17 Nanocomposites for Permanent Magnets
429
Nd 2 Fe 14 B crystals in the highly stressed amorphous matrix. As with single-phase
materials, more detailed studies are required to elucidate the texture inducing mechanism in nanocomposites during high-pressure compaction and hot deformation, and
the applicability of such routes to inducing texture in chemically synthesised precursors needs to be assessed. Magnetic field alignment was used to induce texture in
SmCo 5 /α-Fe nanocomposites compacted by SPS [69]. In this case, alignment can be
explained by the strong out-of-plane (001) texture of the starting SmCo 5 nanoflakes.
17.3.4 Advanced Magnetic Characterisation of Hard-Soft
Nanocomposites
Measurement of the major hysteresis loop of a magnet gives information about
its extrinsic magnetic properties (remanence, coercivity). In the case of hard-soft
nanocomposite magnets, exchange coupled systems are characterised by singlephase hysteresis loops, while exchange decoupled systems are characterised by twophase hysteresis loops [4]. It is worth noting that dipolar interactions have been
shown to mask the presence of anisotropic soft phase inclusions in a hard magnetic
matrix [70]. Information about switching field distributions and magnetic interactions between soft and hard phase components can be accessed by tracing minor
loops, such as in first-order reversal curve (FORC) analysis mentioned above, and in
recoil loop analysis. In the latter case, a sample previously saturated in positive field
is exposed to a negative field of a certain value; the field is reduced to zero, and then
brought back to the negative field value. The thus traced recoil loop is characterised
by its slope and it may be “open” or hysteretic, meaning the ascendant and descendant curves do not coincide. Successive recoil loops can be measured by applying
progressively stronger reversed fields. Irreversible magnetisation reversal in singlephase hard magnets leads to closed recoil loops characterised by low susceptibility
while reversible magnetisation reversal in the soft phase of hard-soft nanocomposite
magnets gives rise to high susceptibility recoil loops [4]. Hysteresis in the recoils
loops of hard-soft nanocomposite magnets has been attributed to a breakdown in
exchange coupling between the soft and hard phase, but mean field modelling indicates that distributions in coercivity values and the relative volume fraction of the
two phases can also contribute to recoil loop hysteresis [71]. Recoil loop analysis
has been applied to hard-soft nanocomposites made by bulk metallurgical synthesis
[72, 73], and physical vapour deposition [8, 27, 74], and its application to hard-soft
nanocomposites made by chemical synthesis should help shed light on magnetisation
reversal processes in these materials, and thus serve to guide the enhancement of their
magnetic performance through improved processing. What is more, micromagnetic
modelling could be used as a support tool in FORC [54] and recoil curve analysis
[8, 74] of chemically synthesised hard-soft nanocomposites.
Element selective magnetic measurement techniques allow studying magnetisation reversal in hard and soft phases independently. X-ray magnetic circular dichroism
429
Nd 2 Fe 14 B crystals in the highly stressed amorphous matrix. As with single-phase
materials, more detailed studies are required to elucidate the texture inducing mechanism in nanocomposites during high-pressure compaction and hot deformation, and
the applicability of such routes to inducing texture in chemically synthesised precursors needs to be assessed. Magnetic field alignment was used to induce texture in
SmCo 5 /α-Fe nanocomposites compacted by SPS [69]. In this case, alignment can be
explained by the strong out-of-plane (001) texture of the starting SmCo 5 nanoflakes.
17.3.4 Advanced Magnetic Characterisation of Hard-Soft
Nanocomposites
Measurement of the major hysteresis loop of a magnet gives information about
its extrinsic magnetic properties (remanence, coercivity). In the case of hard-soft
nanocomposite magnets, exchange coupled systems are characterised by singlephase hysteresis loops, while exchange decoupled systems are characterised by twophase hysteresis loops [4]. It is worth noting that dipolar interactions have been
shown to mask the presence of anisotropic soft phase inclusions in a hard magnetic
matrix [70]. Information about switching field distributions and magnetic interactions between soft and hard phase components can be accessed by tracing minor
loops, such as in first-order reversal curve (FORC) analysis mentioned above, and in
recoil loop analysis. In the latter case, a sample previously saturated in positive field
is exposed to a negative field of a certain value; the field is reduced to zero, and then
brought back to the negative field value. The thus traced recoil loop is characterised
by its slope and it may be “open” or hysteretic, meaning the ascendant and descendant curves do not coincide. Successive recoil loops can be measured by applying
progressively stronger reversed fields. Irreversible magnetisation reversal in singlephase hard magnets leads to closed recoil loops characterised by low susceptibility
while reversible magnetisation reversal in the soft phase of hard-soft nanocomposite
magnets gives rise to high susceptibility recoil loops [4]. Hysteresis in the recoils
loops of hard-soft nanocomposite magnets has been attributed to a breakdown in
exchange coupling between the soft and hard phase, but mean field modelling indicates that distributions in coercivity values and the relative volume fraction of the
two phases can also contribute to recoil loop hysteresis [71]. Recoil loop analysis
has been applied to hard-soft nanocomposites made by bulk metallurgical synthesis
[72, 73], and physical vapour deposition [8, 27, 74], and its application to hard-soft
nanocomposites made by chemical synthesis should help shed light on magnetisation
reversal processes in these materials, and thus serve to guide the enhancement of their
magnetic performance through improved processing. What is more, micromagnetic
modelling could be used as a support tool in FORC [54] and recoil curve analysis
[8, 74] of chemically synthesised hard-soft nanocomposites.
Element selective magnetic measurement techniques allow studying magnetisation reversal in hard and soft phases independently. X-ray magnetic circular dichroism