M. Veverka

Magnetic heating by cobalt ferrite nanoparticles

In the quest for suitable materials for hyperthermia we explored the preparation and properties of nanoparticles of Co ferrite. The material was produced by coprecipitation from water solution of Co and Fe chlorides and afterwards annealed at 400, 600 and 800 °C. The resulting particles were characterized by XRD, TEM, Mossbauer spectroscopy, and dc and ac magnetometry. The heating experiments in ac magnetic fields of various amplitudes were performed with diluted systems of particles suspended in agarose gel and the results were interpreted on the basis of the ac magnetic losses measured at various temperatures. The increase of magnetic losses and consequently of the heating efficiency with increasing temperature is explained by the strong dependence of the constant of magnetocrystalline anisotropy of Co ferrite on temperature.

Silica encapsulated manganese perovskite nanoparticles for magnetically induced hyperthermia without the risk of overheating

Nanoparticles of manganese perovskite of the composition La(0.75)Sr(0.25)MnO(3) uniformly coated with silica were prepared by encapsulation of the magnetic cores (mean crystallite size 24 nm) using tetraethoxysilane followed by fractionation. The resulting hybrid particles form a stable suspension in an aqueous environment at physiological pH and possess a narrow hydrodynamic size distribution. Both calorimetric heating experiments and direct measurements of hysteresis loops in the alternating field revealed high specific power losses, further enhanced by the encapsulation procedure in the case of the coated particles. The corresponding results are discussed on the basis of complex characterization of the particles and especially detailed magnetic measurements. Moreover, the Curie temperature (335 K) of the selected magnetic cores resolves the risk of local overheating during hyperthermia treatment.

Electrical resistivity and thermopower measurements of the hole- and electron-doped cobaltites Ln CoO 3

Two extreme members of the cobaltite series, LaCoO3 and DyCoO3, were investigated by the electrical resistivity and thermopower measurements up to 800-1000 K. Special attention was given to effects of extra holes or electrons, introduced by light doping of Co sites by Mg2+ or Ti4+ ions. The experiments on the La based compounds were complemented with magnetic measurements. The study shows that both kinds of charge carriers induce magnetic states on surrounding CoIII sites and form thus thermally stable polarons of large total spin. Their itinerancy is characterized by low temperature resistivity, which is of Arrhenius type r~exp(EA/kT) for the hole (CoIV) doped samples, while an unusual dependence r~1/Tn (n=8-10) is observed for the electron (CoII) doped samples. At higher temperatures, additional hole carriers are massively populated in the CoIII background, leading to a resistivity drop. This transition become evident at ~300 K and 450 K and culminates at TI-M=540 and 780 K for the La and Dy based samples, respectively. The electronic behaviours of the cobaltites are explained considering two excitation processes in parent compounds. The first one is related to a local excitation from the diamagnetic LS CoIII to close-lying paramagnetic HS CoIII state. Secondarily, a metallic phase of the IS CoIII character is formed through a charge transfer mechanism between LS/HS pairs. The magnetic polarons associated with doped carriers are interpreted as droplets of such IS phase.

Distribution of cations in nanosize and bulk Co–Zn ferrites

The structural and magnetic properties of Co(1-x)Zn(x)Fe2O4 ferrites (Co-Zn ferrites) are investigated in a narrow compositional range around x = 0.6, which is of interest because of applications in magnetic fluid hyperthermia. The study by x-ray and neutron diffraction, Mössbauer spectroscopy and magnetization measurements is done on nanoparticles prepared by the coprecipitation method and bulk samples sintered at high temperatures. In spite of the known preference of Zn2+ for tetrahedral (A) sites and Co2+ for octahedral [B] sites, the cations are distributed nearly evenly over the two sites of spinel structure and there is also a variable number of [B] site vacancies (see text), making cobalt ions trivalent. In particular for x = 0.6, the cationic distribution is refined to [Formula: see text] and [Formula: see text] for the 13 nm particles (T(C) = 335 K) and bulk sample (T(C) = 351 K), respectively.

