The NANOPYME project: Up-scalability of ferrite-based exchange-spring magnets

As long as the size of the magnetically soft phase remains below a certain threshold, i.e. in single-domain state, exchange-coupled hard/soft nanocomposites hold the promise of increasing the energy stored in a permanent magnet. [1–3] According to early models, soft contents up to 90% could lead to enhanced energy products. [4] However, in reality, an increasing amount of ideally coupled soft phase leads to a fast decay in coercivity that becomes the limiting factor. [5] This occurs profusely when the soft phase particles are multi-domain as they act as nucleation centers for domain walls, but it may also happen in single-domain configurations due to the fact that the different easy axes orientations at the interfaces can be regarded as intragrain domain walls liable to propagation. [6,7] On the other hand, perfect decoupling enhances coercivity. Thus, a compromise needs to be met and coercivity generally increases at the expense of remanence. [8,9] Even though considerable advances have been achieved, understanding the coercivity mechanisms in exchange-spring magnets remains a challenging task even in epitaxial thin-film systems with controlled interfaces, for instance as a function of the soft layer thickness or the pinning fields at the interfaces. [5,10–13]