Anna Slawska-Waniewska

Local atomic structure and magnetic ordering of iron in Fe-chitosan complexes.

The iron crosslinked chitosan (Ch-Fe-CL) and N-carboxylmethyl chitosan (N-CM-Ch-Fe) complexes were studied by complementary techniques: structurally sensitive Mössbauer and X-ray absorption methods, as well as static magnetic measurements. A detailed and consistent description of these complexes including, besides the overall magnetic behavior, the spin ordering and local atomic structure around Fe ions is presented. Fe atoms in the investigated samples are mostly penta-coordinated and appear in a high spin Fe (3+) ionic state. In Ch-Fe-CL, two kinds of Fe near neighbors are equally probable and several Fe atoms are situated in the second coordination sphere. The magnetic interactions between these Fe ions lead to a sperimagnetic-like ordering. In N-CM-Ch-Fe, only one Fe neighborhood was found. Other Fe atoms were identified neither in the first nor in the second coordination sphere, but the third coordination sphere indicates the presence of Fe atoms. The magnetic coupling between these atoms is antiferromagnetic, but the dominant part of Fe in this sample remains in a paramagnetic state.

Nanocrystalline metallic glass-an unusual particulate medium

The expectation that nanocrystalline Fe-based metallic glass may show a variety of magnetic phases is confirmed experimentally. It is shown that, depending on the annealing conditions (resulting in controlled crystallite size as well as the volumetric fraction of the crystalline phase), the particles exhibit superparamagnetic or superferromagnetic behavior at temperatures above the Curie point of the amorphous matrix. At sufficiently low temperatures (around room temperature), the samples behave ferromagnetically as expected for typical nanocrystalline metallic glass. >

Structural and magnetic properties of iron nanowires and iron nanoparticles fabricated through a reduction reaction

Summary The main goal of this work is to study the structural and magnetic properties of iron nanowires and iron nanoparticles, which have been fabricated in almost the same processes. The only difference in the synthesis is an application of an external magnetic field in order to form the iron nanowires. Both nanomaterials have been examined by means of transmission electron microscopy, energy dispersive X-ray spectrometry, X-ray diffractometry and Mössbauer spectrometry to determine their structures. Structural investigations confirm that obtained iron nanowires as well as nanoparticles reveal core–shell structures and they are composed of crystalline iron cores that are covered by amorphous or highly defected phases of iron and iron oxides. Magnetic properties have been measured using a vibrating sample magnetometer. The obtained values of coercivity, remanent magnetization, saturation magnetization as well as Curie temperature differ for both studied nanostructures. Higher values of magnetizations are observed for iron nanowires. At the same time, coercivity and Curie temperature are higher for iron nanoparticles.

Magnetoresistance in nanocrystalline Fe-based metallic glass

The ferromagnetic anisotropy of resistivity as well as the effective resistivity of nanocrystalline Fe-based metallic glasses with different volumetric fraction of crystalline phase have been studied. It has been shown that the ferromagnetic anisotropy of resistivity can be considered as a sum of two independent contributions of the opposite sign-positive, originating from the residual amorphous matrix and negative from the crystallites. >

Some aspects of simultaneous stress- and field-annealing of metallic glasses

Abstract Magnetic anisotropy induced in the Co 57.4 Fe 5.6 Ni 10 Si 11 B 16 amorphous ribbons by annealing in the presence of tensile stress and/or magnetic field is investigated. The results of an analysis of possible contributions to the observed stress and/or field induced anisotropy are presented showing that in the case of the metallic glass investigated the recently reported enhancement effect does not occur.

