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.     

Some magnetic properties of the cores of various ferritins

⁵⁷Fe Mössbauer spectra of the mineral cores of various ferritins show both paramagnetic and superparamagnetic relaxation effects and in some cases a combination of both. Correlations have been observed between data on the crystallinity, phosphate content and percentage iron in the cores and their magnetic ordering and mean blocking temperatures derived from the Mössbauer spectra.     

Simple theory of superparamagnetism and spin-tunneling in Mössbauer spectroscopy

The magnetic relaxation of isolated small (< 100 Å) monodomain magnetic particles is due to superparamagnetic relaxation (predominant at high temperatures) and eventually quantum tunneling of the magnetic moment (at low temperatures). The superparamagnetic relaxation process can be formally described by an (multiple phonon absorption and emission) Orbach process with an anisotropy Hamiltonian due to crystalline or form anisotropy \widehat_Ion = S² _z and a usual dynamical spin-Hamiltonian for the spin--phonon interaction. From this Mössbauer spectra can be calculated using ab-initio or stochastic methods. Phonon-assisted tunneling and its influence on Mössbauer spectra are discussed.     

Relaxation Mössbauer spectra of polymer composites with magnetic nanoparticles

It is shown that the shape of Mössbauer spectra of composites based on single-domain magnetic particles is defined to a greater degree by interparticle interactions rather than the superparamagnetic properties of individual particles. The characteristic features of the spectra of such systems are described and shown to be adequately described within a three-level relaxation model.     

Influence of solvent on the superparamagnetic relaxation of nanocrystalline Fe3O4 in ferrofluids

Mössbauer spectra for 5 nm Fe₃O₄ nanocrystallites coated with different surfactants were measured and show a significant influence on superparamagnetic relaxation with and without the solvent in ferrofluids.     

A study of the sump beaver's dental enamel

The nature of the iron-containing phase in the dental enamel of sump beaver incisors has been studied by use of Mössbauer spectroscopy. All iron atoms are found to be trivalent and to be present in particles exhibiting superparamagnetic relaxation. The iron-containing phase has Mössbauer parameters similar to those of ferritin.     

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.     

Spin fluctuctuation phenomena in small particle magnetic systems: A Mössbauer spectroscopic study of collective excitations and superparamagnetic relaxation in haemosiderin

Mössbauer spectra obtained from a sample of the iron-storage protein haemosiderin have been analysed in both the collective excitation and superparamagnetic relaxation temperature regions. The results indicate that the low temperature spectra cannot be explained in terms of collective excitations, and that there must be other effects leading to the observed distribution of hyperfine fields.     

Transparency of a thin absorber in Mössbauer optics with allowance for electron relaxation

A model of Mössbauer absorption in the nuclear level anticrossing regime with allowance for electron relaxation has been proposed. The role of quantum interference in the creation of particular transparency on Mössbauer transitions under these conditions has been determined and the dependence of this characteristic on the relaxation and mixing parameters has been obtained using the stochastic theory of Mössbauer relaxation spectra.     

Simulation of Mössbauer Spectra of Superparamagnets under External Fields Using a Many-State Model

Hyperfine Interactions - The magnetization in a uniaxial superparamagnetic particle is modelled by spin-phonon relaxation of a giant spin. A multi-level formalism is used to simulate Mössbauer...     

Mössbauer investigations on glass-forming organic liquids

Glycerol forms a molecular glass near 180K. Fe²⁺ dissolved in glycerol allows the study of the dynamics of the system by Mössbauer spectroscopy. Recently it has been shown that the Mössbauer spectra can be understood in a way consistent with the results of dielectric and ultrasonic viscoelastic relaxation measurements. A jump diffusion model of Sinqwi and Sjolander with a jump rate distribution according to Davidson and Cole allowed us to fit the Mössbauer spectra of Fe in glycerol. First attempts to compare mode coupling theory with Mössbauer spectra are reported.     

Mössbauer spectra of a paramagnetic ion in an axial crystal field in the presence of spin-lattice relaxation

The influence of spin-lattice relaxation on the Mössbauer hyperfine spectra of the Fe³⁺ ion is investigated in the case of an axial crystal field. All the various influences of the relaxation process on the spectra can be explicitly described using two relaxation parameters. A detailed analysis of the Mössbauer relaxation spectra for the various temperatures, relations between the relaxation parameters and external magnetic field directions is carried out. When the magnetic field direction is nearly collinear with the symmetry axis and one of the relaxation parameters is small, the dynamics of Mössbauer spectra is shown to have anomalous features. The influence of random magnetic fields is shown to give an unconventional development of patterns as a function of the relaxation parameters.     

