Synthesis of the aluminum-substituted hexaferrite SrFe 9.5Al 2.5O 19

The phase-formation processes involved in the synthesis of the hexagonal ferrite SrFe 9.5Al 2.5O 19 by solid-state reaction at 900°C for 5 min to 8 h were studied by x-ray diffraction and Mössbauer spectroscopy. The formation of the hexagonal ferrite at this temperature was found to take 3 h. The resultant material also contained SrAl 2O 4 and SrFeO 3-x, which suggests that, for the synthesis to reach completion, the heat-treatment temperature should be higher. The aluminum cations in the hexaferrite phase were shown to occupy, for the most part, positions 12k and 4f 1. © 1999 MAHK "Hayka/Interperiodica"

Noether's second theorem in a general setting. Reducible gauge theories

We prove Noether's direct and inverse second theorems for Lagrangian systems on fiber bundles in the case of gauge symmetries depending on derivatives of dynamic variables of an arbitrary order. The appropriate notions of reducible gauge symmetries and Noether's identities are formulated, and their equivalence by means of certain intertwining operator is proved.Comment: 20 pages, to be published in J. Phys. A (2005

Maximum Renyi entropy principle for systems with power--law Hamiltonian

The Renyi distribution ensuring the maximum of a Renyi entropy is investigated for a particular case of a power--law Hamiltonian. Both Lagrange parameters, $\alpha$ and $\beta$ can be excluded. It is found that $\beta$ does not depend on a Renyi parameter $q$ and can be expressed in terms of an exponent $\kappa$ of the power--law Hamiltonian and an average energy $U$. The Renyi entropy for the resulted Renyi distribution reaches its maximal value at $q=1/(1+\kappa)$ that can be considered as the most probable value of $q$ when we have no additional information on behaviour of the stochastic process. The Renyi distribution for such $q$ becomes a power--law distribution with the exponent $-(\kappa +1)$. When $q=1/(1+\kappa)+\epsilon$ ($0<\epsilon\ll 1$) there appears a horizontal "head" part of the Renyi distribution that precedes the power--law part. Such a picture corresponds to observables.Comment: LaTeX, 7 pages, 4 figure

Fractal geometry, information growth and nonextensive thermodynamics

This is a study of the information evolution of complex systems by geometrical consideration. We look at chaotic systems evolving in fractal phase space. The entropy change in time due to the fractal geometry is assimilated to the information growth through the scale refinement. Due to the incompleteness of the state number counting at any scale on fractal support, the incomplete normalization $\sum_ip_i^q=1$ is applied throughout the paper, where $q$ is the fractal dimension divided by the dimension of the smooth Euclidean space in which the fractal structure of the phase space is embedded. It is shown that the information growth is nonadditive and is proportional to the trace-form $\sum_ip_i-\sum_ip_i^q$ which can be connected to several nonadditive entropies. This information growth can be extremized to give power law distributions for these non-equilibrium systems. It can also be used for the study of the thermodynamics derived from Tsallis entropy for nonadditive systems which contain subsystems each having its own $q$. It is argued that, within this thermodynamics, the Stefan-Boltzmann law of blackbody radiation can be preserved.Comment: Final version, 10 pages, no figures, Invited talk at the international conference NEXT2003, 21-28 september 2003, Villasimius (Cagliari), Ital

Magnetic microstructure in strontium hexaferrites with correlated nonisomorphous substitutions

The influence of the correlated nonisomorphous substitution of Mn2+ and Ti4+ ions for Fe3+ ions in M-type hexaferrites on the shaping of their magnetic microstructure is studied. Conclusions regarding the possibility of the distribution of the substituent cations in the structure and their influence on the principal magnetic properties of the hexaferrites studied are drawn on the basis of data from Mössbauer spectroscopy. © 1997 American Institute of Physics

Mössbauer study of some perovskite-related oxides

Complex perovskite-related oxides with various structure types: orthoferrites, 4-1-5, 8-8-20, and 1-2-3 were studied using Mössbauer spectroscopy. Correlations between magnetic ordering of iron atoms and their structural coordination, as well as between oxygen deficiency and iron ion valence, were found. © 2006 by Allerton Press, Inc

Superexchange interactions and magnetic microstructures of oxyfluoride ferrospinels

A Mössbauer study has been performed on a series of oxyfluoride ferrites based on nickel ferrite. Anion substitution results in new magnetically nonequivalent positions for the iron ions, which are characterized by lower local magnetic fields at the nucleus. The ratios between the different types of cations affect the cation distribution in the octahedral positions. The Néel temperatures calculated in this way describe the experimental values satisfactorily and indicate that the fluorine weakens the superexchange interaction between sublattices, which is confirmed also by the temperature dependence of the local magnetic fields at the iron ions with different anionic environments. © 1985 Plenum Publishing Corporation

Mössbauer studies of sublattice magnetization curves of ferromagnets with several species of characterizing ions

The paper gives a model of a local molecular field describing the magnetization curves in iron garnets separately according to magnetic sublattices. It takes account of the relation between the ion distribution over the lattice sites as well as the values of the local magnetic field and the parameters of the superexchange interaction in such materials. The relations obtained are compared with the data of Mössbauer and NMR spectroscopy. © 1980 Plenum Publishing Corporation

Investigation of the cation distribution in perovskite-like oxides with mössbauer spectroscopy

Complex perovskite-like oxides, such as LnFe(2/3)Mo(1/3)O"3 orthoferrites, Ln(8 -) ySryCu(8 -) xFexO"2"0 (8-8-20), Pr"4BaCu"4FeO(13 - δ) (4-1-5), YBa(2 -) yLayCu(3 -) xFexO(7 - δ), and Y(1 -) yCayBa(2 -) yLayCu(3 -) xFexO(7 - δ) (1-2-3), are studied by means of Mössbauer spectroscopy. At room temperature, the spectra of the orthoferrites contain only magnetic components. The spectra of the 1-2-3 compounds contain only magnetically disordered components: iron atoms substitute for copper at Cu(1) sites, taking various configurations: planar squares, quadratic pyramids, and octahedra. Cuprates 8-8-20 and 4-1-5 have a wide diversity of spectra. In the 8-8-20 oxides, a phase related to the pyramidal environment of the iron cations is present at any iron concentration. In all the perovskites, iron cations become magnetically ordered only at octahedral sites of the structure. © 2005 Pleiades Publishing, Inc