Papers by Author: Janusz Przewoźnik

Abstract: Magnetic field (up to 8T) and temperature (10-300K) dependence of size of Zn doped magnetite samples Fe_3-xZn_xO₄ (x=0.008, 0.022, i.e. falling within first and second order Verwey transition regime) were measured by the strain gauge method. Both samples experienced shrinking on cooling through the Verwey transition along monoclinic c axis, while the strong expansion was found for the 1^st order sample in the a-b direction, unlike in the 2^nd order sample. Magnetostriction of both samples is very small and limited to low fields only, concomitant with magnetization process. However, field application perpendicular to c axis and at T slightly below the Verwey transition temperature TV results in a large change of dimensions, coinciding with the axis switching process.

Abstract: The results of magnetic and specific heat studies are reviewed and discussed for the series of RZnSn₂ (R = Y, Er and Lu) compounds crystallizing in the same tetragonal HfCuSi₂ - type of structure. A sharp peak in the specific heat of ErZnSn₂ compound is indicative for magnetic ordering of Er moments at 5.0 K, being in agreement with its established antiferromagnetic nature. The estimated magnetic entropy S_magn contribution is less than that expected for the ground state J = 15/2 multiplet of Er³⁺.

Abstract: Bismuth layer-structured compounds in the Bi-Ti-Fe-O system known as Aurivillius phases are single phase multiferroics. It was stated that substitution of some rare earth elements for bismuth ions in such structure can modified its magnetic properties. Powders of Bi7Fe3Ti3O21 and Bi6.3Sm0.7Fe3Ti3O21 were prepared by co-precipitation – calcination method and then were sintered to dense polycrystalline materials. Low field DC susceptibility was measured in the zero field cooled (ZFC) and field cooled (FC) modes at 10÷350 K. For selected temperatures magnetisation curves and hysteresis loops were also measured. The FC and ZFC curves of both samples diverge at temperatures below 250 K indicating a spin glass-like behaviour. The compound with samarium exhibits magnetic hysteresis already at room temperature with the coercive field increasing to 870 Oe at 10 K. The low temperature hysteresis loops of the samarium containing compound are shift with respect to zero field which can be attributed to a magneto-electrical coupling of the samarium sublattice "exchange biased" by the iron one, which orders anti-ferromagnetically at a higher temperature than the samarium sublattice.

Abstract: The multiferroic Aurivillius phases in the Bi-Fe-Ti-O system are built from alternate (Bi2O2)2+ and (Bin-1XnO3n+1)2 layers, where X = Fe3+, Ti4+ and “n” refers to the number of perovskite-like layers between Bi2O2 layers. Detailed magnetic studies should be done to understand electromagnetic interactions and multiferroic coupling effects. In the present paper, a powder composed of the Aurivillius phase with n = 5, Bi6Fe2Ti3O18, was successfully prepared by the hydrothermal method. The powder was sintered, obtaining dense polycrystalline materials. It was stated that both powder and sintered bodies were paramagnets with a possible antiferromagnetic ordering or a spin-glass state at the liquid helium temperatures.

Abstract: A combined XRD, Mössbauer, SEM, STXM and NMR study of naturally oxidised, ball milled iron powders is presented. The XRD patterns show the peaks of the bcc-Fe phase with the line widths increasing with the milling time. This corresponds to a flattening of the crystallites, as confirmed by SEM, and increased strain due to the accumulation of defects. The effect is consistent with the variation of the Mössbauer line-widths with the milling time. Scanning Transmission Xray Microscopy (STXM) measurements provided oxygen maps of the particles and revealed that the dominant oxide in the nanometric oxide layer is magnetite. The 57Fe spin echo NMR study reveals a dominant signal corresponding to a bcc-Fe core and a much weaker resonance corresponding to a magnetite amount of less than 1%.