intermediate state below about 500 K, which becomes metastable at RT as shown by the changes with aging. The transition to the increased-IS state at , however, is independent of the thermal history. A discussion of the observed phenomena is presented, which is based on thermally induced changes in the Sn-Sn distribution (short-range order) that preserve the long-range order of the alloy.

EXAFS Study of a Decomposed Fe-Grafoil System

Polarization dependence of the EXAFS is a useful tool for measuring the anisotropy in bonding of oriented samples [1]. We measured the EXAFS of an Fe-Grafoil sample with x-ray polarization both parallel and perpendicular to the Grafoil surfaces to determine whether the Fe was in a two-dimensional state, with the goal of studying two-dimensional ferromagnetism.