Double resonance experiments in57Fe Mössbauer spectroscopy

Splitting of Mössbauer lines has been observed in conventional energy domain spectra of Permalloy samples when an external radio frequency (rf) magnetic field was tuned to be in resonance with the separation of Zeeman sublevels of either the ground or the excited state of the⁵⁷Fe nucleus. Time domain measurements were performed with the frequency of the rf field close to the natural linewidth of the 14.4 keV Mössbauer state. A nonmagnetostrictive magnetically soft Permalloy sample was used in the experiments.     

On the origin of discontinuity of the hyperfine fields at Fe nuclei in bulk iron and aerosol Fe nanoparticles

Advancing the early work in which a discontinuity of hyperfine fields at ⁵⁷Fe nuclei in bulk iron and in aerosol Fe nanoparticles has been revealed by analyzing their Mössbauer spectra the present Letter evidences that the existence of several peaks in the hyperfine distribution (HFD) for bulk Fe is caused with the internal magnetic fields owing to its multidomain structure whereas aerosol Fe nanoparticles are single-domain and show only a unique peak in HFD. This argument has been corroborated by transformation of the HFD pattern for Fe foil after applying the external magnetic field of 0.03 T.     

The rf stimulation of amorphous metals

The effect of the rf modulation of the Mössbauer gamma radiation (rf sidebands) and the effect of the fast relaxation of the magnetic hyperfine field induced by the rf field (rf collapse) are described. The coexistence and separation of these effects is discussed. The rf collapse of the magnetic hyperfine structure allows a direct observation of the quadrupole interaction in ferromagnetic amorphous metals providing the method for studying the short range order. Examples of FeSiB, FeNiSiB, FeCoSiB and FeB are discussed with respect to structural models. The effect of radiofrequency enhanced crystallization of amorphous metals and its relation to rf sidebands is demonstrated.     

Goldanskii-Karyagin effect and external magnetic field method as tools to measure anisotropy of the recoilless fraction in amorphous materials

Diffraction of X-rays or neutrons can not be used to obtain details about the atomic vibrational anisotropy in the case of amorphous materials due to the lack of well-defined Bragg reflections. Mössbauer spectroscopy can yield some information in such cases, either via the Goldanskii-Karyagin effect or by applying a magnetic field, preferably along the beam axis. The latter method can be applied to the (preferably diamagnetic) samples exhibiting an electric quadrupole interaction (preferably non-axial) and the magnetic field should be chosen in such a way as to produce significant off-diagonal elements in the hyperfine hamiltonian. The external magnetic field method is capable of yielding much more information than the Goldanskii-Karyagin effect in most cases, provided sufficiently strong magnetic fields are available. Some examples of the¹²⁹I Mössbauer spectra have been calculated to show the usefulness and sensitivity of the external magnetic field method.     

A Mössbauer spectroscopy study of antiferromagnetic iron arachidate Langmuir-Blodgett films

Langmuir-Blodgett multilayers of ferric arachidate (abbreviated: FeA) on silicon wafers have been investigated by means of conversion electron Mössbauer spectroscopy (CEMS) and absorption Mössbauer spectroscopy at different temperatures between room temperature and 4.2 K, without and with external magnetic fields up to 5 T. The films show a quasicrystalline structure with ac-axis orientation perpendicular to the plane and a random distribution of orientations in the plane of the films. All Mössbauer spectra show Fe³⁺. At low temperatures, antiferromagnetic ordering has been observed. The lines in both the room temperature and the low-temperature spectra are significantly broadened, which is due to a distribution of electric field gradients and hyperfine fields. The reason for this is a slightly disordered iron environment. The orientation and the distribution function of the internal magnetic fields have been determined.     

61Ni Mössbauer spectra of fine Ni particles

We report the first measurements of fine Ni particles from⁶¹Ni Mössbauer spectroscopy. The average hyperfine field of the 10 nm particles at 4.2K is measured to be 7.7(4) T, compared to the measured value for Ni foil of 7.5(3) T. Application of an external field of 6 T to the fine particles causes a reduction of the hyperfine splitting to 1.5(6) T, a consequence of the negative hyperfine field at Ni nuclei. These results are discussed in terms of fine particle effects.     

Electron spin relaxation studies of impurity iron ions in ferroelectric lithium niobate

The nuclear hyperfine structure of dilute impurity iron ions in ferroelectric lithium niobate is investigated via⁵⁷Fe Mössbauer effect (ME). The ME spectra exhibit typical slow electron spin relaxation effects associated with Fe³⁺ ions in the⁶S_5/2 state, which are analysed using spin-Hamiltonian formalism. For Fe³⁺ ions, the principal axis of EFG tensor is found to be parallel to the crystallographic c-axis. The strong external magnetic field of 47 KOe is used to study magnetic and crystal field effects on the ME hyperfine structure.     

