Mixed hyperfine interactions in Fe-Mg-O: magnetic behaviour

The complex Mössbauer spectra exhibited by Fe_xO (x0.91) and (Fe_1–y Mg _y )_xO (y=0.15–0.85) powder samples at liquid helium temperature have been analysed by a Hamiltonian treatment to allow for the significant electric field gradients present at the Fe²⁺ defect sites. The magnetic behaviour of the defect clusters are considered in terms of antiferromagnetic couplings, consistent with the spin glass-like behaviour reported recently for magnesiowüstite.     

A study of platinum-supported catalysts through hyperfine interactions

The effect of indium addition on alumina-supported platinum catalysts was investigated by measurements of hyperfine interactions. Via lime differential perturbed angular correlation spectroscopy (TDPAC) on¹¹¹Cd, Pt/Al₂O₃ catalysts were studied in the flow of a heptane/H₂ gas stream. The results indicate that some amount of indium sticks to platinum which is then dispersed on the support surface. The amount of In that is free from platinum is mobile under reaction conditions, being capable of diluting platinum particles and draining off coke precursors from the platinum surface.Researcher from Consejo Nac. Invest. Cient. Tecn., CONICET, Argentina.     

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.     

A neural model of hysteresis in amorphous materials and piezoelectric materials

A new approach to constructing hysteretic operator (HO) is proposed in this paper. Based on the HO, the input space of neural networks is expanded from one-dimension to two-dimension and the multi-value mapping of hysteresis is transformed into a continuous mapping comprised of one-to-one mapping and multiple-to-one mapping. Based on the expanded input space, a neural network is employed to approximate hysteresis. The results of experimental examples suggest the proposed approach is effective.     

Dimensionless coordinates for simulations and theory of hyperfine interactions in materials

For problems involving simulations or theory of hyperfine interactions in materials, it is convenient to work in dimensionless coordinates of reasonable magnitude. Such a system is described here that constitutes a consistent and useful “natural” dimensionless unit system that simplifies point-charge approximations, scaling between different crystal structures, scaling charges of various defects in materials, and other computations involving hyperfine interactions. We present the proposed system with examples of its use for data analysis as well as for simulations and theory.     

NMR: hyperfine interactions in oxides and metals

A NMR characterisation is given of various polymorphs of TiO₂ (anatase, rutile and brookite), Ti₂O₃, perovskites CaTiO₃ and BaTiO₃, FeTiO₃, TiB₂, titanium metal, the titanium aluminides Ti₃Al, TiAl, TiAl₂, TiAl₃, and TiAg. Values of chemical or Knight shift, nuclear quadrupole coupling constant and asymmetry parameter were derived from the (1/2, −1/2) powder lineshapes. For TiB₂, titanium metal, TiAl, and TiAl₃, where ±(1/2, 3/2), and higher satellite transitions were observed, a value for the axial component of the Knight shift was obtained.     

Laser spectroscopy applied to the study of hyperfine interactions

The impact of laser-spectroscopy techniques on the study of hyperfine interactions is illustrated by chosen examples. A brief survey of the theory of hyperfine interactions is given as well as an overview of the available experimental techniques. Results from alkali atoms, alkaline-earth elements and Group III atoms are given with special emphasis on the studies of electronic properties in sequences of Rydberg states and the investigation of nuclear properties for long chains of isotopes of the same element. Recent developments on the theoretical side, particularly what regards the many-body perturbation approach, are also discussed.     

Mixed hyperfine interactions and defect structure in magnesiowüstite Fe−Mg−O

An analysis of the 4.2 K spectra of Fe_xO (x∼0.91) and (Fe_0.4Mg_0.6)_xO is presented in which it is considered that because of the large electric field gradient at Fe²⁺ defect sites, the spectra cannot be described by Lorentzian sextets. It is assumed that the magnetic hyperfine field vector is oriented at random in the coordinate system defined by the EFG main axis, and that the EFG coordinate system is also distributed randomly. The simplifying assumption of the asymmetry parameter η=0 allows an analytical formula to be used to describe the complex spectra. Distributions of both magnetic hyperfine field and quadrupole interaction were progressively refined resulting in reasonable fits to the spectra with the main features being well reproduced. The magnetic hyperfine field distribution is rather broad with several features present while distinct values were obtained in the distribution of quadrupole interactions. These latter values are considered to correspond to the defect configurations around the Fe²⁺ sites. The distribution of hyperfine fields is considered to reflect the varying strengths of superexchange due to the high defect concentration as well as the effects of magnetic relaxation.     

A composite model of the plastic flow of amorphous covalent materials

A model based on the data available in the literature on the computer simulation of amorphous silicon has been proposed for describing the specific features of the plastic flow of amorphous covalent materials. The mechanism of plastic deformation involves homogeneous nucleation and growth of inclusions of a liquidlike phase under external shear stress. Such inclusions experience plastic shear, which is modeled by glide dislocation loops. The energy changes associated with the nucleation of these inclusions at room and increased temperatures have been calculated. The critical stress has been found, at which the barrierless nucleation of inclusions becomes possible. It has been shown that this stress decreases with an increase in temperature. According to the calculations, the heterogeneous (homogeneous) plastic flow of an amorphous material should be expected at relatively low (high) temperatures. Above the critical stress, the homogeneous flow is gradually replaced by the heterogeneous flow.     

Mössbauer study of hyperfine field relaxations in soft magnetic amorphous alloys

Superimposed asymptotic exponential relaxation between 125 and 200°C of average hyperfine inductions and their orientations were detected by the⁵⁷Fe Mössbauer spectroscopy in the ferromagnetic Fe₄₀Ni₄₀B₂₀ and Fe₇₀Co₁₀B₂₀ amorphous alloys. From their Arrhenius type temperature dependence, average activation enthalpies ΔH were derived in agreement with the resistometric and coercivity data. The interpretation is proposed in terms of individual irreversible processes: stress relief, free volume shrinking and short-range ordering.     