Magnetic properties of Fe - Cr-based nanocrystalline alloys

FeCrCuNbSiB nanocrystalline alloys, with low fractions of the crystalline phase (10 - 25%), have been studied by means of static magnetic measurements and Mossbauer spectroscopy over a wide temperature range (4.2 - 800 K). As a reference the temperature dependences of the hyperfine parameters measured in ordered Fe - Si alloys were used. It has been shown that at temperatures close to the Curie point of the amorphous matrix the coercivity exhibits a maximum that corresponds to a kink in the thermal evolution of the hyperfine field of the crystalline phase. These effects originate from the lessening of the interphase exchange interactions at the ferro - paramagnetic phase transition of the amorphous matrix and superparamagnetic fluctuations of the magnetization in single-domain grains. A mechanism considering different energy contributions as well as their changes with temperature in such a mesoscopic system is discussed.

Indication of Intrinsic Macroscopic Forces Affecting Magnetic Properties of Fe-Nb/Mo-Cu-B-Si Ribbons

Si-rich (at.% Si >; 12) Fe-Nb/Mo-Cu-B-Si nanocrystalline ribbons are already successfully used in industry. Still the impact of intrinsic macroscopic stresses on magnetic anisotropy is not known in enough detail. Domain structure, hysteresis loops and surface chemistry have been studied in the as-annealed (nanocrystalline) state as well as the response to surface removal by etching. Creep-induced anisotropy established during annealing was found to be the essential source of the observed anisotropy. Apparently smooth gradient of transversal to longitudinal creep-induced anisotropy was identified from air-side center across the ribbon width as well as across the ribbon thickness. The relative rise of transversal anisotropy due to surface removal points to different surface etching efficiency. Raman spectroscopy confirmed the presence of ferrous oxides on both the ribbon surfaces. The existence of a defined oxide layer generating the macroscopic stress was not indicated.

Magnetic properties of partially devitrified metallic glasses

This paper presents a review of selected experiments which have been carried out using partially devitrified Fe- and Co-based metallic glasses. It is shown that by a suitable choice of the starting metallic glass composition and of annealing conditions, a variety of magnetic phases can be created in these systems. It is also shown that by controlling annealing processes, the coercivity of metallic glass can be altered by several orders of magnitude, giving the possibility of tailoring materials with magnetic properties ranging from extremely soft up to semihard. The magnetostriction of nanocrystalline materials is discussed as well, showing that size-dependent phenomena and the symmetry restriction at the particle boundaries create a surface contribution to the effective magnetostriction. Finally, the magnetic interactions between particles in partially devitrified samples are considered and the observed influence of temperature and particle density on them is discussed.

Effect of micro structure on magnetization processes in nanocrystalline Fe-Zr-B alloys

Abstract The nanocrystalline alloys produced by devitrification of the Fe89Zr7B4 amorphous alloy exhibit paramagnetic behaviour for the fractions of the crystalline phase lower than 13%. This is due to dendritic growth of crystals and paramagnetic properties of the amorphous matrix. A rapid increase of the nanocrystalline phase was observed for the 1 h annealings at temperatures between 480 and 540 °C. A formation of regular nanocrystals with diameters of about 50 nm and the increase of the Curie temperature of the amorphous matrix were accompanied by transition from para- to ferromagnetic properties. These properties were characterized by increasing saturation magnetization, magnetic permeability and Barkhausen noise intensity and decreasing coercivity of the alloy. Both the microstructure and magnetic properties vary only slightly for higher crystalline fractions up to 87%.

The influence of superparamagnetic particle size distribution and ferromagnetic phase on GMR in melt spun Cu-Co granular alloys

Magnetic characteristics and structure of granular Cu/sub 90/Co/sub 10/ alloy were investigated in order to explain their relationship with electric transport properties. The annealed sample contains mainly Co-particles with sizes of a few nanometers and some much larger particles or agglomerations, which are not superparamagnetic. It is shown that the ferromagnetic phase contribution to the total magnetization (calculated applying the new, invented method) and differentiated surface to volume ratio of superparamagnetic particles are the main sources of the observed deviation of the magnetoresistance characteristics from that expected theoretically for superparamagnetic particles of equal size.