17 Nanocomposites for Permanent Magnets
425
decreases, all in a monotonic fashion. All the nanocomposites showed apparent twophase magnetic behaviour, despite the fact that the Fe inclusions should be small
enough to be fully exchange coupled to the hard magnetic matrix. The authors
suspected that the low field kink may be due to rotation of loosely packed Fe inclusions, and thus, they pressed the nanocomposites at room temperature under 1.5 GPa.
This compaction step did indeed lead to an increase in the saturation magnetisation
and suppression of the low field kink (Fig. 17.22b), which is indicative of exchange
coupling between the soft inclusions and the hard matrix.
This case study has demonstrated the possibility to use chemical synthesis to
fabricate hard-soft nanocomposites in which a high magnetisation phase (α-Fe) is
stabilised within a hard matrix based on a RE-TM high anisotropy phase (SmCo 5 ).
The soft-in-hard structure favours maximisation of coercivity while the temporary
use of a SiO 2 shell to limit growth of Fe nanoparticles may facilitate the future
use of novel consolidation methods being studied to densify and texture magnetic
nanocomposites.
17.3 Challenges and Future Prospects for Hard-Soft
Nanocomposites
17.3.1 Outline of the Challenges Faced in Producing
Hard-Soft Nanocomposites
Key challenges facing the development of hard-soft nanocomposite magnets using
chemically synthesised nanoparticles are identified in Fig. 17.23. The two most urgent
challenges concern compaction to full density and alignment of the easy axes of the
hard magnetic grains, both of which impact directly on the remanence and thus
maximum energy product achievable. Both of these objectives are particularly challenging with nanoparticles. The coercivity values presently achieved in chemically
synthesised hard magnetic nanoparticles are far below the theoretical upper limit
given by the magnetocrystalline anisotropy of the hard magnetic phase. The coercivities reported for nanocomposites made using these hard magnetic nanoparticles are
further reduced. Increasing the coercivities of the hard magnetic precursors and the
final nanocomposites will increase the maximum energy product attainable and will
increase the upper temperature limit for application of the nanocomposite magnets.
Understanding magnetisation reversal in hard nanoparticles and hard/soft nanocomposites, with the aim to increase coercivity, will require advanced structural (e.g.
high resolution TEM, atom probe) and magnetic characterisation (see below) and
should be complemented with theoretical studies (e.g. micromagnetics).
The density, degree of alignment and coercivity values achieved in nanocomposite
magnets are all influenced by the compaction process used, through its impact on the
final nanostructure. A critical point here is to avoid grain growth which would lead to
decoupling of the hard and soft phases. Thus, high-temperature sintering, now used to
425
decreases, all in a monotonic fashion. All the nanocomposites showed apparent twophase magnetic behaviour, despite the fact that the Fe inclusions should be small
enough to be fully exchange coupled to the hard magnetic matrix. The authors
suspected that the low field kink may be due to rotation of loosely packed Fe inclusions, and thus, they pressed the nanocomposites at room temperature under 1.5 GPa.
This compaction step did indeed lead to an increase in the saturation magnetisation
and suppression of the low field kink (Fig. 17.22b), which is indicative of exchange
coupling between the soft inclusions and the hard matrix.
This case study has demonstrated the possibility to use chemical synthesis to
fabricate hard-soft nanocomposites in which a high magnetisation phase (α-Fe) is
stabilised within a hard matrix based on a RE-TM high anisotropy phase (SmCo 5 ).
The soft-in-hard structure favours maximisation of coercivity while the temporary
use of a SiO 2 shell to limit growth of Fe nanoparticles may facilitate the future
use of novel consolidation methods being studied to densify and texture magnetic
nanocomposites.
17.3 Challenges and Future Prospects for Hard-Soft
Nanocomposites
17.3.1 Outline of the Challenges Faced in Producing
Hard-Soft Nanocomposites
Key challenges facing the development of hard-soft nanocomposite magnets using
chemically synthesised nanoparticles are identified in Fig. 17.23. The two most urgent
challenges concern compaction to full density and alignment of the easy axes of the
hard magnetic grains, both of which impact directly on the remanence and thus
maximum energy product achievable. Both of these objectives are particularly challenging with nanoparticles. The coercivity values presently achieved in chemically
synthesised hard magnetic nanoparticles are far below the theoretical upper limit
given by the magnetocrystalline anisotropy of the hard magnetic phase. The coercivities reported for nanocomposites made using these hard magnetic nanoparticles are
further reduced. Increasing the coercivities of the hard magnetic precursors and the
final nanocomposites will increase the maximum energy product attainable and will
increase the upper temperature limit for application of the nanocomposite magnets.
Understanding magnetisation reversal in hard nanoparticles and hard/soft nanocomposites, with the aim to increase coercivity, will require advanced structural (e.g.
high resolution TEM, atom probe) and magnetic characterisation (see below) and
should be complemented with theoretical studies (e.g. micromagnetics).
The density, degree of alignment and coercivity values achieved in nanocomposite
magnets are all influenced by the compaction process used, through its impact on the
final nanostructure. A critical point here is to avoid grain growth which would lead to
decoupling of the hard and soft phases. Thus, high-temperature sintering, now used to