Chemistry Reference
In-Depth Information
Fig. 4.16 Schematic 2D atomic representation of a nanocrystalline alloy with following caption:
yellow circles correspond to Fe atoms located in the core of the crystalline grains; red circles Fe
atoms in the amorphous phase; blue, dark blue, and black Fe atoms of the interfacial layer as
second and first neighbours of the Zr/Nb-rich layer in the crystalline and amorphous phases,
respectively; white and green circles represent B and Zr atoms respectively [
131
]
The role of the unconventional radio frequency Mössbauer technique [
143
] has to
be emphasized in the case of the nanocrystalline alloys: indeed, it allows to distin-
guish the magnetically soft amorphous and nanocrystalline phase from the mag-
netically harder microcrystalline Fe, to determine the anisotropy fields in each phase
as a function of the rf field intensity. It was found that the magnetic anisotropy of the
amorphous matrix is significantly smaller than that encountered in the nanocrys-
talline phase. Finally, the rf sidebands effect reveals a strong reduction of magne-
tostriction related to the formation of the nanocrystalline phase [
144
,
145
].
In conclusion, the Mössbauer studies versus temperature and/or external magnetic
field performed on nanocrystalline alloys allow to well estimate the hyperfine
characteristics of the different components: this contributes to understand the evo-
lution of magnetic properties with the role of amorphous remainder and the interface
(estimated at about 2 atoms thick) in coupling crystalline grains, crucial feature to
understand the extremely soft magnetic properties of these alloys.
4.6 Magnetic Nanostructured Powders
Nanocrystalline materials or nanostructured powders are characterized by a
microstructure composed with ultrafine grain sizes of 10-100 nm, originated thus
high density of defects, interfaces, mainly grain boundaries, i.e. a large volume
