Papers by Author: H. Szymczak

Abstract: The magnetocaloric effect in La0.6Ca0.4MnO3 manganite has been investigated. The isothermal magnetization versus applied magnetic field at various temperatures in the vicinity of Curie temperature was measured, and the temperature dependence of magnetic entropy change was determined using Maxwell’s relation. This value is comparable to that in Gd. Nevertheless, the relative cooling power of La0.6Ca0.4MnO3 was shown to be considerably lower than that of Gd. The experimental results have been analyzed in frames of a phenomenological statistical model. This model considers explicitly Jahn-Teller interactions and allows prediction of the field dependences of the magnetic entropy change and the relative cooling power.

Abstract: The main objective of the performed investigations was to enhance sensitivity of a current sensor to weak changes of magnetic field. New design of the sensor of current based on magnetoresistance effect – MRE (MRE = (RH - R0)/R0 , where RH is the resistance in magnetic field and R0 is the resistance without magnetic field) was developed. The sensor was produced in the form of an annular magnet with a gap, in which the (La0.7Sr0.3)0.8Мn1.2О3 manganite film possessing large negative MRE was inserted. Nominal current in a controllable electric circuit can change from a few tenths parts of ampere to a hundred of amperes. The limit detectable change of current value depends on the size of gap in the annular magnet. The operation time of sensor at current overload and short circuit is less than 0.3 sec. These magnetoresistors are thermally stable over the temperature range from (- 50 ° С) to (+ 50 ° С). Proposed sensors based on MRE can be applied in many electrical arrangements and devices.

Abstract: Nanocrystalline La0.50Ba0.50MnO3 manganite was synthesized by an optimized sol-gel method. The initial sample was subjected to step-by-step heat treatment under air atmosphere. The ion stoichiometry, the morphology of crystallites of ceramics, and the magnetic properties were studied. It is established that the average crystallite size increases with increasing annealing temperature. All of the samples studied are characterized by a perovskite-like cubic structure, with the unit cell parameter a increasing continuously with the average crystallite size. The most significant lattice compression occurs in the sample with an average crystallite size of ~ 30 nm. The increase in the average crystallite size causes a nonmonotonic increase in the Curie temperature and in the spontaneous magnetic moment. The anomalous behavior of the magnetic properties of the La0.50Ba0.50MnO3 manganites obtained is explained by the competition between two size effects, namely, the frustration of the indirect exchange interactions Mn3+ – O – Mn4+ on the nanocrystallite surface and the crystal lattice compression due to the crystallite surface tension.

Abstract: Peculiarities of penetration of electromagnetic waves through the bulk and thin film samples of the La0.67Sr0.33MnO3 manganite were investigated for the frequency range from 50 kHz to 20 MHz. Strong variations of penetration coefficient in the applied dc magnetic field were observed both for ferromagnetic and paramagnetic states. Local dynamic magnetic ordering in the paramagnetic temperature range was observed. The experimental data reveal weak dependence of electromagnetic properties on microstructure of the films. The upper frequency limit of the dynamic polaron correlations in the paramagnetic phase was estimated.

Abstract: We report the study of the magnetoresistance (MR) and resistivity as a function of temperature in epitaxial thin films of the La0.67Ca0.33MnO3 type (LCMO). The films deposited onto Y-cut of single-crystalline LiNbO3 (LNO) substrates by pulsed-laser deposition in an on-axis geometry. For comparison, the same manganite film deposited onto LaAlO3 (LAO) substrate is studied. The MR of LCMO/LNO film is found to be anisotropic and exhibits a hysteresis loops in low magnetic fields. When magnetic field is parallel to the film plane a number of jumps are observed in the MR below magnetic fields H~6 kOe. These jumps reflect the presence of pinning centers for domain walls. Evidently, a lattice deformation produced by a mismatch of lattice constants LCMO film and LNO, which lead to distortion of the film, is the origin of such strong pinning centers.

Abstract: A-site ordered PrBaMn2O6 manganites were obtained by a two-step synthesis process. The chemical composition, crystal structure, magnetic and magnetotransport properties have been investigated with regard to the degree of ordering of the Pr3+ and Ba2+ cations. A-site disordered Pr0.50Ba0.50MnO3 has a cubic perovskite-like unit cell (SG = Pm3m , Z = 1), while the A-site ordered PrBaMn2O6 has a tetragonal unit cell (SG = P4/mmm, Z = 2). The A-site ordered state in PrBaMn2O6 remains stable on heating to 1300 0C in an oxidizing medium (P[O2] = 1 bar). The cation ordering leads to a significant increase in the critical temperatures of phase transitions. Asite ordered PrBaMn2O6 is a metallic ferromagnet with the TC ~ 320 K, whereas TC of a fully disordered sample is ∼ 140 K. The samples with intermediate degrees of ordering contain two magnetic phases. Slightly below TC, all samples exhibit a metal–insulator transition and a peak of the magnetoresistance, amounting to approximately 10 % and 65 % in a magnetic field of 9 kOe for the fully ordered PrBaMn2O6 and disordered Pr0.50Ba0.50MnO3, respectively. The results are interpreted in terms of the degree of ordering of the Pr3+ and Ba2+ cations.

Abstract: Nanocomposites (nanocrystals) of KY(WO4)2 and KY(WO4)2+1% mol Yb were synthesized using a Complex Sol-Gel Process (CSGP). A chemical treatment with concentrated nitric acid and hydrogen peroxide was used to reduce the decarbonisation temperature. The expected monoclinic phase C2/c of the KYW of the nanocomposite powder was confirmed using XDR. From the X-ray diffraction measurements, the unit cell parameters and the size of nanoparticles was determined. Electron spin resonance studies in the X band were carried out on KYW and KYW:Yb nanocrystals. The sintered samples were made with using the high pressure technique at temperatures up to 600oC. In addition chemical analysis, X-ray diffraction measurements and ESR investigations were carried out on the sintered samples.