Neutronenstreuuntersuchungen zur magnetischen Mikrostruktur Nd-Fe-B-basierter Nanokomposite

Exchange-coupled magnetic nanocomposites are considered to be promising candidates
for future permanent magnet applications. These nanocomposites consist of a
hard magnetic phase (based on rare-earth metal alloys), which provides high magnetic
anisotropy and which is exchange coupled to a soft magnetic rare earth free phase,
which provides a high saturation magnetization and Curie-temperatur. For suitable
microstructures one could achieve on the one hand side dramatically increased energy
products, in comparison to state-of-the-art Dy-Nd-Fe-B magnets. On the other
hand side – due to the reduced amount of rare earth metals – expenses could be saved
significantly. The understanding of the coercivity mechanisms in these materials
is crucial for the development of nanocomposite permanent magnets. In the present
work, the magnetization reversal of a two-phase Nd2Fe14B/Fe3B nanocomposite has
been studied by means of magnetic small angle neutron scattering (SANS). This
technique exclusively provides access to information of the magnetic microstructure
in the bulk of the material and on the relevant length scales of 1 − 100 nm. Direct
Fourier-transformation allowed the calculation of the correlation function of the
spin misalignment C(r). From this data, the correlation length lc has been obtained.
The parameter lc is a measure for the spatial extent of magnetization inhomogeneities.
The field dependence of lc indicates a magnetization reversal process, which
is widely governed by homogeneous rotation of the magnetization within the hard
magnetic Nd2Fe14B particles. In addition, in terms of a micromagnetic approach,
the exchange stiffness constant A was determined experimentally.