Synthesis of HgBa2CuO4+δ by sol–gel method under controlled oxygen pressure; electron and thermal transport properties

Abstract Samples of mercury superconductor HgBa2CuO4+δ were prepared employing highly homogeneous and reactive precursor Ba2CuO3+x obtained by the sol–gel method. Mixture of Mn3O4/Mn2O3 was used to adjust p(O2) during the synthesis. This approach allows to achieve the appropriate p(O2) at the reaction temperature 720°C and multizone furnace is not required. The enhancement of the thermal conductivity below Tc, measured for the first time in Hg-type superconductor, indicates unusually strong phonon–electron coupling. The enhancement is accompanied by a sharp resistivity and thermopower decrease.

Structure and physical properties of YCoO{sub 3} at temperatures up to 1000 K

The crystal structure and oxygen stoichiometry in two-layer Na{sub 0.74}CoO{sub 2} and Na{sub 0.38}CoO{sub 2} at room temperature are analyzed by powder neutron diffraction. Two sets of diffraction data for each sample, taken at different incident neutron wavelengths, {lambda}=1.1968 A and {lambda}=1.5403 Ang , are analyzed simultaneously by the Rietveld method, allowing for the independent refinement of all structural parameters. The fractional oxygen site occupancies are found to be 1.01(1) for Na{sub 0.74}CoO{sub 2} and 0.99(2) for Na{sub 0.38}CoO{sub 2}, respectively. These results indicate that the oxygen content of these phases is stoichiometric to a precision of 1 to 2%, and therefore the formal cobalt oxidation state is determined solely by the sodium content. The analysis also reveals that both types of sodium ions in the structure are in off-center distorted trigonal prismatic geometry.We prepared a series of Mg{sub 1-x}(AlLi){sub x}B{sub 2} samples with 0{<=}x{<=}0.45 in order to compensate with Li the electron doping induced by Al. Structural characterization by means of neutron and x-ray diffraction confirms that Li enters the MgB{sub 2} structure even though in an amount less than nominal one. We performed susceptibility, resistivity, and specific heat measurements. Vibrational properties were also investigated by means of Raman spectroscopy. We compare these results with those obtained on a homologous series of Mg{sub 1-x}Al{sub x}B{sub 2} samples. The systematic success of scaling the relevant properties with the Al content rather than with the electron doping suggests that lattice deformation plays an important role in tuning the superconducting properties.High-resolution x-ray powder diffraction and extended x-ray-absorption fine-structure (EXAFS) measurements have been performed on the iso-structural framework crystals Cu{sub 2}O and Ag{sub 2}O as a function of temperature. According to diffraction, both compounds exhibit a negative thermal expansion (NTE) of the lattice parameter over extended temperature intervals (from 9 to 240 K for Cu{sub 2}O, up to 470 K for Ag{sub 2}O) and anisotropic thermal displacements of M atoms (M=Cu,Ag). EXAFS measures a positive expansion of the nearest-neighbors M-O pair distance and a perpendicular to parallel anisotropy of relative motion, much stronger than the anisotropy of the absolute M motion. The M-O bond is much stiffer against stretching than against bending. According to EXAFS, out of the 12 M-M next-nearest-neighbor pairs, the 6 connected via a bridging oxygen undergo negative expansion, while the 6 lacking the bridging oxygen undergo positive expansion. These results show a rather complex local behavior, which, while confirming the connection of NTE to strong perpendicular vibrations, is inconsistent with rigid unit modes models and suggests a more flexible model based on rigid M-O rods.The crystal structure of the PbMg{sub 1/3}Ta{sub 2/3}O{sub 3} relaxor ferroelectric was studied under hydrostatic pressure up to {approx}7 GPa by means of powder neutron diffraction. We find a drastic pressure-induced decrease of the lead displacement from the inversion center, which correlates with an increase by {approx}50% of the anisotropy of the oxygen temperature factor. The vibrations of the Mg/Ta are, in contrast, rather pressure insensitive. We attribute these changes being responsible for the previously reported pressure-induced suppression of the anomalous dielectric permittivity and diffuse scattering in relaxor ferroelectrics.The ground state energy and pairing gap of the interacting Fermi gases calculated by the ab initio stochastic method are compared with those estimated from the Bardeen-Cooper-Schrieffer pairing Hamiltonian. We discuss the ingredients of this Hamiltonian in various regimes of interaction strength. In the weakly interacting (1/ak{sub F} or approx. 0, it becomes part of the bare Hamiltonian. However, the bare BCS Hamiltonian is not adequate for describing atomic gases in the regime of weak to moderate interaction strength -{infinity}<1/ak{sub F}<0 such as ak{sub F}{approx}-1.

