Karel Závěta

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.

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.

A domain study of magnetization processes in a stress-annealed metallic glass ribbon for fluxgate sensors

Abstract The magnetization processes in a metallic glass ribbon at different places with different magnetization curves have been studied together with the magnetic domain structure observed by the scanning electron microscopy technique. In a hysteresis-free region the domain walls are fixed as expected, but regions with hysteric behaviour reveal reproducible as well as non-reproducible types of domain wall movements.

The magnetic transition in ε-Fe2O3 nanoparticles: Magnetic properties and hyperfine interactions from Mössbauer spectroscopy

The nanoparticles of e-Fe2O3 enriched with 57Fe isotope in amorphous silica matrix were prepared by sol-gel technique starting from a single molecular precursor for both Fe2O3 and silica. From the X-ray powder diffraction pattern e-Fe2O3 was identified as the major phase and α-Fe2O3 and β-Fe2O3 were observed as minor iron oxide phases. Using the log-normal distribution for fitting the experimental data from the TEM micrographs, the characteristic size of particles d0 ∼ 25 nm was derived. The rather high coercivity of ∼2.1 T at room temperature was confirmed for our nanoparticle system. From the dependences of magnetization on temperature a two-step magnetic transition spread between 100 K and 153 K was indicated. From the 57Fe Mossbauer spectra measured in the temperature range of 4.2–300 K, the hyperfine parameters for one tetrahedral and three octahedral sites of e-Fe2O3 structure were identified. The in-field spectra in the external magnetic fields up to 6 T were taken both above and below the indicate...

Structure and magnetic properties of ball-milled iron nitride powders

Abstract In the present work we studied the influence of ball milling of iron nitrides–α′ martensite+γ austenite in the NH 3 atmosphere on the content of nitrogen, the phase composition, and the magnetic moment of nitrides. Only a small increase in the nitrogen content (from 6.5 to 7.0 at.% N) was found after milling. XRD and Mossbauer spectroscopy analyses of milled samples showed that at first austenite was transformed into nanocrystalline α-Fe and amorphous γ′-Fe 4 N and FeN x during milling. The decrease in the austenite content was gradually accompanied also by the change of martensite into the above mentioned compounds. The structure of the samples milled for the longest time (8 h) was so destroyed that these samples were not able to be transformed into α″-Fe 16 N 2 by subsequent annealing. The magnetic moment of milled samples firstly slightly increased and then decreased with increasing milling time.

The effect of electron beam current in SEM on the observed domain structure of metallic glasses

Abstract An excessive increase of the intensity of electron beam current in the scanning electron microscope may lead to a distinct change of the magnetic domain structure observed by means of type II magnetic contrast. This effect was studied on two types of amorphous alloys, one with very low and the other with a rather large positive magnetostriction. From the calculations of the heating effect of the electron beam together with the analysis of the experiments, it was concluded that the results may be explained by the local anisotropy induced by the strain distribution around the area heated by the beam, favouring a normal component of magnetization close to the surface and consequently an associated closure domain pattern.

Low temperature behavior of hyperfine fields in amorphous and nanocrystalline FeMoCuB

Low temperature (4.2 K) magnetic behavior of Fe76Mo8Cu1B15 metallic glass was studied by 57Fe Mossbauer spectrometry (MS) and 57Fe NMR. Distributions of hyperfine magnetic fields P(B) were determined for as-quenched and annealed (nanocrystalline) samples with relative fraction of the grains about 43%. P(B) distributions were derived for both the amorphous matrix and nanocrystalline grains. NMR of alloys with natural and 57Fe enriched Fe enabled to assess the contribution of 11B to the total NMR signal. P(B) distribution of the as-quenched alloy derived from MS matches reasonably well the one from NMR of the enriched sample. NMR signal from the sample with natural Fe exhibits contributions from 11B nuclei. The principal NMR lines of the annealed alloys at 47 MHz correspond to bcc Fe nanocrystals. Small asymmetry of the lines towards higher frequencies might be an indication of possible impurity atoms in the bcc structure. The observed differences between natural and enriched samples are attributed to highe...

Hydrostatic pressure experiments with granular Co-Cu alloys

Abstract Rapidly quenched ribbon of the composition Co 15 Cu 85 was used to produce by annealing the sample consisting of the fcc-Co nanometer-sized particles embedded in a copper matrix. Its magnetic characteristics were measured in a superconducting quantum interference device (SQUID) magnetometer equipped with a miniature pressure cell. The measurements were carried out as a function of temperature (10–300 K) in fields up to 30 kOe and under 7 kbar hydrostatic pressure and after release of the pressure. The difference between the zero-field-cooled (ZFC) and field-cooled (FC) dependences of magnetization indicates mictomagnetic behavior at low temperatures. With an increase in temperature, the sample gradually becomes superparamagnetic; however, in the range of temperatures used, only 25 vol.% of the particles reach this magnetic state (even at 10 K, the fraction of particles exhibiting superparamagnetic properties equals 15 vol.%). The analysis of the remanence magnetization and demagnetization points to the presence of interactions, whose sensitivity to pressure depends on the temperature and the details of the initial magnetic state.

The effect of hydrostatic pressure on magnetic properties of partially crystallized Co-based amorphous alloy

Abstract Melt-spun Co66Nb9Cu1Si12B12 metallic glass ribbon (T C ≈150 K ) was used to produce nanocrystalline samples by annealing at 590°C and 610°C for 1 h. Their magnetic properties were measured in a superconducting quantum interference device (SQUID) magnetometer as a function of temperature (10–250 K) in fields to 3 kOe at zero and ∼7 kbar hydrostatic pressure. The pressure shifts the Curie point of the amorphous matrix to lower temperature by −0.55 K/kbar. The difference between the zero-field-cooled (ZFC) and field-cooled (FC) dependences of the magnetization indicates mictomagnetic properties at low temperatures (T

FeOx/Al2O3 catalysts for high-temperature decomposition of N2O under conditions of NH3 oxidation in nitric acid production

The catalytic activity of a series of FeOx/Al2O3 prepared under various conditions was evaluated for high temperature decomposition of N2O (HT-deN2O) in a complex gas mixture produced by oxidation of ammonia. Thus the relevant step in the industrial nitric acid production process was simulated. The catalyst stability during long-term exposure (12 days) to reaction conditions relevant to HT-deN2O was studied. Both fresh and aged FeOx/Al2O3 exhibited more than 90% conversion of N2O in the 750–900 °C temperature range. Structural analysis showed that the prepared FeOx/Al2O3 catalysts contained various proportions of δ, θ and α alumina phases and up to four individual iron species. In the δ and θ Al2O3 phases, well stabilized Fe(III) in Td or Oh coordination was identified as the active species in HT-deN2O.