Narrow spectral lines of modulation harmonics of gamma radiation in unordered magnetic materials

The spectra of gamma-resonance lines excited using the amplitude modulation of the pumping field of the Mössbauer effect were studied in unordered magnetic materials with a strong spread of magnetic hyperfine fields at nuclei of Mössbauer isotopes. The analysis demonstrates that the spectra of modulation harmonics consist of narrow lines, and their form and intensity are determined by the contribution of magnetic local hyperfine fields, which are excited at a pumping frequency. The inversion of intensities for odd harmonics with the change in sign of detuning of the pumping frequency relative to the average Larmor frequency of the Mössbauer nucleus was noted. The application of this method to the spectral analysis of inhomogeneous magnetic materials is discussed.     

Electric field gradient sign determination in low magnetic fields perpendicular to the gamma rays in57Fe Mössbauer spectroscopy

The determination, by Mössbauer⁵⁷Fe spectroscopy, of the sign and the asymmetry parameter of the electric field gradient (EFG), in a non-magnetic powder sample, needs the use of an external magnetic field. Usually a “parallel-magnetic-field-to-the-gamma-rays” configuration is employed. In the present work, theoretically generated spectra in both configurations, parallel an perpendicular, are compared. The result is that the spectra in the perpendicular configuration show always a larger asymmetry for the same fields intensitics. In consequence, the sign determination of the EFG, is possible with magnetic fields significantly smaller in the perpendicular case than in the parallel one. Furthermore, the practical consequences are important, because EFG studies with conventional electromagnets, allowing only fields lower than 25 kGauss are feasible. As an example. Mössbauer experimental results obtained at room temperature for natural pyrite (FeS₂) in perpendicular fields of 15 and 20 kGauss, created by a conventional electromagnet, are shown here.     

Investigation of amorphous and crystalline Sc3Fe alloys

Amorphous Sc₃Fe alloy has been prepared by splat-cooling with levitation melting. The quadrupole splitting distributions for Sc₃Fe were determined from Mössbauer spectra: the zero-field distribution has a Gaussian form. The analysis of Mössbauer spectra taken in applied magnetic field show that both positive and negative signs of principal component (V_z2) of electrical field gradients (EFG) are present. The crystallization process has also been investigated. Heat treatment of the amorphous alloys causes crystallization first into a metastable, and then into the stable phase.     

The electric field gradient at the iron sites inβFeOOH

The temperature behaviour of the hyperfine parameters of iron in paramagnetic chlorine containingΒFeOOH is studied by⁵⁷Fe Mössbauer spectroscopy. Applying external magnetic fields, the sign and asymmetry parameterη of the electric field gradient (EFG) at the compound's two iron sites is determined. It is shown that an external field enhances the distributive character of the hyperfine parameters. Finally, heat treatment experiments allow new data on the nature of both iron coordination types to be presented.     

Mössbauer study of Al65Cu20Fe15 quasicrystals

Al₆₅Cu₂₀Fe₁₅ quasicrystalline ribbons exhibit similar paramagnetic spectra in the temperature range 45 mK-300 K, evidencing for no magnetic change down to 45 mK. Analysis of in field Mössbauer spectra by both negative and positive EFG distribution shows an only slightly dominant negative contribution (59%).     

Mixed hyperfine interactions in amorphous materials: A model study

⁵⁷Fe Mössbauer absorption profiles were calculated supposing distributions of all hyperfine parameters: hyperfine magnetic fields, isomer shifts, and electric field gradients. The effect of mixed hyperfine interactions was taken into account in all orders of perturbation theory. The shapes of the spectra were systematically studied for varying average values and widths of the hyperfine magnetic field distribution (HMFD). From the simulated spectra, the shapes of the HMFD were reconstructed using standard techniques of Mössbauer spectra processing which neglect the effects of random isomer shifts and electric field gradients. It has been shown that the reconstructed shapes of the HMFD differ qualitatively from the original single-peaked distributions and exhibit a double-peaked structure similar to the distributions found in many experiments on amorphous alloys with low iron content. A brief review of various mechanisms responsible for either apparent or real double-peaked structure of HMFD has been given.     

