ALBERTO BOLLERO REAL

1 Introduction
A permanent magnet can be defined as a ferromagnetic material which is prepared in
a metastable state where it retains some net magnetisation. In this way, a magnet can be
considered as an energy-storage device which provides a magnetic field in a particular
volume of space. From 1983, when high-performance Nd 2 Fe 14 B-type magnets were
successfully prepared in Japan and the USA, the production of rare-earth (R) magnets, and
Nd 2 Fe 14 B-type magnets in particular, has seen a spectacular growth. The wide range of
applications for these magnets, from everyday appliances like loudspeakers to the hightech
applications of the aerospace industry, and new areas of application like magnetic
resonance imaging (MRI), have fueled this increase in production. One promising area is
the use of permanent magnets in automotive applications, particularly in control systems.
The search for optimised compositions and improved processing techniques, which has
lead to improved properties and low-cost NdFeB-type magnets, has been the challenge
facing researchers over recent years.
An effective permanent magnet requires as a starting point adequate intrinsic
magnetic properties, but this is not enough to guarantee a high-quality magnet. The
interplay between the intrinsic properties and the microstructure will determine the
magnetic behaviour of permanent magnets. Depending on the R-content three different
prototypes of magnets can be defined (see Fig. 1.1): (a) R-rich magnets constituted by
decoupled grains separated by a thin paramagnetic layer giving rise to high coercivities
(type I); (b) magnets based on R 2 T 14 B stoichiometric composition where the grains are
exchange-coupled resulting in high remanence values J r (type II) and (c) nanocomposite
magnets constituted by two different magnetic phases, hard and soft, mutually exchangecoupled
(type III). The latter type has attracted much attention in recent years due to the
potentially very high (BH) max value despite of a relatively low content of hard magnetic
phase based on the magnetically single-phase behaviour of these two-phase materials and a
remanence enhancement due to the higher saturation polarisation J s of the soft magnetic
phase and the exchange-coupling effect between the hard and the soft magnetic grains.
This effect results in larger remanences than those predicted for systems of isotropically
oriented, magnetically uniaxial, non-interacting single-domain particles where the limit J r /
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