Phase equilibria in the Hg-Ba-Cu-O system

The already published data of temperatures and enthalpies of decomposition for mercury-based phases were combined with our experimental results from DSC and low-temperature calorimetry and the data for the Ba-Cu-O system taken from the literature. The consistent set of thermodynamic data of all phases in the Hg-Ba-Cu-O system was used for the calculation of the equilibrium phase composition under varying experimental conditions. The selected sections of the p -p -T stability phase diagram were constructed for the ratio of non-volatile components Ba:Cu s 2:1 and the OH g 2

Clusters of Magnetic Nanoparticles as Contrast Agents for MRI: Effect of Aggregation on Transverse Relaxivity

The effect of aggregation of magnetic nanoparticles on the transverse relaxivity (r2) is analyzed with respect to the size of clusters. The nanoparticles employed are based on La0.75Sr0.25MnO3 ferromagnetic phase with the mean size of crystallites dXRD = 26 nm synthetized via sol-gel route followed by thermal treatment and mechanical processing. The subsequent silica coating provides colloidally stable particles whose magnetic cores are mostly composed of compact clusters of manganite crystallites. The product has been subjected to repeated differential centrifugation and several size fractions, possessing the same dXRD but differing in the effective size of magnetic cores, are isolated. Thorough analyses of their size distributions by transmission electron microscopy and dynamic light scattering measurements are carried out together with SQUID magnetometry. The concentration of particles in aqueous suspensions is accurately determined by atomic absorption spectroscopy and a detailed study of transverse relaxation at the magnetic field B0 = 0.5 T is performed. The highest r2 values are clearly observed for midsized clusters and the temperature dependence of r2 resembles the evolution of magnetization with temperature. Supplemental samples with different thicknesses of the silica shell are also synthetized and thoroughly analyzed.

Preparation of Hg0.8Cr0.2Ba2CuO4+δ under controlled oxygen and mercury partial pressures

Abstract Ba 2 CuO 3 /HgBa 2 CuO 4+ δ and Mn 3 O 4 /Mn 2 O 3 couples were employed to control the mercury and oxygen partial pressures, respectively, during the synthesis of chromium doped Hg-1201 phase in the sealed quartz tube. Suitably adjusted conditions allowed us to prepare series of single phase samples of the final composition Hg 0.7 Cr 0.2 Ba 2 CuO 4+ δ determined by weight changes and EMA. The oxygen stoichiometry was modified by the oxygen partial pressure in the range of p O 2 =0.05–1.59 atm. The X-ray diffraction and magnetic susceptibility study revealed linear dependence of the structural parameters and parabolic one of T c vs. log p O 2 . Multiphase samples, however, form for too high oxygen activity, namely for p O 2 =5.16 atm.

Cobaltites as perspective thermoelectrics

The recent material research of mixed cobalt oxides is strongly motivated by the potential of some of them to be used as chemically stable high temperature thermoelectric material. This fact together with both the theoretical and experimental ambitions to fulfill the severe criteria needed for efficient thermoelectric conversion intensified both their theoretical and experimental research. Nonetheless, despite the investigations of the prototype materials represented by 3D perovskites Ln 1−x A x CoO 3 (Ln = La, Y, rare-earth, A = alkaline-earth) and 2D cobaltites of Na x CoO 2 type, the concise physical background of their transport and magnetic properties remain still a matter of debate. This is likely due to a fact that cobalt ions can be stabilized either in low-spin state (diamagnetic for “pure” Co 3+ ), with filled t 2 g levels and empty eg states, or magnetic ones, with filled eg states. As the energy difference between respective states is due to comparable strength of crystal field and Hunds energies rather small, the thermodynamically most stable ground-state, with eventually different character of charge carriers, can be critically influenced by an interplay of additional degrees of freedom - orbital and charge. The challenge for unequivocal theoretical model represents the thermoelectric power of mixed cobaltites where, up to now, somewhat ambiguous models based either on “classical” approach, associated with diffusion of itinerant charge carriers, or more exotic - based on configurational entropy of quasi-itinerant carriers - are often used for similar materials. Simultaneously, the open question remains the assessment of the dominant mechanism of phonon scattering in 2D cobaltites.