The crystallisation of amorphous phases yields nanocrystalline structures when co-pious nucleation and hindered growth of the phase formed by primary crystallisation oc-cur together. The crystal growth in these systems is controlled by soft impingement of the diffusion profiles which makes impurity rejection from the bcc phase more and more difficult with the increase of the bcc grain size. This process on one hand stops further growth of the bcc phase at very low grain sizes and on the other hand excludes perfect partitioning. The formation mechanism makes evident that the B and the Early Tran-sition Metal compoTran-sition of the bcc phase is significantly reduced in comparison with the amorphous precursor and the impurity distribution is heterogeneous in the grains.
The bcc phase contains a well observable amount of impurities, which was in fact de-tected by different methods. The impurities may reduce the magnetic anisotropy and this way may have a dominant influence to the advantageous soft magnetic properties.
The present Mössbauer spectroscopy results show the impurity content of the bcc phase through temperature and composition dependence studies. The impurity content of the bcc phase increases with the increase of the total Zr +B (i.e., decreasing Fe) content.
This conclusion is drawn from the observed initial decrease of the Curie temperature of the bcc phase when the Fe content of the amorphous precursor is reduced. The following increase of this Curie temperature and the shift of the satellite position in the Mössbauer
spectrum are well explained by the gradual shift from the dominantly Zr neighbourhood at low B content to the more B rich environments at higher B content. The deduced change in the Curie temperature of the bcc phase does not correlate with the decreasing grain size observed for increased alloying content.
The bcc grain size has been shown to decrease with decreasing Fe content of the amorphous precursor in many different systems. This trend was shown in the Fe92−xZr7BxCu1alloy system in an exceptionally wide composition range, 26x 623.
The fraction of the bcc phase determines the composition of the residual amorphous phase and it was shown to be Fe2(B1−yZry). The average characteristic size of the resid-ual amorphous phase can be estimated from the fraction and the grain size of the bcc phase. It is found to be constant, approximately 4 nm, in the whole investigated com-position range. It is assumed that this value corresponds to the soft impingement of the diffusion profiles which stops the further growth of the bcc phase by excluding further impurity partitioning.
The estimated composition of the residual amorphous tissue, Fe2(ByZr1−y) is ver-ified by comparison to melt quenched ribbons in the 0 6y 60.55 composition range.
The observed isomer shift values support that the B/Zr ratio of the residual amorphous phase is in good agreement with that of the amorphous precursor.
Above the Curie point of the residual amorphous phase it is possible to observe a small fraction (approximately 4%) of Fe sites which cannot be assigned either to the bcc or to the residual amorphous phase. These intergrain sites has a significantly reduced magnetisation and the temperature dependence of this magnetisation follows that of the bcc phase.
Comparison of the nanosized amorphous phase and the macrosized amorphous ribbon of similar composition reveals, that the deviation between their magnetic proper-ties does not scale with the spatial dimensions. The deviation increases with increasing volume fraction of the bcc phase, therefore the role of atomic volume changes are em-phasised. These changes are probably caused by tensile stresses due to the misfit of the different phases.
It is established that beyond the low grain size other factors must also be taken into account to explain the observed soft magnetic properties. At and above 12 at.% boron content isolated fine particle behaviour is observed via high temperature superparamagnetic relaxation of the Mössbauer spectra. Since these nanostructures show good soft magnetic behaviour at low temperatures the magnetic coupling of nanograins is evidently important for obtaining good soft magnetic properties. The magnetic dipolar coupling is an important and frequently neglected factor in this con-text.
Acknowledgements
This work was supported by the Hungarian Research Fund (OTKA T-022413, T030753, F029164) by grant FKFP 0453/1997 and by the Hungarian Academy of Sci-ences (AKP 98-25 2,2).
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