17 Nanocomposites for Permanent Magnets
409
(a) Bulk metallurgical synthesis
Examples of bulk metallurgical processing include work on melt spinning to produce
NdFeB–Fe [10–12], mechanical alloying to produce SmFeN–Fe [13], splat cooling
to produce RE 2 TM 14 B/Fe 3 B [14] and ball milling to produce SmCo–Fe [15, 16] and
NdFeB–Fe [17]. In all these studies, the influence of sample composition, preparation and annealing conditions on phase formation, phase content and microstructure
(grain size) was probed. While cold welding limits the lower particle size achievable
in classical ball milling to some hundreds of nm and more, surfactant assisted ball
milling was developed to produce nanometre sized coercive SmCo and NdFeB hard
magnetic particles [18, 19]. The motivation for this pioneering work was to fabricate
nanocomposite magnets in a bottom-up approach by blending hard and soft magnetic
nanopowders. The very small grain size reached constituted a significant advance
of this type of approach, but the isotropic nature of the samples greatly limited the
energy product value achieved. An increase in remanent magnetisation by nanoparticle alignment under magnetic field was reported [20]. More details on advances in
bulk metallurgical synthesis of nanocomposites, including the use of novel powder
consolidation techniques, can be found in a recent review by Yue et al. [21].
(b) Thin film physical vapour deposition synthesis
Thin film fabrication techniques (sputtering, pulsed laser deposition, molecular beam
epitaxy) have been used to study the fabrication of hard-soft nanocomposites. Thin
film deposition has the advantage that full density samples can be produced and the
crystallographic texture of the hard magnetic layer can be controlled through deposition conditions. Deposition onto heated substrates produces out-of-plane texture in
NdFeB films [22, 23], and most generally in-plane texture in SmCo films [24]. On
polycrystalline substrates, FePt films normally grow with [111] texture, but [001]
texture with the easy magnetisation axis perpendicular to the film plane may be
obtained by appropriate selection of the substrate [25]. The deposition of multilayer
structures allows us to combine materials of our choice, thus by-passing the phase
diagram, and the relative thickness of the different layers can be controlled by varying
the deposition conditions. Exchange coupling has been demonstrated in a number
of hard-soft multilayer stacks of different type, based on RFeB/Fe or RFeB/FeCo
[26–28], SmCo/Fe or SmCo/Co [29, 30], and FePt/Fe 3 Pt [31].
The deposition of in-flight-formed clusters has also been applied to the preparation of model hard magnetic materials. Cluster deposition may permit the crystallisation of the hard magnetic phase without grain growth. It was initially applied to
the deposition of FePt nanoparticles [32]. More recently, YCo 5 and SmCo 5 nanomagnets were prepared by this approach [33, 34]. The nanoparticles are essentially
monodisperse and they may be oriented under a magnetic field before landing on
the substrate surface. Hard nanocomposites have been obtained by embedding hard
HfCo 7 nanoparticles in a soft FeCo matrix [35]. To date, the materials prepared
with this approach suffer from the fact that the matrix is soft, whereas soft magnetic
inclusions in a hard magnetic matrix are predicted to permit much higher values of
coercivity [6].
409
(a) Bulk metallurgical synthesis
Examples of bulk metallurgical processing include work on melt spinning to produce
NdFeB–Fe [10–12], mechanical alloying to produce SmFeN–Fe [13], splat cooling
to produce RE 2 TM 14 B/Fe 3 B [14] and ball milling to produce SmCo–Fe [15, 16] and
NdFeB–Fe [17]. In all these studies, the influence of sample composition, preparation and annealing conditions on phase formation, phase content and microstructure
(grain size) was probed. While cold welding limits the lower particle size achievable
in classical ball milling to some hundreds of nm and more, surfactant assisted ball
milling was developed to produce nanometre sized coercive SmCo and NdFeB hard
magnetic particles [18, 19]. The motivation for this pioneering work was to fabricate
nanocomposite magnets in a bottom-up approach by blending hard and soft magnetic
nanopowders. The very small grain size reached constituted a significant advance
of this type of approach, but the isotropic nature of the samples greatly limited the
energy product value achieved. An increase in remanent magnetisation by nanoparticle alignment under magnetic field was reported [20]. More details on advances in
bulk metallurgical synthesis of nanocomposites, including the use of novel powder
consolidation techniques, can be found in a recent review by Yue et al. [21].
(b) Thin film physical vapour deposition synthesis
Thin film fabrication techniques (sputtering, pulsed laser deposition, molecular beam
epitaxy) have been used to study the fabrication of hard-soft nanocomposites. Thin
film deposition has the advantage that full density samples can be produced and the
crystallographic texture of the hard magnetic layer can be controlled through deposition conditions. Deposition onto heated substrates produces out-of-plane texture in
NdFeB films [22, 23], and most generally in-plane texture in SmCo films [24]. On
polycrystalline substrates, FePt films normally grow with [111] texture, but [001]
texture with the easy magnetisation axis perpendicular to the film plane may be
obtained by appropriate selection of the substrate [25]. The deposition of multilayer
structures allows us to combine materials of our choice, thus by-passing the phase
diagram, and the relative thickness of the different layers can be controlled by varying
the deposition conditions. Exchange coupling has been demonstrated in a number
of hard-soft multilayer stacks of different type, based on RFeB/Fe or RFeB/FeCo
[26–28], SmCo/Fe or SmCo/Co [29, 30], and FePt/Fe 3 Pt [31].
The deposition of in-flight-formed clusters has also been applied to the preparation of model hard magnetic materials. Cluster deposition may permit the crystallisation of the hard magnetic phase without grain growth. It was initially applied to
the deposition of FePt nanoparticles [32]. More recently, YCo 5 and SmCo 5 nanomagnets were prepared by this approach [33, 34]. The nanoparticles are essentially
monodisperse and they may be oriented under a magnetic field before landing on
the substrate surface. Hard nanocomposites have been obtained by embedding hard
HfCo 7 nanoparticles in a soft FeCo matrix [35]. To date, the materials prepared
with this approach suffer from the fact that the matrix is soft, whereas soft magnetic
inclusions in a hard magnetic matrix are predicted to permit much higher values of
coercivity [6].