Program: K5 Nanocrystalline Alloys
K5 Nanocrystalline Alloys

K5.1 Investigation of Magnetic Properties in Iron-Based Nanocrystalline Alloys by Mössbauer Effect and Magnetization Measurements
J. Hesse1, O. Hupe1, C. Hofmeister1, H. Bremers1, M. Chuev2 and A. Afanasev2
1) Institut für Metallphysik und Nukleare Festkörperphysik, Technische Universität Braunschweig, Braunschweig, Germany
2) Institute of Physics and Technology, Russian Academy of Sciences, Moscow, Russia

The combination of both the local nuclear measurement technique Mössbauer effect spectrometry and the global technique of magnetization measurements deliver a founded insight in the properties of modern nanostructured materials. As an example we present investigations of nanostructured alloys exhibiting alpha-iron as bcc-Fe-nanograins embedded in an amorphous matrix.

We present examples from the ferromagnetic Fe(86-x)Cu1NbxB13 (x = 4, 5, 7) nanostructured alloys studied by both above mentioned experimental techniques in a wide temperature range, below and above the Curie temperature of the amorphous matrix. Different annealing temperatures for the as quenched amorphous samples result in different amounts of crystallites, which affect the inter-granular coupling and the magnetic behaviour of the alloy. The Curie temperature of the ferromagnetic amorphous matrix is lower than that of the embedded bcc-Fe-nanograins. Fitting the magnetization curves measured versus temperature in different external magnetic fields with a model based on an extended mean-field-theory provides information about the particle-particle coupling.

Our model for fitting the magnetization measurements considers contributions from different ferromagnetic phases and also from a superparamagnetic phase coupled to the ferromagnetic amorphous one.

The magnetic coupling between the bcc-Fe-nanograins for T > TCurie-amorphous and the interaction between the nanograins and the amorphous matrix for T < TCurie-amorphous can be quantitatively analysed within this model.

At temperatures, above the Curie temperature of the amorphous phase, a line-broadening is observed in the Mössbauer spectra. We compare the magnetization measurements with the Mössbauer spectra. Superparamagnetic behaviour and particle-particle interaction can be observed depending on annealing conditions.


K5.2 Microstructure and Magnetic Behaviour of FeNbB Nanocrystalline Alloys
I. Škorvánek1, J.-M. Greneche2, P. Švec3, J. Kováè1 and J. Kötzler4
1) Institute of Experimental Physics, Slovak Academy of Sciences, Košice, Slovakia
2) Laboratoire de Physique de l'Etat Condensé, UMR CNRS 6087, Université du Maine, Le Mans, France
3) Institute of Physics, Slovak Academy of Sciences, Bratislava, Slovakia
4) Institute of Applied Physics, University Hamburg, Hamburg, Germany
The nanocrystalline FeNbB-based alloys prepared by devitrification of melt-spun amorphous precursors belong to an important group of novel extremely soft magnetic materials, called also NANOPERM. Except of their technological importance these materials are attractive also for the fundamental magnetic studies of the nanosized magnetic systems where the ultrafine bcc-Fe grains are embedded in a residual amorphous matrix. In our study, the formation of a nanocrystalline structure and its influence on the magnetic properties in a ternary Fe80.5Nb7B12.5 alloy is investigated using a variety of complementary methods. The experimental data obtained by DSC calorimetry have confirmed a two-stage nature of the primary crystallization process in this alloy. The changes in microstructure upon annealing are examined by transmission electron microscopy and x-ray diffraction. The 57Mössbauer spectrometry provides relevant structural and magnetic data on each of the presented phases, thanks to its local probe character. The temperature and field dependencies of magnetization for the amorphous and selected nanocrystalline samples have been obtained by using a combination of the VSM and SQUID magnetometry. A special attention is focused on the temperature region, where the residual amorphous phase displays a phase transition from a ferromagnetic to a paramagnetic state. One observes a broadening of this transition when the fraction of nanocrystalline particles increases. Striking differences in the magnetic hardening regime at elevated temperatures have been observed for the samples with different volume fractions of nanocrystalline particles. The strongest magnetic hardening effects are visible for the samples exhibiting medium degree of crystallinity, while the best soft magnetic properties are obtained for the samples where the primary crystallization process is nearly finished.


