National Institute Of Materials Physics - Romania

Melt spun ribbons of FexCu100-x were obtained by different procedures, in order to maximize the dispersion of the Fe atoms in the Cu matrix. Subsequent thermal and mechanical treatments were used for a controlled crystallization process of the soft magnetic phase. A detailed analysis of structural aspects and crystallization dynamics and their relation with the magnetic behavior of the Fe-Cu ribbons has been obtained via Mossbauer spectroscopy, X-ray diffraction (XRD) and magnetic measurements. In order to obtain composite permanent magnets with shape anisotropy, the optimal conditions and the suitable processing were established. © 2004 Elsevier B.V. All rights reserved.

The investigation of the structure, magnetic and Mossbauer properties for the series of R-3(Fe,MO)(29) compounds has been performed, where R = Y, Nd, Sm, Gd, Tb, and Dy. The crystallographic structure of the ternary phase compounds has been investigated by Rietveld refinement of the X-ray diffraction patterns obtained at room temperature. The quality of the single-phase compounds was also checked by thermomagnetic measurements, from room temperature to above the Curie temperatures. From the magnetic isotherms for the free powder samples, measured at 4.2 K, the saturation magnetizations and the iron average magnetic moments have been derived. Fe-57 Mossbauer spectra of the R3 (Fe,MO)29 compounds have been measured at 15 K. The analysis of spectra, in a model which takes into account both the Fe atom nearest neighbor numbers and the Fe-Fe nearest neighbor bond lengths, indicates that the transferred contribution to the hyperfine field at the iron sites, due to rare earth moments, can be correlated with the rare earth effective spin. (c) 2004 Elsevier B.V. All rights reserved.

Scanning Hall probe microscopy has been used to make a microscopic study of flux structures and dynamics in yttrium barium copper oxide thin-film disks containing a regular 10-mu m-period square array of 2.5-mu m-sized holes (antidots). Images obtained after field cooling the sample to 77 K in very low fields reveal that the holes can trap two flux quanta at this temperature. Scans obtained after zero-field cooling (ZFC) to 77 K and a subsequent applied field cycle clearly display preferential flux channeling along chains of antidots in the direction of maximum induction gradient. Remarkably, upon reversal of field sweep direction, we observe flux "streaming" out of the holes towards the sample edges with almost uniform density flux "stripes" bridging the holes in the exit direction. We estimate that the antidots can preferentially trap about 15 flux quanta in these ZFC experiments. Classical electrodynamics simulations of our samples appear to be in good qualitative agreement with our results, indicating that many of the observed phenomena may be geometrical effects that depend primarily on the shape and topology of the sample, and potential applications are discussed.

The electronic structure of LaNiSn and NdNiSn compounds and their hydrides has been studied by first principles calculations and variable temperature Sn-119 Mossbauer spectroscopy and the nature of the hydrogen-metal bond is discussed. The analysis of the electronic density of states (DOS) in both compounds before and after hydrogenation indicates an hybridization of the Sn, Ni, and H orbitals. The partial Sn-p DOS of LaNiSnH2 gives evidence for a lower symmetry of electron density around tin atoms compared to LaNiSn, according to the larger quadrupole splitting in the corresponding Mossbauer spectrum. Theoretical and experimental Mossbauer parameters agree very well for all samples.

Antiferromagnetic (AF) FeSn2(001) epitaxial and polycrystalline layers were grown in ultrahigh vacuum under various conditions on clean InSb(001) substrates and covered by polycrystalline ferromagnetic Fe layers, forming a new system with exchange bias. Isotopically enriched Fe-57- and (FeSn2)-Fe-57-tracer layers were placed on either side of the Fe/FeSn2 interface for a microscopic investigation of the spin structure and atomic interdiffusion phenomena in near-interfacial regions by Fe-57 conversion electron Mossbauer spectroscopy (CEMS) at room temperature and T=10 K. Several spectral components, assigned to pure bcc Fe, Sn-containing bcc Fe, AF-ordered FeSn2, and paramagnetic FeSn2 were resolved in the CEM spectra. Evidence is provided for interdiffusion across the interface. The temperature dependence of the exchange-bias field H-E was measured by magnetometry. The CEMS data provide evidence for a correlation between H-E at low T and chemical disorder (defects) in the FeSn2 films via the intensity of the paramagnetic line. These results support the assumption that exchange bias is related to the presence of AF domains formed via magnetic defects in the antiferromagnet. The T dependence of H-E suggests interfacial exchange-coupling energies higher than the AF wall energy. (C) 2005 American Institute of Physics.

