Theoretical study on pressure dependence of isomer shift and hyperfine field

The pressure dependence of the isomer shift and the hyperfine field are theoretically investigated. By expanding the Gibbs free energy of the system under proper circumstances, the explicit form of the pressure dependence of the isomer shift is obtained. At low temperatures, by applying some well-known relations, the pressure dependence of the hyperfine field is approximately derived. Finally, the theoretical results are compared with the experimental data.     

Self-consistent field cluster study of hyperfine properties of normal and anomalous muonium in elemental semiconductors

Using the Unrestricted Hartree Fock (UHF) Cluster Procedure, it is shown that for the normal muonium (Mu) center, the tetrahedral site is the most favorable in the two systems diamond and silicon investigated, while for the anomalous muonium (Mu^*) center, a site displaced in the <111> direction with respect to a vacancy in a double-positively charged environment is the appropriate one for all three elemental semiconductors. Using our calculated electronic wave-functions, one is able to explain all features of the observed hyperfine properties of both centers and, in a number of cases, obtain good quantitative agreement with experiment.     

Charge transfer and the57Fe isomer shift in iron alloys

This is a brief report on the present state of our experimental studies on the influence of the valence electron distribution on the⁵⁷Fe isomer shift in iron-based alloys. The studies are performed by Mössbauer and positron annihilation spectroscopies.     

Mössbauer isomer shift and hyperfine fields of ordered and disordered Fe3Al alloys

The Mössbauer technique was used to measure the hyperfine magnetic field and isomer shift of⁵⁷Fe nucleus in the ordered and disordered Fe₃Al alloys. The Mössbauer spectra were analyzed to yeld the hyperfine magnetic field distribution curves. A linear correlation has been revealed between the average hyperfine field and the average number of Al atoms in the first two nearest neighbour shells of⁵⁷Fe nucleus,N _Al, for both ordered and disordered alloys. The measured values of the mean isomer shift agree very well with the values expected from the Miedema-Van der Woude model.     

Ligand field and hyperfine interaction of the [Fe(H2O)6]2+ complex in different compounds

A series of compounds all containing the ferrous hexaquo complex were investigated by Mössbauer spectroscopy at 4.2K and in magnetic fields up to 5T and far infrared techniques. The results are interpreted in terms of a T_2g ligand field model for which normal distortions of the water octahedron are used as parameters. The ferrous hexaquo complex can be described in all compounds by the same covalency, spin-orbit coupling and hyperfine coupling constants.     

Isotope shift and hyperfine structure measurements at the242f Am fission isomer

Istope shift and hyperfine structure measurements have been performed for the^242fAm fission isomer with target production rates of only a few per second. The method is based on resonance ionization spectroscopy (RIS) in a buffer gas cell with radioactive decay detection of the ionization process (RADRIS). A relative isotope shift ratioX _exp=IS^242f,241/ IS^243,241=41.7±0.9 has been measured for the 500.02 nm transition corresponding to a nuclear parameter ^242f,241=5.4±0.3 fm². The analysis of the quadrupole moment based on the deformed Fermi-model of the nuclear charge distribution including second order corrections results inQ ₂₀=38.2 ±1.4( _–0.8 ^+0.4 )_model eb. The measurement of the hyperfine structure splitting of the transition at 466.28 nm yields a negativeg-factor and a nuclear spin ofI=2 orI=3.Work supported by the Bundesministerium für Bildung und Forschung under contract 06 MZ 5661.     

Mössbauer effect study of the hyperfine fields and isomer shift of ordered Fe−Co alloys

The influence of Co concentration on the⁸⁷Fe hyperfine magnetic field and isomer shift in ordered and disordered FeCo alloys in the composition range (34–70) at .% is studied. It is shown that the model which assumes an additivity of the hyperfine field perturbation from solutes in the first two nearest neighbour shells of Fe nucleus very well describes the experimental data.     

The effect of radiofrequency modulation of 57Fe hyperfine interaction by rotating magnetic field

The effect of ⁵⁷Fe hyperfine interaction radiofrequency (rf) modulation by external rotating magnetic field was studied in thin Permalloy foil by means of Mössbauer spectroscopy. The rf effect was investigated as a function of intensity for several rf field frequencies. The experiments show that the external rotating rf field causes considerable changes in the hyperfine pattern. The obtained spectra are in disagreement with those obtained by Perlow [Phys. Rev. 172 (1968) 319]. They also are inconsistent with magnetostriction hypothesis. Proceeding from the Mössbauer spectrum analysis one may conclude that the magnetization of investigated foil changes its direction in a complex manner. However, the undertaken experiments show that the essential number of Mössbauer nuclei experience the rotating magnetic field influence.     

The hyperfine fields and isomer shift at Fe-Co metal inclusions in synthetic diamond grains

The thermal changes taking place at the sites of Fe-Co metal inclusions in synthetic diamond grains have been investigated using Mössbauer spectroscopy on as-grown samples and samples annealed at 970–1370 K. The spectra show the presence of Fe-Co and (Fe-Co)-C complexes. The temperature-dependent results suggest that the latter are due to reactions at the metal inclusion-diamond interface, and may be a signature of the processes leading to strength loss in the grains.     