K5.3 Corrosion of Fe-Based Nanocrystalline Alloys
J. Sitek, K. Sedlaèková and M. Seberíni
Department or Nuclear Physics and Technology, Slovak University of Technology, Bratislava, Slovakia

Nanocrystalline materials have attracted attention because they show excellent soft magnetic properties. However, in comparison with magnetic and structural features, to their corrosion resistance has not yet been given sufficient attention. Amorphous and nanocrystalline FINEMET and NANOPERM were investigated after corrosion treatment. An accelerated laboratory corrosion test was realized in the sulfur acid solution. The samples were simultaneously exposed to the outdoor atmosphere in a SO2 rich, industrial and moisture, rural environments. To evaluate the changes in the structure and magnetic behavior the technique of Mössbauer spectrometry taken in transmission geometry was used. Furthermore, the Conversion Electron Mössbauer Spectrometry was employed to obtain information from the corroded superficial layers.

The laboratory tests have shown that corrosion resistance of investigated materials is strongly dependent on their composition and structure. The rapid mass loss of NANOPERM confirmed its low corrosion resistance in comparison with FINEMET, whose higher dissolution resistance can be attributed to the formation of protective superficial SiO2 film. Moreover, addition of some elements may prevent corrosion process. The improving of corrosion resistance with Ni addition was observed.

The obtained results point at different behavior of amorphous and nanocrystalline samples either. From the relative decrease of the absorption area indicating the mass loss of the Fe-containing phases it is obvious that in the case of FINEMET, nanocrystalline sample is more corrosive resistant than their amorphous precursor. This tendency was found to be opposite in the case of NANOPERM.

Analysis of outdoor exposed samples confirmed this behavior and identified the iron compound preferably formed on the corroded sample surfaces as lepidocrocite.


K5.4 Nanocrystalline Fe81-xNixZr7B12 (x = 0 - 40) Investigated by the Mössbauer Spectroscopy
M. Kopcewicz1 and B. Idzikowski2
1) Institute of Electronic Materials Technology, Warszawa, Poland
2) Institute of Molecular Physics, Polish Academy of Sciences, Poznañ, Poland

Novel nanocrystalline Ni-Fe based alloys exhibiting extremely soft magnetic behaviour and improved mechanical properties, as compared with the NANOPERM alloys, with nominal composition Fe81-xNixZr7B12 (x = 0 - 40) were prepared by melt spinning in protective Ar atmosphere. The nanostructure was formed by heat treatment of the amorphous precursors in the temperature range Ta = 440 - 620oC. Formation of the nanocrystalline phase was studied by the XRD, DSC, and Mössbauer techniques. The composition of the nanocrystalline phase strongly depends on the Ni-content in the alloy. While for x < 30 the dominating nanocrystalline phase is the bcc-Fe, similarly to the NANOPERM alloys, the alloy with x = 40 behaves in a clearly different way. Annealing of the Fe41Ni40Zr7B12 alloy at Ta = 520 - 620oC leads to the formation of the nanograins of magnetically ordered cubic FeNiB phase, as identified by the Mössbauer and XRD measurements. However, at highest annealing temperatures, exceeding 590o C, the Mössbauer spectra of the annealed samples are dominated by a single-line nonmagnetic component while the XRD measurements reveal that the nanocrystalline phase formed at 570o C and 620o C is very similar. The Mössbauer measurements performed at low temperatures reveal a superparamagnetic origin of this spectral component. Conversion electron Mössbauer spectra do not contain a single-line component, which suggests that superparamagnetic relaxation at the sample surfaces is restricted by the surface anisotropy. Annealing at Ta > 700oC causes complete crystallization of the residual amorphous matrix and the Mössbauer spectra reveal the magnetic hyperfine structure only.

Magnetic properties of FeNiZrB alloys were also studied by the rf-Mössbauer technique, which is very sensitive to local anisotropy. The nanocrystalline FeNiB phase is magnetically very soft. The complete rf-collapse of the magnetic hyperfine structure is observed for the nanocrystalline Fe41Ni40Zr7B12 alloy in clear distinction to the similar earlier studies of NANOPERM alloys, in which only partial rf-induced narrowing of the magnetically split spectra was observed.