We discuss the development of a correct theory of Mössbauer spectra of an iron borate single crystal, starting with a simple, but desperately needed technique of correction of any Mössbauer spectrum for the absorber (crystal) thickness, which is followed by the treatment of the Mössbauer spectra in the framework of the Hamiltonian of combined hyperfine magnetic and quadrupole interaction of 57Fe nuclei in ground and excited states. The latter results in appearance of two additional spectral lines to a conventional magnetic sextet and describes the specific asymmetry of pairs of lines observed in low-temperature experimental spectra of FeBO3 crystals. We demonstrate and explain also the characteristic difference in the shape of Mössbauer spectra of the FeBO3 single crystal and nanoparticles. A drastic difference in the shapes of magnetization curves of the FeBO3 single crystal and nanoparticles is demonstrated and evaluated in the framework of the generalized Stoner-Wohlfarth model which is extended for antiferromagnetic nanoparticles.
Principal difference of magnetic nanoparticles from the bulk matter which cannot be ignored when constructing upon them combined metamaterials and modern devices is the essential influence on their behavior thermal fluctuations of the environment. These disturbances lead to specific distributions of the particles characteristics and to stochastic reorientations of their magnetic moments. On the basis of quantum-mechanical representation of the particle possessing intrinsic magnetic anisotropy and being placed onto the external magnetic field we developed general approach to describe equilibrium magnetization curves and relaxation Mössbauer spectra of magnetic nanoparticles for diagnostics of magnetic nanomaterials in the whole temperature or external field ranges. This approach has universal character and may be applied not only to the systems under thermal equilibrium, but may in principle describe macroscopic dynamical phenomena such as magnetization reversal.
In order to extract quantitative information about characteristics of the magnetic nanoparticles in a media it is necessary to define a model of the magnetic dynamics for treating self-consistently the whole set of the experimental data, particularly, the evolution of Mössbauer spectral shape with temperature and external magnetic field as well as the magnetization curves. We have developed such a model and performed such an analysis of the temperature- and magnetic field-dependent spectra and magnetization curves by the example of nanoparticles injected into laboratory mice. This allowed us to reliably evaluate changes in the characteristics of the residual particles and their chemical transformation to paramagnetic ferritin-like forms in animals organs as a function of time. Actually, the approach makes it possible to quantitatively characterize biotransformation and biodegradation of magnetic nanoparticles delivered in a living organism.
A quantum-mechanical model for describing thermodynamic properties of an ensemble of ideal antiferromagnetic nanoparticles with axial magnetic anisotropy is developed in the first approximation of slowly relaxing macrospins of magnetic sublattices. This model clarifies principally the difference in thermodynamic behavior of ferromagnetic and antiferromagnetic particles revealed in spectroscopic measurements. In particular, one can now qualitatively describe specific (non-superparamagnetic) temperature evolution of the 57Fe Mössbauer spectra of antiferromagnetic nanoparticles, which has been often observed for almost half a century and looks like a quantum superposition of well resolved magnetic hyperfine structure and single line (or quadrupolar doublet of lines) with the temperature-dependent partial spectral areas. This approach can be easily generalized for describing uncompensated antiferromagnetic and ferrimagnetic nanoparticles as well as magnetic relaxation processes, which would allow one to take directly into account the magnetic nature inherent to the particles in analyzing a large amount of experimental data collected so far.
High-temperature magnetic effects observed in the conventional 'low-frequency' magnetization measurements and Mössbauer spectra of nanoparticles are discussed in the framework of a general model for magnetic dynamics of ensemble of single-domain particles in which the uniform magnetization precession orbits are considered as stochastic states. Qualitative analysis and numerical calculations within the 'multilevel' model evidence for the existence of the
asymptotic high-temperature behavior of the magnetization of an ensemble of particles in a weak magnetic field, which is earlier predicted to be different from the classical Langevin limit for ideal superparamagnetic particles. Based on this approach, a simplified three-level stochastic model is developed in order to describe the Mössbauer absorption spectra of an ensemble of magnetic nanoparticles in a weak magnetic field. In particular, this model predicts the appearance of 57Fe magnetic sextets with a small hyperfine splitting in a weak magnetic field and at high temperature, which look like effective "doublets" of lines often observed in experimental spectra.