Mössbauer studies of iron-rich metallic glasses

Iron-based metallic glasses have recently become an important class of ferromagnetic materials exhibiting excellent soft magnetic properties coupled with good mechanical properties. These glasses are usually prepared by rapid quenching techniques and are produced in thin long ribbon form with widths ranging from a few mm to 150 mm or more.⁵⁷Fe Mössbauer spectroscopy has been extensively used to study hyperfine interaction parameters in these metallic glasses to understand ferromagnetism in amorphous structure. In particular, Mössbauer spectra have been carefully analyzed to reveal information about the distribution of hyperfine fields resulting from the randomness of the atomic arrangement and to understand the temperature dependence of hyperfine fields, spin-wave excitations, magnetic structure, thermal stability and crystallization, the quenched-in magnetization axis, the Curie temperature and its dependence on compositions, the effect of stress and pressure on the magnetic properties, corrosion behaviour, local order and atomic arrangement, phase transformation, etc. This paper reviews the application of⁵⁷Fe Mössbauer spectroscopy to magnetic studies on metallic glasses mainly based on the iron-boron alloy system, and some of the significant results obtained which are characteristic of the glassy/amorphous state.     

Effects of dzialoshinskii exchange interaction in Mössbauer spectra of trimeric clusters

An interaction of the electronic shell of trimetric exchange cluster with Mössbauer nuclei has been examined. A new type of quadrupole splitting depending essentially on the distortions and magnetic field has been predicted. Noncollinear spin structure of the exchange cluster and the effective magnetic fields on Mössbauer nuclei have been uncovered. The theory explains qualitatively the peculiarities of Mössbauer spectra of iron three-nuclear carboxilate crystals.     

Characterization of Zn−Fe intermetallic compounds

Zn?Fe intermediate phases have been investigated. The distribution of quadrupole splittings P(QS) was determined from the Mössbauer spectrum: the P(QS) gives evidence for two different iron sites in the γ phase. The analysis of Mössbauer spectrum taken in an applied magnetic field shows the negative sign of electrical field gradients is present.     

Radiofrequency forward Mössbauer scattering method in study of magnetic materials

Mössbauer spectra in forward scattering scheme were measured for iron borate (FeBO ₃) exposed to radiofrequency (rf) field below the Neel temperature. The spectra have satellites spaced by doubled rf field frequency. The semiclassical model of Mössbauer transmission through a thick magnetic sample under rf reversals of a hyperfine field is proposed. This model reproduces all features of the measured spectra. Experiments and model calculations indicate additional possibilities of this measurement scheme for study the soft magnetic materials.     

Radiofrequency mössbauer forward scattering spectra in magnetic materials

Radiofrequency (RF) Mössbauer spectra in experiments on forward scattering by thick samples of iron borate (FeBO₃) below the Neel temperature are measured. The spectra have satellites spaced by doubled RF-field frequency. A semiclassical model of Mössbauer transmission through a magnetic absorber exposed to RF reversals of a hyperfine field on nuclei is proposed. The model reproduces all features of the measured spectra. Experiments and modeling calculations indicate additional possibilities for studying soft magnetic materials using this measurement scheme.     

Lattice sum calculations and a Mössbauer study on electric field gradient in phlogopite

Lattice sum contributions have been calculated at the two octahedral sites in a phlogopite mica assuming a systematic distribution of octahedral cationic charges. This, unlike the case of a random distribution of charges, is able to reproduce broad features of quadrupole doublet spectra in Mössbauer experiments suggesting that the doublet assignments in terms of the two structural sites,M ₁ (trans) andM ₂(cis), are quite valid. Angle dependence of Mössbauer spectra has also been studied for a ferric-rich phlogopite mica sheet to determine the orientation of the EFG principal axis component and the sign of the quadrupole coupling constant. Quadrupole splitting values and the positive sign of coupling constants match the theoretical predictions quite well but the model fails to predict the observed EFG orientation.     

Collapse in the Stoner-Wohlfarth noninteracting-particle model

A theory of Mössbauer spectra of noninteracting Stoner-Wohlfarth (SW) particles interacting with rf magnetic fields is developed. The theory makes it possible to calculate the absorption spectra for arbitrary frequency and amplitude of the rf field. The main features of the Stoner-Wohlfarth model are discussed. The Liouville superoperator formalism is used to generalize the results to the case of arbitrarily time-varying hyperfine fields at a nucleus. To understand the qualitative features of the collapse effect that are observed in the Mössbauer spectra of SW particles the particular case of a circularly polarized hyperfine field is studied, and an analytical expression is obtained describing the Mössbauer spectra for this case. An analysis is also made for weak rf magnetic fields and in this case the resonance behavior of the Mössbauer lines is traced as a function of the frequency of the rf field.     