Study of the magnetic structure and57Fe hyperfine interaction in FeMnP

The⁵⁷Fe Mössbauer spectra of FeMnP show a remarkable magnetic distribution for T 175K, although all iron atoms are crystallographically equivalent. It seems possible to explain this distribution by a spin modulation described by coexistence of an antiferro — and a helimagnetic probe. A theoretical calculation performed, justifies this explanation.     

A cluster calculation of the muon hyperfine field in nickel

We have performed a partially self-consistent, first-principles calculation of the hyperfine field at a ⁺ at the octahedral interstitial site in ferromagnetic nickel. Using a package of programs made available by the Department of Theoretical Chemistry of the University of Mexico, various calculations were performed of the spin-dependent charge density in the central region of a spherically averaged superposition of atomic charge densities. In this way it was found that: (1) The electronic configuration of bulk Ni contains a significant amount of 4p-states, in agreement with other calculations¹. (2) The ⁺ produces only a weak perturbation on its nearest-neighbor Ni atoms. (3) The hyperfine field at the ⁺ is opposite to the bulk magnetization and has the value –590 G, in good agreement with the measured² value of –641 G.     

Mixed hyperfine interaction in amorphous Fe-Zr sputtered films in external magnetic field — A57Fe Mössbauer study

Conventional⁵⁷Fe-Mössbauer spectroscopy provides only information about the magnitude of the splitting QS in the case of electric quadrupole hyperfine interaction, but not on the sign of the main component of the electric field gradient (EFG), V_zz, or the asymmetry parameter, η, which are sensitive to the local environment of the⁵⁷Fe nuclei. This kind of information is obtained by measurements in external magnetic fields. In the case of amorphous Fe-Zr sputtered films mixed hyperfine interaction leads to a clear change in the behaviour of the Zr-rich and the Fe-rich alloys, indicating the existence of magnetic clusters in the Fe-rich samples.     

Reduced magnetic hyperfine field at the surface of α-Fe2O3 and FeBO3 single crystals

The surface magnetic ordering of an α-⁵⁷Fe₂O₃ and ⁵⁷FeBO₃ single crystal was studied by Depth Selective Conversion Electron Mössbauer Spectroscopy (DCEMS). The depth selective Mössbauer data of ⁵⁷FeBO₃ up to the Néel temperature (T _N = 348.35 K) reveal the coexistence of the canted antiferromagnetic bulk phase and a magnetic surface phase that shows time relaxation. Both phases are separated by a wall (interim layer) and the thickness of the magnetic surface layer increases as the crystal is heated to T _N.     

First-principles calculation of electric field gradients and hyperfine couplings in YBa2Cu3O7

The local electronic structure of YBa₂Cu₃O₇ has been calculated using first-principles cluster methods. Several clusters embedded in an appropriate background potential have been investigated. The electric field gradients at the copper and oxygen sites are determined and compared to previous theoretical calculations and experiments. Spin polarized calculations with different spin multiplicities have enabled a detailed study of the spin density distribution to be made and a simultaneous determination of magnetic hyperfine coupling parameters. The contributions from on-site and transferred hyperfine fields have been disentangled with the conclusion that the transferred spin densities essentially are due to nearest neighbour copper ions only with marginal influence of ions further away. This implies that the variant temperature dependencies of the planar copper and oxygen NMR spin-lattice relaxation rates are only compatible with commensurate antiferromagnetic correlations. The theoretical hyperfine parameters are compared with those derived from experimental data. Received 10 April 2001 and Received in final form 19 June 2001     

The magnetic hyperfine interaction of111Cd in the spinels Fe3O4 and Co3O4

The perturbed angular correlation (PAC) method was applied to study the temperature dependence of the magnetic hyperfine fields in¹¹¹In-doped polycrystalline Fe₃O₄ and Co₃O₄. The critical behaviour near the magnetic phase transitions has been investigated. The changes occuring near the Vervey transition in Fe₃O₄ and the path of the supertransferred magnetic fied in Co₃O₄ are discussed. The results are compared with results obtained from Mössbauer spectroscopy.     

Anomalous behaviour of the magnetic hyperfine field in the microcrystalline YFeO3-Fe2O3 mixed phase system

The magnetic hyperfine field in 120 Å YFeO₃ microcrystals just below the superparamagnetic blocking temperature, as well as at 5 K are found to be surprisingly low. In addition, the hyperfine field fails to saturate even at the lowest recorded temperature. The observed features cannot be understood in terms of either collective magnetic citations or superferromagnetism. A negative equivalent pressure arising out of decreased particle size is proposed as a plausible explanation.     

Molecular orientation dependence of ortho-para H2 conversion on Fe(OH)3 cluster induced by hyperfine contact interaction

We investigate the molecular orientation dependence of the (o)rtho-(p)ara H₂ conversion on Fe(OH)₃ cluster. Taking Fermi's contact interaction as perturbation, we find that the o-p H₂ conversion yield for the molecular orientation angle θ= 10^○ with respect to the surface normal is larger than that for θ= 70^○.