Magnetic relaxation effects revealed in Moessbauer spectra and magnetization measurements of nanoparticles are
discussed in the framework of a general model for magnetic dynamics of ensemble of single-domain particles. The
phenomenological model is based on a generalization of the well-known Stoner-Wohlfarth model within more accurate
description of relaxation processes and corresponding time-dependent hyperfine interactions in the magnetic system.
This model allows one to treat numerically both Moessbauer spectra in radiofrequency magnetic field and magnetization
curves in alternative low-frequency magnetic field as well as temperature demagnetization FC and ZFC curves in a self-consistent
way within the same set of physical parameters inherent to the system studied. Besides that, a number of
qualitative effects can be explained or predicted within the approach, which include interaction effects, relaxation-stimulated
resonances in Moessbauer spectra under radiofrequency field excitation, specific shapes of Moessbauer
spectra within precession of particle's uniform magnetization, and asymptotic high-temperature magnetization and
susceptibility behavior different from the classical Langevin's high-temperature limit for ideal superparamagnetic
particles. Corrections to the above-mentioned effects within more general models based on the Landau-Lifshitz-Gilbert
or Braun kinetic equations are also discussed.
Electron transport, optical and structure properties of the shallow pseudomorphic quantum wells (QW) GaAs/InGaAs/GaAs are studied by electrophysical, photoluminescence and X-ray double-axes diffractometry methods. It is revealed that insertion of a thin AlAs potential barrier in the center of QW leads to efficient changes of subband structure and mobility. In the case of shallow and narrow quantum wells the observed decrease of mobility is due to appearance of the different scattering mechanism. X-ray diffractometry study is undertaken in order to distinguish whether it is interface roughness scattering from the introduced barrier or another scattering mechanism.
The vast majority of experimental Moessbauer spectra of nanomagnets are treated within a static hyperfine structure for which the line positions and intensities can be described in terms of static magnetic fields acting on nuclei. As has been recently found, the rotation of magnetic moments of nanoparticles results in a renormalisation of the nuclear g-factors, accompanied by a qualitative transformation of absorption spectra. In particular, along with magnetic sextet well known in the Moessbauer spectroscopy of 57Fe isotope, partial spectra consisting of 'magnetic' quintuplet, quartet, triplet and even doublet of lines can be formed. Such a transformation of Moessbauer spectra depends not only on the magnitude of the rotation frequency but also on its sign, which, in turn, may be dictated by the shape and constants of magnetic anisotropy. The predicted effects can essentially modify the conventional scheme of analysis of experimental Moessbauer spectra of nanomagnets.
KEYWORDS: Heterojunctions, X-rays, Crystals, Diffraction, Quantum wells, X-ray diffraction, Data modeling, Scattering, Crystallography, Chemical species
A general mathematical approach is realized for simultaneous treatment of several X-ray rocking curves from different crystallographic planes. The corresponding analysis of experimental X-ray rocking curves for (004), (113) and (115) reflections from the single quantum well GaAs-InxGa1-xAs/GaAs(001) heterostructure have been carried out. This approach allows one to restore the depth profiles of the lattice mismatch and mean-square displacements of atoms from regular positions for particular layers as well as to estimate the anisotropy of in-plane and normal-to-plane random atom displacements.
We analyze the possibility for simultaneous adequate treatment of angular dependencies of the X-ray diffraction reflectivity and photoelectron yield (X-ray standing waves method) in order to extract the structural characteristics of semiconducting materials with ultra fine inclusions. Facilities of such an approach for evaluation of the degree of structural perfection of the layers, the phase shift of upper layers with respect to the buffer, the lattice parameters of particular layers and interfaces between them are demonstrated within the analysis of heterostructures based on the Si matrix with the Si1-xGex quantum wells and on the GaAs matrix with the InAs quantum dots.
Structural characteristics of semiconducting heterostructures AlGaAs/GaAs/AlGaAs containing either single quantum well or two quantum wells separated with a thin AlAs layer are estimated by means of the double-crystal X-ray diffractometry. It is found that an additional Si-doping of outer barrier layers results in the formation of abrupt (less than 2 nm) interfaces at quantum wells.
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