Magnetic properties of ferrihydrite

The magnetic properties of a sample of synthetic ferrihydrite have been investigated by use of Mössbauer spectroscopy. The area ratios of the Mössbauer lines are not significantly changed by application of magnetic fields at 5 K, indicating an amorphous magnetic structure. An average magnetic moment per particle of 3.1×10^?21 JT^?1 was estimated from the dependence of the hyperfine splitting of the Mössbauer spectra on external magnetic fields at 80 K.     

57Fe Mössbauer study of new multiferroic AgFeO2

The present work reports results of the ⁵⁷Fe Mössbauer measurements on AgFeO₂ powder sample recorded at various temperatures including the points of both magnetic phase transitions. The ⁵⁷Fe Mössbauer spectra of AgFeO₂ measured in the paramagnetic range (T > T _N1) consist of one quadrupole doublet with rather high quadrupole splitting of Δ_300K = 0.66 ± 0.01 mm/s for Fe³⁺ ions. In order to predict the sign of electric field gradient (EFG) at ⁵⁷Fe nuclei, we calculated the lattice contribution to the electric field gradient (EFG) at ⁵⁷Fe nuclei, which emphasized the importance of the dipolar contributions, with resultant oxygen polarizabilities in the range of α _O = 0.83 ų, in agreement with the results obtained previously for other delafossite-like oxides. In the temperature range of T _N2 < T < T _N1, Mössbauer spectra gave clear evidence for the existence of a distribution of the hyperfine magnetic fields H _hf at ⁵⁷Fe nuclei. We present the results of a model fitting of the spectra based on an assumption of the cycloid magnetic structure of AgFeO₂ at T < T _N2. The obtained data were analysed in comparison with published data on Mössbauer studies of oxide multiferroics.     

Method for decomposition of Mössbauer spectra of amorphous magnetic materials

A mathematical method for decomposition of symmetrical Mössbauer spectra of amorphous magnetic materials is proposed. The method consists of two procedures: positioning and synthesis by convolution followed by substration of 2,3,4,5-lines beginning from the outside 1,6-lines. The resultant spectrum containing only the 1,6-components makes it easy to find the hyperfine magnetic field distribution in the sample.     

Magnetism in amorphous Fe-Si alloys

Fe-Si alloys have been obtained by the vapor quenching technique. Resistance measurements, electron microscopy and diffraction results are typical of an amorphous system. The Mössbauer spectra of these amorphous alloys, fitted in terms of a distribution of hyperfine fields, show the existence of a local magnetic order with, however, a proportion of weakly or even no coupled Fe atoms which is temperature and Fe-concentration dependent.     

Synthesis and hyperfine interactions of the amine intercalates of FeOCl

Some amine derivatives were intercalated into the van der Waals gap of layered compound FeOCl. These compounds were characterized by X-ray powder diffraction and Mössbauer spectroscopy. The isomer shift and the quadrupole splitting are characteristic of the high-spin ferric iron. The magnetic structure of FeOCl was affected by intercalation of amine molecules in the interlayer region of FeOCl. The magnetic hyperfine field in the low temperature is larger in the intercalates than it is in unintercalated FeOCl. The sign of the EFG tensor at the iron atom is dependent of the type of amine intercalated.     

Zero-field and in-field Mössbauer spectroscopy as a tool for structural and magnetic characterization of maghemite (γ-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>) nanoparticles

Nowadays, nanoparticles of maghemite (γ-Fe₂O₃) represent one of the most useful materials in modern advanced nanotechnological applications due to their superior magnetic properties. For their characterization,⁵⁷Fe zero-field and in-field Mössbauer spectroscopy have proved themselves to be very powerful and effective tools which are crucial for an investigation of the local surrounding of iron atoms and observation of dynamic effects. The structural and magnetic characteristics of maghemite and its nanoparticles are thus discussed with regard to their zero-field and in-field Mössbauer spectra recorded at various temperatures and applied external magnetic fields. In addition, a special attention is also devoted to remarkable physical phenomena (superparamagnetism, spin canting) occurring largely in maghemite nanosized particles.     

On the advantages and validity of the method of decomposition of Mössbauer spectra from amorphous and disordered magnetic materials

A method for decomposition of Mössbauer spectra of amorphous and disordered magnetic materials is tested. Its algorithm is discussed as well as the adaptation of the method for processing of more complex spectra including correlation between the isomer shift and the magnetic hyperfine field and also quadrupole effect.