ALBERTO BOLLERO REAL

3.1 Models
magnetic moments deviating from the easy axis is necessary to have a high degree of
remanence enhancement. This situation is schematically illustrated in Fig. 3.2 where the
remanent state of an ideal nanostructure with grain sizes of 20 nm can be observed. The
shaded areas represent the regions where the magnetic moments deviate from the local
easy axis by more than 20 o (darkest regions) and 10 o , respectively. Smaller grains will
result in a more effective exchange-coupling, i.e. enlarged remanence by comparison with
that of non-interacting particles.
A mean grain size < 20 nm and a homogeneous microstructure, with a very small
range in grain size, were established as requirements to obtain an enhanced remanence
preserving a high coercivity in NdFeB based magnets [33,42].
3.1.2 Two-phase nanocomposite magnets
In 1988, Coehoorn et al [43] reported for the first time that two-phase nanocomposite
magnets, containing 85% soft magnetic phases (73% Fe 3 B and 12% bcc-Fe) and 15% hard
phase (Nd 2 Fe 14 B), could be prepared by crystallisation of amorphous melt-spun alloys with
approximate compositions of Nd 4 Fe 78 B 18 having remarkable permanent magnet properties
(µ 0 H c ≈ 0.4 T, (BH) max = 95 kJm -3 ). Eckert et al [44] showed that it is the interaction
between the soft and the hard magnetic phases which results in the specific hysteresis loop
and the very steep recoil loops [45]. These permanent magnets consist of two different
ferromagnetic and mutually exchange-coupled phases, one of which is hard magnetic to
provide a high coercive field, while the other is soft magnetic providing a high saturation
polarisation.
One of the main results derived from the models is that a small enough grain size
improves both remanence and coercivity of isotropic two-phase magnets. Too large grains
will result in a non-effective exchange-coupling of the soft magnetic phase due to a short
range of the exchange interactions unable of aligning completely the magnetic moments of
the grains of the soft phase. However, too small grains could result in a drop in coercivity
due to exchange interactions between hard magnetic grains. Thus, the aim is to develop a
nanostructure consisting of small enough grains to increase the remanence while
preserving a high coercive field, which will result in remarkable high-energy products.
Numerical calculations show an effective exchange hardening of the soft magnetic phase
when the corresponding grains are about twice the domain wall width of the hard phase.
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