Phase Transitions in Magneto-Electric Hexaferrites

Hexaferrites have long been the object of extensive studies because of their great possibility for applications-permanent magnets, high-density recording media, microwave devices, in biomedicine, to name but a few. Lately, many researchers' efforts have been focused on the existence of the magneto-electric effect in some hexaferrite systems and the appealing possibility of them being used as single-phase multiferroic and magneto-electric materials. As indicated by theoretical analyses, the origin of the large magneto-electric effect can be sought in the strong interaction between the magnetization and the electric polarization that coexist in insulators with noncollinear magnetic structures. The hexaferrites' magnetic structure and, particularly, the specific magnetic spin ordering are the key factors in observing magneto-electric phases in hexaferrites. Some of these phases are metastable, which hampers their direct practical use. However, as the hexaferrites' phase diagrams reveal, chemical doping can be used to prepare a number of noncollinear stable magnetic phases. Since the magneto-electric effect has to do with the magnetic moments ordering, it seems only logical that one should study the cation substitutions' influence on the magnetic phase transition temperature. In this paper, we summarize recent examples of advances in the exploration of magnetic phase transitions in Y-type hexaferrites. In particular, the effect is emphasized by substituting in Y-type hexaferrites the nonmagnetic Me²⁺ cations with magnetic ones and of the magnetic Fe³⁺ cations with nonmagnetic ones on their magnetic properties and magnetic phase transitions. The work deals with the structural properties of and the magnetic phase transitions in a specific Y-type hexaferrite, namely, Ba(Sr)₂Me₂Fe₁₂O₂₂.

BaFe0.875Re0.125O3−δ and BaFe0.75Ta0.25O3−δ as potential cathodes for solid-oxide fuel-cells: a structural study from neutron diffraction data

In this work, two new perovskites of composition BaFe0.875Re0.125O3−δ and BaFe0.75Ta0.25O3−δ , designed from ab-initio calculations to fulfill different requisites of cathode materials for solid-oxide fuel cells (SOFC), were prepared and studied from the structural point of view from neutron powder diffraction (NPD) data. They are both derivatives of BaFeO3 hexagonal perovskite (space group P6 3 /mmc), typified as the 6H polytype, stabilized when the perovskite tolerance factor slightly overpasses the unity. Whereas BaFe0.875Re0.125O3−δ keeps this structural type, as demonstrated in this crystallographic study from NPD data at 295 and 4 K, with unit-cell parameters a = 5.70177(7); c = 14.0334(2) Å at 295 K, the second material, BaFe0.75Ta0.25O3−δ , is cubic and can be defined in the Pm-3m space group, corresponding of the perovskite arystotype, with a = 4.05876(3) Å. A conspicuous oxygen deficiency is observed, with a refined stoichiometry of 2.86(3) per formula unit. The anisotropic displacement factors for oxygen atoms in this last material are flattened disks perpendicular to the (Fe,Ta)-O-(Fe,Ta) direction, suggesting a dynamic tilting of the octahedra that could be related to the oxygen motion via oxygen vacancies across the structure. This is a pre-requisite for functional mixed-ionic-electronic (MIEC) materials performing as cathodes in SOFC.

Data supporting the results of the characterization of the phases and structures appearing during the synthesis process of Ba0.5Sr1.5Zn2-xNixFe12O22 by auto-combustion

