A. M. Afanas’ev

Mössbauer spectra of Stoner-Wohlfarth particles in rf fields in a modified relaxation model

A theory of Mössbauer absorption spectra in the presence of external rf fields is developed for Stoner-Wohlfarth particles in an extended relaxation model with a more realistic description of the relaxation process. Calculating in this model, we track the transformation of the Mössbauer spectra as a function of relaxation processes for the transitional region and frequencies and amplitudes of the rf field where the well-resolved hyperfine structure transitions into an isolated central peak with satellites. In this transitional region new types of resonance effects are found which have no analogy with previously known resonance phenomena.

New relaxation model for superparamagnetic particles in Mössbauer spectroscopy

A new model is suggested for the relaxation in a system of superparamagnetic particles. The model takes into account the interparticle interaction and ensuing smearing of energy levels for each individual particle, such that the relaxation between the particle states with opposite directions of magnetic moment never occurs as a transition between the states of the same energy. This generalization of the relaxation model accounts for the diversity of relaxation Mössbauer absorption spectra, allowing all the nonstandard features that were observed previously in the experimental spectra of systems with small-sized particles to be described on a qualitative level.

Structural characterization of quantum-well layers by double-crystal X-ray diffractometry

It is demonstrated that double-crystal X-ray diffractometry, which has been known for more than 50 years, is a powerful tool for the characterization of modern materials used in microelectronics and nanoelectronics that makes it possible to thoroughly investigate very thin layers and interfaces of nanometer sizes in complex heterostructures. The physical and mathematical principles of this method are analyzed, and the most prominent examples of its application are given. It is established that, in a number of cases, quantum-layer displacements as small as several hundredths of a nanometer can be reliably determined in heterostructures.

Alternative approach for evaluation of Mössbauer spectra of nanostructured ferromagnetic alloys within generalized two-level relaxation model

The simplest treatment of the complex 57Fe Mössbauer absorption spectra of nanostructured Fe-Cu-Nb-B alloys within the recently developed generalized two-level relaxation model has been successfully performed. This model applied for a system of superparamagnetic particles allows one to take into account the interparticle interaction in a simpler form and to describe qualitatively a specifically asymmetric shape of Mössbauer lines with sharp outer and smeared inward sides when the conventional two-level relaxation model fails. The approach is actually an alternative way in order to evaluate the Mössbauer spectra of nanostructured ferromagnetic alloys without taking into consideration a rather wide and diverse distribution over the particle sizes.

Double-crystal X-ray diffractometry in the role of X-ray standing-wave method

It is shown that the atomic displacements (induced by foreign layers) comparable with or smaller than the interatomic distances can be detected in perfect multilayer systems by double-crystal X-ray diffractometry alone. It was earlier thought that the detection of displacements as small as those was accessible only to the specific methods such as the X-ray standing-wave method. The measurements were carried out on a GaAs/InAs/GaAs system, where InAs was a foreign layer. Its thickness did not exceed three monolayers, while the structure was of the insular type and represented a set of separate quantum dots. The displacement of the capping GaAs layer relative to the GaAs buffer was measured with an accuracy of less than 0.1 of the thickness of the atomic layer.

Identification of Iron Oxidation States in the Products of Interaction of Na 2 O 2 and Fe 2 O 3 by Mössbauer Absorption Spectroscopy

The first stage of the solid-phase reaction of Na2O2 and Fe2O3 yields a tetravalent iron derivative. The product is unstable and disproportionates to form compounds with different oxidation states of iron. Analysis of their Mössbauer spectra was performed with the DISCVER program based on the Afanas’ev-Chuev method. At the early stage of analysis, the program identifies the maximal possible number of well-defined lines in the spectrum with a specified statistical quality and, thus, discerns a large number of known and unknown iron derivatives (phases) in samples of complex composition. Previously unknown highest oxidation states of iron from +5 to +8 were identified.

Magnetic “doublets” in Mössbauer spectra of superparamagnetic particles

It is shown that the Mössbauer magnetic hfs spectra of a superparamagnetic particle are cardinally affected by the rotation of its magnetic moment about the magnetic anisotropy field. This rotation renormalizes the nuclear g factors and qualitatively transforms the spectra. In particular, apart from the well-known magnetic sextet, five, four, three, and even two (magnetic “doublet”) lines can arise in the 57Fe absorption spectra.

Mössbauer Effect in Iron-Based Nanocrystalline Alloys

Nanocrystalline ferromagnetic alloys of the Fe-Cu-Nb-B type consist of a ferromagnetic amorphous matrix in which nanograins are embedded. These nanograins normally are so small (typical diameter in the range of 5-30 nm) that, if they were free, their magnetic relaxation time would fall into the time window of the 57Fe Mossbauer spectrometry and relaxation phenomena would become visible. It is well known that Mossbauer spetroscopy easily perceives relaxation processes on the time scale (10-11 s to 10-6 s). Normally the nanograins are coupled to the ferromagnetic matrix and an interaction between these two components must be regarded, too. This coupled situation influences the thermally driven fluctuation of the nanograins’ magnetization and often led to the very common interpretation of Mossbauer spectra in terms of hyperfine field distributions (also distributions of quadrupole splitting and isomer shift). Due to the high degree of complexity such spectra consist of many spectral components and an analysis becomes a rather complex task. Recently Miglierini and Greneche [1,2] proposed a new idea to interpret such hyperfine field distributions. From their paper follows that in the spectra information is available about the so-called interface between the nanograins and the residual matrix, too. Nevertheless contrary arguments (Balogh et al. [3]) and also the idea of relaxation influence on the linewidth are present in literature (Kemeny et al. [4]).

New Trends in Mössbauer Spectroscopy Focused on Nanostructured Magnetic Materials

Mossbauer spectroscopy proved to be a powerful technique for characterisation of ironbased nanocrystalline magnetic alloys (NCMA) due to, first of all, its local character which allows one to elucidate the nature of hyperfine interactions of the iron nuclei in different crystallographic sites and to probe the nature of their immediate surroundings. Mossbauer spectra of NCMA and amorphous materials as a whole consist usually of a great number of overlapping lines which are due to a variation of hyperfme parameters from site to site, so that extracting the parameters of hyperfme structure requires a corresponding mathematical processing.

Kossel lines as a new type of X-ray source

The radiation intensity distribution within the Kossel line corresponding to the extremely asymmetric pattern of X-ray diffraction has an anomalous form of a clearly manifested peak exceeding the background intensity by more than two orders of magnitude. A detailed theoretical analysis of this effect is carried out and versions of experimental observation of anomalous Kossel lines are proposed. The possibility of the employment of the effect for obtaining a new source of X rays with a narrow angular collimation is discussed.