Chemistry Reference
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these different components, one can use the magic-angle configuration which may
a priori lead to some simplifications. In addition, f-factors for the resonant atoms
located in various structural positions are assumed to be equal, when determining
their relative contribution, to the total spectrum area. The fitting procedure of the
hyperfine structure has been discussed in various papers [ 119 - 125 ]. Finally, it is
obvious to mention that other techniques have to be used in addition to Mössbauer
spectrometry but great attention has to be made to compare some results.
Such a strategy allows first to estimate accurately the crystalline fraction in
terms of Fe content: its value can be compared to that determined from other
techniques, but it requires a careful analysis of the physical meaning of each data.
Then, from the fitting model, a third component occurs with a temperature
dependence close to that of the nanocrystalline grains: it is attributed to a magnetic
interfacial layer occupying approximately 2-3 atomic layers between crystalline
grains and amorphous remainder [ 126 - 132 ]. Indeed, this layer resulting from a
symmetry restriction consists mainly of Fe atoms which can be structurally
ascribed to the periphery of crystalline grains but magnetically influenced by the
vicinity of a Zr-rich layer in the amorphous phase which prevents from their
growth, and Fe atoms located within the amorphous phase but in close contact with
the crystalline grains. Magnetic structures can be described from in-field
Mössbauer spectrometry. Figure 4.15 compares zero-field spectrum to in-field
ones when external field is applied parallel to the direction of the incident c-rays
and perpendicular to the ribbon plane of the nanocrystalline Fe 89 Zr 7 B 4 alloy at
4.2 K [ 129 ]. Out of field, one concludes that the magnetic Fe moments of the
grains are not randomly distributed but are preferentially oriented within the rib-
bon plane at about 70 from the normal to the ribbon plane. In presence of an
external field of 1 T, one observes a decrease of the in-plane magnetization
component giving rise to a roughly random orientation of magnetization in the
grains, as the external field slightly overcomes the demagnetizing field [ 129 ]. On
the contrary, the magnetic Fe moments are preferentially oriented along to the
intense applied field but a careful analysis allows to evidence that a non collinear
spin arrangement in the topologically disordered region ascribed to the crystalline
grain-amorphous matrix interface [ 129 ]. The spin glass-like disorder of the
interfacial layer is explained by a wide distribution of the Fe-Fe nearest neighbor
distances originating competing ferromagnetic and antiferromagnetic interactions
and by a large surface/interface anisotropy arising from the magnetocrystalline,
magnetoelastic and dipolar shape anisotropies. It is important to emphasize that the
estimate of this interface at about 2-3 atomic layers from in-field Mössbauer
spectra is consistent with previous one and further computer Monte Carlo based
modelling give evidence for a non collinear magnetic layer, whose thickness was
found to be with the same order of magnitude [ 133 - 135 ].
The complete description of the hyperfine structure allows to schematize the
atomic structure of a nanocrystalline alloy, as illustrated in Fig. 4.16 : one notes
that small amounts of Zr and B atoms can be included as impurities within the
crystalline grains, as discussed in literature [ 131 ].
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