M. Pękała

Magnetic field dependence of magnetic entropy change in nanocrystalline and polycrystalline manganites La1−xMxMnO3 (M=Ca,Sr)

Experimental results of magnetocaloric effect for several polycrystalline and nanocrystalline manganites La1−xM0.xMnO3 (M=Ca and Sr) are analyzed. Influence of magnetic field is accounted for by the exponent N. The relatively deep N(T) minimum located close to the Curie temperature is found in the polycrystalline manganites. Temperature dependence of N(T) exponent is comparable with those of the soft magnetic and rare earth containing alloys. The slightly higher sensitivity of magnetocaloric effect in nanocrystalline manganites to magnetic fields is revealed by the N exponent.

X-ray diffraction, magnetization and Mössbauer studies of nanocrystalline Fe–Ni alloys prepared by low- and high-energy ball milling

Abstract Fe–Ni alloys were prepared both by low- and high-energy ball milling processes. Structure and magnetic properties were studied by using X-ray diffraction, differential scanning calorimetry, Mossbauer spectroscopy and magnetization measurements. Mechanical treatment influenced the magnetic properties of Fe–Ni alloys as compared with the equilibrium alloys. Reduction of grain size resulted in the increase of magnetization. Invar anomaly for 35 at% Ni was not detected.

Synthesis, structural, and magnetic characterisation of magnesium-doped lithium ferrite of composition Li0.5Fe2.5O4

Abstract Spinel-related magnesium-doped Li 0.5 Fe 2.5 O 4 has been synthesised by heating magnesium-substituted corundum-related α-Fe 2 O 3 with Li 2 CO 3 at 600°C which is ca. 600°C lower than the temperature normally used to prepare metal-doped Li 0.5 Fe 2.5 O 4 by conventional sintering methods. Rietveld structure refinement of the X-ray powder diffraction data shows that the Mg 2+ ions substitute for Fe 3+ ions on the tetrahedral sites whilst the Li + ions are located on the octahedral sites. The 57 Fe Mossbauer spectrum and magnetic measurements are consistent with this structural model.

Magnetocaloric effect in nano- and polycrystalline manganite La0.7Ca0.3MnO3

La0.7Ca0.3MnO3 samples were prepared in nano- and polycrystalline forms by the sol–gel and solid state reaction methods, respectively, and structurally characterized by synchrotron X-ray diffraction. The magnetic properties determined by ac susceptibility and dc magnetization measurements are discussed. The magnetocaloric effect in this nanocrystalline manganite is spread over a broader temperature interval than in the polycrystalline case. The relative cooling power of the poly- and nanocrystalline manganites is used to evaluate a possible application for magnetic cooling below room temperature.

Hyperfine interactions in nanocrystalline Fe-Al alloys

Nanocrystalline powder samples of Fe-30 at.% Al, Fe-40 at.% Al and Fe-50 at.% Al alloys were prepared by the mechanical alloying method. X-ray diffraction studies indicated that the solid solution with bcc structure was formed with increasing milling time for all investigated compositions. The magnetic ordering temperature of the nanocrystalline mechanically synthesized alloys was larger than that of the corresponding alloys on a micrometric scale. The magnetization curves as well as the Mossbauer spectra revealed that the Fe-Al alloys formed during the low energy ball milling process contained different magnetic phases.

Magnetocaloric and transport study of poly- and nanocrystalline composite manganites La0.7Ca0.3MnO3/La0.8Sr0.2MnO3

Magnetocaloric and transport properties are reported for novel poly- and nanocrystalline double composite manganites, La0.8Sr0.2MnO3/La0.7Ca0.3MnO3, prepared by the sol-gel method. Magnetic field dependence of magnetic entropy change is found to be stronger for the nano- than the polycrystalline composite. The remarkable broadening of the temperature interval, where the magnetocaloric effect occurs in poly- and nanocrystalline composites, causes the relative cooling power (RCP(S)) of the nanocrystalline composite to be reduced by only 10% compared to the Sr based polycrystalline phase. The RCP(S) of the polycrystalline composite becomes remarkably enhanced. The low temperature magnetoresistance is enhanced by 5% for the nanostructured composite.