Effect of atomic environment on the Fe hyperfine structure and the thermal stability of magnetic order in L10 ordered Fe-Pt alloys

The ⁵⁷Fe Mössbauer effect measurements were made for the L1₀ ordered Fe-Pt alloys with 39-62 at% Pt and the effect of local atomic environment on the hyperfine structure was investigated. Furthermore, the thermal stability of magnetic order was investigated for the alloys with high Pt concentration. From the analyses of the observed Mössbauer spectra, we found that dipole-field-like anisotropic transferred hyperfine fields are mainly responsible for the large difference in hyperfine field between Fe-site and Pt-site in the Fe-rich alloys. In the Pt-rich region far from stoichiometry, the existence of many Fe-sites occupied by excess Pt atoms causes a distribution of exchange fields. Therefore, the iron atoms in different local environments may have their several hyperfine fields with different temperature dependence. The anomalous temperature dependence of the averaged hyperfine field and line broadening observed for the 61, 62 at% Pt alloys can be understood from the co-existence of various sub-spectra with different temperature dependence. As a result, the thermal stability of magnetic order is largely reduced as the Pt concentration exceeds 60 at%.     

External influence on magnetic properties of Fe-based nanocrystalline alloys

Amorphous and nanocrystalline ribbons of NANOPERM, FINEMET and HITPERM were studied by Mössbauer spectroscopy (MS) after the influence of external factors: different annealing atmospheres, tensile stress and several kinds of corrosion. MS is a suitable tool for such studies because the spectral parameters are very sensitive to changes in the vicinity of the probe — ⁵⁷Fe nuclei. The most sensitive parameters were hyperfine magnetic field in crystalline component, average hyperfine field in amorphous component and direction of net magnetic moments. Influence of external factors modified also the structure of the alloys, i.e. new or modified phases were identified by MS phase analysis.     

Changes in the hyperfine interactions in the Fe80Nb3Cu1B16 metallic glass under tensile loading

The influence of tensile stress on the changes in hyperfine parameters obtained by Mössbauer spectroscopy of the ferromagnetic amorphous NANOPERM-type Fe₈₀Nb₃Cu₁B₁₆ alloy has been investigated. The bulk changes are obtained by Mössbauer spectroscopy in a transmission geometry using γ-rays; the surface properties are studied in a scattering geometry using conversion electrons. The Mössbauer spectra are analyzed by two distributions of hyperfine inductions, which result in high-field and low-field components corresponding with a model of clusters formed by predominantly iron atoms and intermediate phase containing mainly the Nb, Cu and B atoms. The obtained results are completed by the bulk and surface magnetic measurements, with observations of surface morphology by AFM and microstructure by SEM. The investigations yield approximately linear increase of hyperfine parameters and slight deterioration of the bulk as well as surface magnetic characteristics with tensile loading.     

Investigation of the magnetic hyperfine field at197Au impurities in the heavy Rare Earth metals

The Mössbauer effect has been used to measure the magnetic hyperfine field at the site of¹⁹⁷Au impurities in the heavy Rare Earth metals Gd, Tb, Dy, Ho and Er at 4.2 K. The magnetic hyperfine field decreases in a non linear way with decreasing spin of the Rare Earth host. For¹⁹⁷Au this decrease is stronger than for any other impurity investigated up to now. Possible reasons for this behaviour are discussed.     

Features of the hyperfine interaction of tin impurities in metallic holmium

The Mössbauer effect has been used to study the hyperfine fields for the¹¹⁹Sn atom as an impurity in metallic Ho in the 4.5–136 K temperature range. Two values of the field relevant to the features of the Ho magnetic structure have been found in the ferro- and antiferromagnetic regions of ordering. The temperature dependence of the fields differs drastically from the magnetization function and is similar to the case of a Dy host studied earlier. The experiments with a 40 kOe external magnetic field at 25 K have revealed a strong (up to 19%) hysteresis in the hyperfine fields which can be explained by a rearrangement of the magnetic structure. The field sign has been found to be negative, from which fact a peculiar compensation of the local and hyperfine fields is deduced. The experiment is also indicative of a possible new phase transition at 95 K.     

