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"

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

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

Effect of Mn2+ ions on the magnetic microstructure of hexaferrites

Effect of Mn2+ ions on the magnetic microstructure of substituted hexaferrites SrFe12 - 2xMnxTixO19 was studied using the Mössbauer spectroscopy data. A new method is developed for estimating the hyperfine interaction parameters in substituted ferrites, and is based on a quasicontinuous description of their Mössbauer spectra. It is shown that a single substitution of manganese for iron in the second coordination shell of Fe3+ changes the local magnetic field strength at this ion by approximately 20 kOe, this value being independent of the concentration of substituted ions. © 2000 MAIK "Nauka/Interperiodica"