The data presented has to do with identifying the various phases arising during the synthesis of the Y-type hexaferrite series Ba_0.5Sr_1.5Zn_2-xNi_xFe₁₂O₂₂ by auto-combustion that we deem important for their microstructural and magnetic properties. The data and the related analyses support the research paper "Ni-substitution effect on the properties of Ba_0.5Sr_1.5Zn_2-xNi_xFe₁₂O₂₂ powders" [1]. Thus, the parameters are presented of the phases appearing after auto-combustion and after the initial annealing at 800 °C, namely, crystal cell and crystallite size. Also, additional data are provided obtained by EDS concerning the Ba:Sr:Zn:Ni:Fe ratio in Ba_0.5Sr_1.5Zn_2-xNi_xFe₁₂O₂₂ (x = 0.8, 1, 1.5) samples synthesized at 1170 °C for 10 h. The data can be used as a reference in establishing how the phases distinguished during the initial process of auto-combustion affect the Ba_0.5Sr_1.5Zn_2-xNi_xFe₁₂O₂₂ powders, which are candidates for room-temperature multiferroic materials. The data have not been published previously and are made available to permit critical or further analyses.

Structural, Magnetic and Microwave Characterization of Polycrystalline Z-Type Sr3Co2Fe24O41 Hexaferrite

We report results on the structural and microwave properties and magnetic phase transitions in polycrystalline Sr₃Co₂Fe₂₄O₄₁ hexaferrite synthesized by sol-gel auto-combustion and acting as a filler in a composite microwave absorbing material. The zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves revealed a change in the magnetization behavior at 293 K. The reflection losses in the 1-20 GHz range of the Sr₃Co₂Fe₂₄O₄₁ powder dispersed homogeneously in a polymer matrix of silicon rubber were investigated in both the absence and presence of a magnetic field. In the latter case, a dramatic rise in the attenuation was observed. The microwave reflection losses reached the maximum value of 32.63 dB at 17.29 GHz in the Ku-band. The sensitivity of the microwave properties of the composite material to the external magnetic field was manifested by the appearance of new reflection losses maxima. At a fixed thickness t_m of the composite, the attenuation peak frequency can be adjusted to a certain value either by changing the filling density or by applying an external magnetic field.

Study of the Structural and Magnetic Properties of Co-Substituted Ba2Mg2Fe12O22 Hexaferrites Synthesized by Sonochemical Co-Precipitation

Ba₂Mg_0.4Co_1.6Fe₁₂O₂₂ was prepared in powder form by sonochemical co-precipitation and examined by X-ray diffraction, Mössbauer spectroscopy and magnetization measurements. Careful XRD data analyses revealed the Y-type hexaferrite structure as an almost pure phase with a very small amount of CoFe₂O₄ as an impurity phase (about 1.4%). No substantial changes were observed in the unit cell parameters of Ba₂Mg_0.4Co_1.6Fe₁₂O₂₂ in comparison with the unsubstituted compound. The Mössbauer parameters for Ba₂Mg_0.4Co_1.6Fe₁₂O₂₂ were close to those previously found (within the limits of uncertainty) for undoped Ba₂Mg₂Fe₁₂O₂₂. Isomer shifts (0.27-0.38 mm/s) typical for high-spin Fe³⁺ in various environments were evaluated and no ferrous Fe²⁺ form was observed. However, despite the indicated lack of changes in the iron oxidation state, the cationic substitution resulted in a significant increase in the magnetization and in a modification of the thermomagnetic curves. The magnetization values at 50 kOe were 34.5 emu/g at 4.2 K and 30.5 emu/g at 300 K. The zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves were measured in magnetic fields of 50 Oe, 100 Oe, 500 Oe and 1000 Oe, and revealed the presence of two magnetic phase transitions. Both transitions are shifted to higher temperatures compared to the undoped compound, while the ferrimagnetic arrangement at room temperature is transformed to a helical spin order at about 195 K, which is considered to be a prerequisite for the material to exhibit multiferroic properties.