On the Mössbauer hyperfine spectra of paramagnetic clusters

The hyperfine interaction of a system containing two weakly coupled paramagnetic ions is investigated. It is shown that even a very weak interaction cannot be broken by a large magnetic field, if the Zeeman interaction of the two paramagnetic centers is identical. Small differences in theg-values, however, yield immediately a quite different hyperfine pattern which corresponds to two isolated paramagnetic centers in the case of a large external magnetic field. Mössbauer experiments on reduced bacterial ferredoxin where one molecule contains two iron clusters withS=1/2 each are used to analyze the interaction of two paramagnetic centers.     

The external magnetic field dependence of RF splitting of57Fe hyperfine lines. NMR + Mössbauer double resonance experiment

We present the results of an experimental investigation of a RF splitting of⁵⁷Fe hyperfine lines in the regime of NMR and Mössbauer double resonance. The experiments have been performed as a function of RF field intensity and static magnetic field magnitude. The intensity of the RF components and the separation between them are extremely sensitive to the frequency and amplitude of the RF magnetic field. The RF splitting of hyperfine lines is inversely proportional to the strength of the static magnetic field.     

Influence of radiation damage on the hyperfine field of triton irradiated nickel foils

The recently observed Mössbauer effect in⁶³Ni enables the study of the hyperfine field in nickel with high resolution. In the present paper, the line broadening of the Mössbauer resonance is analyzed in order to study the influence of radiation damage on the magnetic hyperfine field in⁶⁴Ni foils. Each of these foils was irradiated by tritons at 15 K with a total current integral of 2.4 A h in order to produce the short-lived⁶³Co sources (T _1/2=27 s). An upper limit of 4.6 kG was estimated for the standard deviation of the magnetic hyperfine field distribution which resulted from the accumulated radiation damage. For the annealed absorber material the standard deviation of the magnetic field is less than 53 G.     

Perturbed nuclear scattering of synchrotron radiation

Mössbauer spectroscopy in the time domain with SR as a source may become a powerful tool in investigations of time-dependent hyperfine interactions caused by external perturbations. This is firstly due to the easy possibility to synchronize the excitation of the nuclei with an external perturbation and secondly due to the insensitivity of Mössbauer time spectra to mechanical vibrations which are generated in samples by these perturbations. As a particular case of perturbed nuclear scattering, the coherent scattering under conditions of 90°- and 180°-switching of the magnetic hyperfine field direction is considered. The results of the first experimental studies are discussed.     

High-frequency magnetic modulation of recoilless gamma radiation of57Fe

Radio-frequency (rf) magnetic modulation has been used to generate sidebands in⁵⁷Fe Mössbauer spectra of Fe_0.18Ni_0.82 Permalloy foils which have the smallest constant of magnetostriction among Fe?Ni alloys. Sidebands in Mössbauer spectra were observed at 30 MHz and 55 MHz. In addition to the generation of sidebands, the external rf magnetic field was found to alter the line positions of the original six line spectrum. An attempt was made to study acoustic vibrations in the foil by means of X-ray diffraction. The rf magnetic field caused changes in diffraction peak intensities and positions. It was found that X-ray diffraction can be used to study the amplitude of acoustic vibrations in Permalloy foils.     

Glass forming tendency in the FeBSn system

The glass forming tendency in the Fe-B-Sn system is studied. Samples were prepared by melt quenching and analyzed by X-ray diffraction. The glasses obtained were also analyzed by⁵⁷ Fe Mössbauer spectroscopy. Mössbauer spectra were fitted using hyperfine field distributions.     

General cases of the pseudoquadrupole interaction in169Tm

Clauser and Mössbauer [1] showed that the second order (in perturbation theory) magnetic hyperfine interaction, the so called pseudoquadrupole inter-action, in the¹⁶⁹Tm Mössbauer effect can produce apparent isomer shifts. While they assumed two isolated singlets and simplified the coupling, we will show that the second order magnetic hyperfine coupling can be observed in general for any kind of crystalline electric field (CEF) wavefunctions, and that the coupling between many states must often be considered in calculating it. Furthermore since the first order magnetic coupling of the excited CEF states is important, the interaction is best accounted for by diagonalization of the full magnetic hyperfine and CEF Hamiltonian. Examples of the effects that may occur are presented.Work supported by the U. S. Department of Energy.     

Mössbauer effect studies on magnetic thin films

The electric and magnetic hyperfine interactions measured in Mössbauer experiments and their general relations to magnetic properties of solids are summarized briefly. The particular possibilities and experimental difficulties, which arise in applications of the Mössbauer method to thin film problems, are discussed. The main results of the so far rather scarce Mössbauer studies of magnetic thin films are reviewed.