Mössbauer effect study of hyperfine interactions in145Nd

The magnetic hyperfine splitting of the 72.5 keV γ rays of¹⁴⁵Nd was investigated in intermetallic compounds of Nd and in the paramagnetic salts Nd _x Y _1-x Cl₃·6H₂O (withx=0.02 andx=0.05) at 4.2 K. With the magnetic hyperfine tensorA of Nd_0.01Y_0.99Cl₃·6H₂O known from EPR spectroscopy, the analysis of the unresolved magnetic hyperfine spectra yieldsI _e =5/2 for the spin of the 72.5 keV state, in contradiction to a previous result. The multipolarity of the 72.5 keV γ transition was found to be essentiallyM1 with δ²=0.010±0.014, and the magnetic moment of the 72.5 keV state was determined as μ(5/2)=?0.319±0.004 nm. For various divalent and trivalent Nd compounds as well as for metallic Nd the isomer shift IS of the 72.5 keV γ line was measured. A value for the change of the mean square nuclear charge radius during the 72.5 keV γ transition of Δ〈r ²〉=+(1.9±0.9)·10^?3fm² was deduced using electron density differences from free-ion Hartree-Fock calculations.     

Magnetic hyperfine interactions in GdAl3

We have studied the magnetic hyperfine interactions in GdAl₃ using¹⁵⁵Gd Mössbauer spectroscopy between the temperatures of 4K and 90K. Previous studies on GdAl₃ have shown that antiferromagnetic ordering occurs at 18K, and that a fit of the susceptibility to 1/(T-θ_p) yields a θ_p value of ?89K. The large ratio of θ_p to T_N is indicative of magnetic frustration between competing ferro-and antiferromagnetic interactions, which may be due to a combination of the oscillatory nature of the RKKY interaction and the geometry of the hexagonal lattice. Our studies show that the saturation magnetic hyperfine field at the Gd site is ?24.0 T, with the moments lying in the basal plane. The efg at the gadolinium site is 2.55(1)×10¹⁷V cm^?2 which is considerably larger than the value predicted by a point charge calculation. This difference may indicate that there is a conduction electron contribution. A helical magnetic structure has been calculated from RKKY theory.     

Muon hyperfine interactions in magnetic oxides

Muon spin rotation studies on magnetic oxides among which-Fe₂O₃ and Fe₃O₄ have shown that at temperatures below approximately 500 K the muons form a muon-oxygen bond, analogous to a hydrogen bond. Generally, the muon hyperfine interactions in magnetic oxides can be explained in terms of supertransfer (hyperfine) and dipole fields. Supertransfer fields result from covalent bonding effects. For the rare earth (R) orthoferrite series (RFeO₃) comparison is made with Mössbauer results on covalency effects in hyperfine interactions. Suggestions for next stages of experimentation in solid state research in oxides bySR are mentioned.     

Anisotropy of hyperfine interactions as a tool for interpretation of NMR spectra in magnetic materials

Approach for interpretation of nuclear magnetic resonance (NMR) spectra in magnetic materials is presented, consisting in employing the anisotropy of hyperfine interaction. The anisotropic parts of hyperfine magnetic fields on ⁵⁷Fe nuclei are calculated ab initio for a model example of lithium ferrite and utilized to assign the experimental NMR spectral lines to iron sites in the crystal structure.     

High pressure Mössbauer study of hyperfine interactions in europium metal

The 21.6 keV Mössbauer resonance of ¹⁵¹Eu has been studied at 4.2 K in europium metal at pressures up to 16 kbar. The measured pressure coefficient of the isomer shift dS/dp = 4.8 (5) × 10^-2 mm/sec kbar can be understood as a congruent compression of 6s conduction electrons. The magnetic hyperfine splitting decreases under pressure. The slope of the (negative) hyperfine field dB/dp = + 0.133(8) T/kbar must arise predominantly from an increase in the conduction electron polarization.     

Theoretical study of hyperfine interactions at the Ta site in Hafnia polymorphs

The electric field gradient resulting from Ta substitutional defect in normal monoclinic phase is studied using all-electron ab initio NFP-LMTO method. Hyperfine parameters in Pbca and Pnma phases have also been calculated to determine the usefulness of quadrupolar interactions in the investigation of phase diagrams under hydrostatic pressure. Predictions for hyperfine parameters in high temperature P42nmc and Fm3m phases were also developed. Given the donor behavior of Ta in HfO₂, two charge states, 0 and +1, have been studied for each phase. Although HFI do not vary significantly with charge, it was determined that for a Ta+1 in P21/c phase hyperfine parameters is consistent with experimental results. Quadrupolar interactions for transitions to denser phases show important variations with respect to that of the normal phase: asymmetry parameter for Pbca and the electric field gradient for Pnma both increase substantially. At high temperature phases, drastic decrease in both EFG and asymmetry parameter in P42nmc is observed, while they almost vanish in Fm3m.     

Nuclear hyperfine interactions in spherical top molecules

The nuclear hyperfine Hamiltonian for XY₄ and XY₆ spherical top molecules in their ground electronic state is built up by a tensor method, and is given by a linear combination of invariant tensor products in the group ^L0(3) × G(G = T_d or 0_h); the case when an external magnetic field is applied is also considered. For the study of a given vibrational state, the appropriate restriction is easily determined; as examples were consider the ground vibronic state and a triply degenerate fundamental state.