RADON LEVELS AND ЕQUIVALENT DOSE RATES AT THE IRT-SOFIA RESEARCH REACTOR SITE

Results from radon measurements by active sampling of indoor air in the buildings within the Nuclear Scientific Experimental and Educational Centre (NSEEC) protected site at the Institute for Nuclear Research and Nuclear Energy (INRNE) are presented. The inspected buildings included in this report are the IRT research reactor structure and several auxiliary formations wherein the laundry facilities and the gamma irradiator GOU-1 (60Co source) are installed as well as the Central Alarm Station (CAS) premises. Besides the reactor hall and the primary cooling loop area, special attention was given to the premises of the First Class Radiochemical Laboratory in the IRT reactor basement. Determination of radon concentration distribution in the premises of the constructions within the site is an important part of radiation surveillance during the operation and maintenance of the NSEEC facilities as well as for their involvement in the educational activities at INRNE.

Investigation of iron-containing products from natural and laboratory cultivated Sphaerotilus-Leptothrix bacteria

Bacterial biomass collected from sheath-forming bacteria of the genera Sphaerotilus and Leptothrix was collected from a high-mountain natural stream water source. The elemental constitution and oxide phases of the products after selective cultivation of the bacteria on two different elective media using neutron activation analysis (NAA), electron microscopy (SEM, TEM), and X-ray diffraction (XRD) were studied. A high enrichment level of iron was revealed by the NAA technique in cultivated isolates as compared to the reference sample from nature. Three types of iron oxide compounds were established after cultivation in Adler's medium: lepidocrocite (γ-FeOOH), magnetite (Fe3O4), and goethite (α-FeOOH). The cultivation in the Isolation medium yielded a single phase, that of goethite, excluding one sample with a distinguishable amount of lepidocrocite. XRD and EM investigations show that the biogenic oxides are nanosized. Our study exemplifies the possibilities of the biotechnology approach for obtaining, under artificial conditions, large quantities of iron-containing by-products that could be of further used in appropriate nano- and biotechnologies.

Radiation protection, radioactive waste management and site monitoring at the nuclear scientific experimental and educational centre IRT-Sofia at INRNE-BAS

This article identifies important components and describes the safe practices in implementing radiation protection and radioactive waste management programmes, and in their optimisation at the Nuclear Scientific Experimental and Educational Centre with research reactor IRT at INRNE-BAS. It covers the instrumentation and personal protective equipment and organisational issues related to the continuous site monitoring. The reactor is under major reconstruction and the measures applied to radiation monitoring of environment and working area focused on restricting the radiation exposure of the staff as well as compliance with international good practices related to the environmental and public radiation safety requirements are also addressed.

Influence of the Bi3+ electron lone pair in the evolution of the crystal and magnetic structure of La(1-x)Bi(x)Mn2O5 oxides

La(1-x)Bi(x)Mn2O5 (x = 0, 0.2, 0.4, 0.6, 0.8 and 1) oxides are members of the RMn2O5 family. The entire series has been prepared in polycrystalline form by a citrate technique. The evolution of their magnetic and crystallographic structures has been investigated by neutron powder diffraction (NPD) and magnetization measurements. All the samples crystallize in an orthorhombic structure with space group Pbam containing infinite chains of Mn(4+)O6 octahedra sharing edges, linked together by Mn(3+)O5 pyramids and (La/Bi)O8 units. These units become strongly distorted as the amount of Bi increases, due to the electron lone pair of Bi(3+). All the members of the series are magnetically ordered below TN = 25-40 K and they present different magnetic structures. For the samples with low Bi content (x = 0.2 and 0.4) the magnetic structure is characterized by the propagation vector k = (0,0,1/2). The magnetic moments of the Mn(4+) ions placed at octahedral sites are ordered according to the basis vectors (Gx, Ay, 0) whereas the Mn(3+) moments, located at pyramidal sites, are ordered according to the basis vectors (0, 0, Cz). When the content of Bi increases, two different propagation vectors are needed to explain the magnetic structure: k1 = (0,0,1/2) and k2 = (1/2,0,1/2). For x = 0.6 and 0.8, k2 is predominant over k1 and for this propagation vector (k2) the magnetic arrangement is defined by the basis vectors (Gx, Ay,0) and (Fx, Cy, 0) for Mn(4+) and Mn(3+) ions, respectively.