Neutron-scattering investigation of the magnetic microstructure of Nd-Fe-B based nanocomposites

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.AFR Project No. 89644