Magnetic susceptibility and thermoelectric power of tungsten intermediary oxides

The magnetic susceptibility of the tungsten intermediary oxides WO3-x (0.05<or=x<or=0.28) with the crystal shear and pentagonal column structures was measured using the Faraday method. A feeble temperature-independent paramagnetic component chi (e) of the total susceptibility was present in each case. Using the Pauli-Peierls-Landau formula, the values of the effective mass were calculated. The ratio m*/m was in the range 1.1-1.55 except for WO2.833(W24O68). Where this value was as large as 4.0. Measurements of the thermoelectric power Q confirm generally the metallic character of the oxides, but the Q versus temperature dependence in the range 20-280 K is more complicated as in the case of tungsten bronzes. Superconductivity was not detected down to 1.5 K.

Temperature-dependent pseudogap-like features in tunnel spectra of high-Tc cuprates as a manifestation of charge-density waves

Temperature, T, variations of the tunnel conductance G(V) were calculated for junctions between a normal metal and a spatially inhomogeneous superconductor with a dielectric gap on the nested sections of the Fermi surface or between two such superconductors. The dielectric gapping was considered to be a consequence of the charge density wave (CDW) appearance due to the electron–phonon (for a Peierls insulator) or a Coulomb (for an excitonic insulator) interactions. Spatial averaging was carried out over random domains with varying parameters of the CDW superconductor (CDWS). The calculated tunnel spectra demonstrate a smooth transformation from asymmetric patterns with a pronounced dip–hump structure at low T into those with a pseudogap depletion of the electron densities of states at higher T in the vicinity or above the actual critical temperatures of the superconducting transition for any of the CDWS domains. Thus, it is demonstrated that both the dip-hump structure and pseudogapping are manifestations of the same phenomenon. A possible CDW-induced asymmetry of the background contribution to G(V) is also touched upon. The results explain the peculiar features of G(V) for Bi2Sr2CaCu2O8+δ and other related high-Tc cuprates.

Phase transformations in Co-B-Si alloys induced by high-energy ball milling

X-ray diffraction analysis, differential scanning calorimetry and magnetization measurements were used to determine the structural changes of the Co 78 B 11 Si 11 alloys prepared by a ball milling of amorphous, crystallized ribbons and a mixture of elemental crystalline powders in vibratory mill. For all starting materials the high-energy ball milling of Co 78 B 11 Si 11 alloy produces a crystalline structure with nanometer sized crystals. The average crystallite size is about 5 nm. Three series of Co 78 B 11 Si 11 alloys are the ferromagnetic materials. Milling of amorphous alloys causes an increase of the room temperature magnetic moment from 0.90 to 1.08 μ B . A similar tendency is observed for alloys produced by milling of the initially crystallized ribbons for which the magnetic moment increases from 0.73 μ B at t = 0 to 1.17 μB after 250 h. Somewhat different dependence is found for alloys milled from powders since magnetization of the alloy subjected to longer milling is reduced by 10% due to structural disorder introduced during a formation of the crystalline Co(Si,B) phase with nanometer sized crystals.

Competition of Superconductivity and Charge Density Waves in Cuprates: Recent Evidence and Interpretation

Explicit and implicit experimental evidence for charge density wave (CDW) presence in high- superconducting oxides is analyzed. The theory of CDW superconductors is presented. It is shown that the observed pseudogaps and dip-hump structures in tunnel and photoemission spectra are manifestations of the same CDW gapping of the quasiparticle density of states. Huge pseudogaps are transformed into modest dip-hump structures at low temperatures, , when the electron spectrum superconducting gapping dominates. Heat capacity jumps at the superconducting critical temperature and the paramagnetic limit are calculated for CDW superconductors. For a certain range of parameters, the CDW state in a -wave superconductor becomes reentrant with , the main control quantity being a portion of dielectrcally gapped Fermi surface. It is shown that in the weak-coupling approximation, the ratio between the superconducting gap at zero temperature and has the Bardeen-Cooper-Schrieffer value for -wave Cooper pairing and exceeds the corresponding value for -wave pairing of CDW superconductors. Thus, large experimentally found values are easily reproduced with reasonable input parameter values of the model. The conclusion is made that CDWs play a significant role in cuprate superconductivity.