Scanning Hall probe microscopy has been used to study flux Structures and dynamics in 5 mu m x 5 mu m YBCO thin film squares, which are mesoscopic with respect to the magnetic penetration depth, lambda(T), at temperatures close to T-C. A number of unusual vortex phenomena are observed in these microstructures which differ qualitatively from the expected behaviour of more macroscopic pieces of film. In field-cooled (FC) experiments to similar to 65 K a full Meissner state is generated for cooling fields less than similar to 6 Oe, reflecting the relatively small demagnetization factors in our samples. Cooling in higher fields, however, results in only a very weak diamagnetic response at low temperatures whose magnitude is almost independent of the cooling field. In contrast we observe considerable trapped flux upon field-removal whose magnitude grows monotonically with cooling field. Remarkably, all FC flux distributions exhibit almost perfect rotational symmetry, and can be nearly completely cancelled in a reversible fashion by tuning the field applied to the initially FC state. Our field-cooled and zero-field-cooled results have been analysed in terms of a Bean-like critical state model containing constant edge and bulk current densities, and most of the observed phenomena can be explained by considering the relative weight of these two components. Not all flux profiles call be described by our simple model, however, and under certain circumstances symmetry-breaking 'dipole'-like flux structures can form in several adjacent YBCO squares. We speculate that these are related to the unidirectional At-ion milling process which was used to pattern the squares and could have broken the expected four-fold symmetry. We note that our results could have important implications for the miniaturization of thin film HTS devices.

Recently we have found superconductivity in a cubic antiperovskite-like compound Li2BPd3. A new pseudo-binary complete solid solution Li2B(Pd1-xPtx)(3), x = 0-1 with similar structure has been synthesized and observation of superconductivity in the entire x-range is reported. Our results strongly suggest that superconductivity is of bulk type. Critical temperature T, is decreasing approximately linearly with amount (x) of Pt from 7.2-8 K for Li2BPd3 to 2.2-2.8 K for Li2BPt3. From isothermal magnetization (M-H) measurements, lower critical fields H-cl (138Oe/(x=0), 38Oe/(x=1)), upper critical fields H-c2(WHH) (3.4 T/(x=0), 1 T/(x=1)), coherence length xi(0) (9.8 nm/(x=0), 17.9 nm/(x=1)) and penetration depth lambda(0) (190nm/(x=0), 364nm(x=1)) were estimated and shown to follow approximately linear dependencies with.x, either. Structure and superconducting similarities with MgCNi3, viewed as a bridge between low and high T-C superconductors are increasing the expectations that Li2B(Pd1-xPtx)(3), x = 0-1 superconductor can be included in the same class of `intermediate' superconductors. For x = 0-1 a weak fishtail effect was observed at low and intermediate fields. Apart from this effect, some samples for x = 1 have shown magnetization jumps at fields close to H-c2.

Effects of electronic charge disproportionation were observed by Fe-57 Mossbauer spectroscopy at low temperatures in substituted LaFeO3 perovskites as (La1-xCax)FeO3 and La(Fe1-xMgx)O-3 (xless than or equal to0.5). The presence of both Fe3+ and Fe5+ species was evidenced in both series of compounds. Direct evidence for disproportionation was supported by correlating the Mossbauer results with the chemical determination of tetravalent iron at RT. The evolution of hyperfine parameters vs. substitution degree was comparatively analysed for Fe3+ and Fe5+ ions in relation to their electronic microscopic origin. Electron delocalisation processes are discussed in terms of ion distribution and valence state. (C) 2004 Elsevier B.V. All rights reserved.

The equiatomic composition of the FeRh alloy was studied by in situ high pressure energy dispersive X-ray experiments. Pressures as high as 19 GPa were generated using a diamond anvil cell. An incomplete bcc --> fcc phase transition was evidenced, both bcc and fcc phases coexisting above 10.0 GPa. The transition is accompanied by a decrease of the lattice parameter of the remaining bcc phase by approx. 1.5%. A substantial low lattice parameter was observed for the newly formed fee phase, reflecting a rather compressed structure. (C) 2004 Elsevier B.V. All rights reserved.