Mössbauer spectroscopic studies of haemosiderins from different sources

Mössbauer spectra obtained from the haemosiderin isolated from ironoverloaded horse and reindeer spleen and liver show superparamagnetic behaviour with a much lower transition temperature than haemosiderin from iron-overloaded human spleen. These data clearly indicate the existence of different forms of haemosiderin, which are primarily distinguishable by Mössbauer spectroscopy.     

Magnetic Study of Nanocrystalline Ferrites and the Effect of Swift Heavy Ion Irradiation

100 MeV Si⁺⁷ irradiation induced modifications in the structural and magnetic properties of Mg_0.95Mn_0.05Fe₂O₄ nanoparticles have been studied by using X-ray diffraction, Mössbauer spectroscopy and a SQUID magnetometer. The X-ray diffraction patterns indicate the presence of single-phase cubic spinel structure of the samples. The particle size was estimated from the broadened (311) X-ray diffraction peak using the well-known Scherrer equation. The milling process reduced the average particle size to the nanometer range. After irradiation a slight increase in the particle size was observed. With the room temperature Mössbauer spectroscopy, superparamagnetic relaxation effects were observed in the pristine as well as in the irradiated samples. No appreciable changes were observed in the room temperature Mössbauer spectra after ion irradiation. Mössbauer spectroscopy performed on a 12 h milled pristine sample (6 nm) confirmed the transition to a magnetically ordered state for temperatures less than 140 K. All the samples showed well-defined magnetic ordering at 5 K, whereas, at room temperature they were in a superparamagnetic state. From the magnetization studies performed on the irradiated samples, it was concluded that the saturation magnetization was enhanced. This was explained on the basis of SHI irradiation induced modifications in surface states of the nanoparticles.     

On the Néel temperature of βFeOOH: Structural dependence and its implications

Using ⁵⁷Fe Mössbauer thermoscanning, the magnetic transition temperature T_N of a number of synthetic βFeOOH samples (akaganeite) is studied. Mössbauer spectra in external fields up to 6 T reveal the sextet-doublet transition to be a true magnetic order-disorder transition and that no superparamagnetic relaxation occurs. It is shown that the value of T_N in particular and the strength of the magnetic interactions in general are highly variable and depend upon the chemical circumstances during synthesis; this is due to spin reduction, induced by a variable amount of interstitial water molecules in the compound.     

On the shape of the gamma resonance spectra of slowly relaxing nanoparticles in a magnetic field

A nonstandard shape of the gamma resonance spectra of nanoparticles in the form of inverted five-step pedestal has been predicted, observed, and analytically described. This shape corresponds to the limit of high temperatures and slow relaxation of the homogeneous magnetization of single-domain particles with axial magnetic anisotropy. To describe the Mössbauer spectra of the ensemble of chaotically oriented nanoparticles in a magnetic field, a continual magnetic-dynamics model has been developed in the limit of slow relaxation. This model adequately describes the polarization effects observed in the experimental absorption spectra. The revealed features significantly expand the methodical capabilities of Mössbauer spectroscopy for the diagnostics of magnetic nanomaterials.     

Mössbauer study of biofilms formed at spring caves of Buda Karst,Hungary

Biofilms formed at spring caves of Buda Karst, Hungary, were investigated by ⁵⁷Fe Mössbauer spectroscopy. 78 K Mössbauer spectra were decomposed into a sextet and two doublets. The subspectra were assigned to goethite, hematite, ferrihydrite and siderite, according to their known Mössbauer parameters. The room temperature spectra indicated that goethite and/or hematite are in the superparamagnetic state at room temperature. The results can be interpreted in terms of karstification of hypogenic caves by the role of biofilms via discharge features.     

Mössbauer and X-ray study of biodegradation of <Superscript>57</Superscript>Fe<Subscript>3</Subscript><Emphasis Type="Italic">O</Emphasis><Subscript>4</Subscript> magnetic nanoparticles in rat brain

Biodegradation of a ⁵⁷Fe₃ O ₄ - based dextran - stabilized ferrofluid in the ventricular cavities of the rat brain was studied by X-ray diffraction and Mössbauer spectroscopy. A two-step process of biodegradation, consisting of fast disintegration of the initial composite magnetic beads into separate superparamagnetic nanoparticles and subsequent slow dissolution of the nanoparticles has been found. Joint fitting of the couples of Mössbauer spectra measured at different temperatures in the formalism of multi-level relaxation model with one set of fitting parameters, allowed us to measure concentration of exogenous iron in the rat brain as a function of time after the injection of nanoparticles.