Biomedical Engineering Reference
In-Depth Information
The ability to control size, size distribution, morphology and composition of
constituent phases is a key point in the synthesis of nanostructured magnetic
materials. Tuning the particle size and preparing nanomaterials with a narrow
particle size distribution is fundamental in order to control the magnetic proper-
ties and to design materials suitable for new applications. The chemical composi-
tion of the constituent phases is also of primary importance. This concerns not
only the average composition but also the compositional gradients within the
material. It must also be taken into account that a reduction in particle size can
lead to changes in the structure and thermodynamic properties of crystalline
phases [5]. In Section 12.3, we report on the synthetic methods that have emerged
during the past 10 years and have provided a noteworthy control of the parameters
determining the magnetic properties of nanomaterials.
Finally, we describe some examples that highlight the strong correlations
between the preparation methods and the structural and magnetic properties, and
complete the chapter with a brief summary and perspectives.
12.2
Magnetism in Nanostructured Metal Oxides
12.2.1
Magnetism in Condensed Matter
In condensed matter, the atomic magnetic moments can mutually act together
(cooperative magnetism) and lead to a different behavior from what would be
observed if all the magnetic moments were reciprocally isolated (noncooperative
magnetism). This, coupled with the different types of magnetic interaction
that can be found, leads to a rich variety of magnetic properties in real systems
[4, 29, 30].
The two basic types of noncooperative magnetism are paramagnetism and
diamagnetism :
•
Paramagnetism arises from identical, uncoupled atomic moments located in
isotropic surroundings. Hence, in a paramagnetic material, there is no long-
range order and, under an external magnetic fi eld, the magnetic moments
partially align.
•
Diamagnetism, instead, is just due to the effect of an external magnetic fi eld on
the motion of the atomic inner electrons [30]. All substances have then a basic
diamagnetism, that is nearly always weak and is very often masked when a
much larger paramagnetism is present.
In cooperative magnetism, the interactions between adjacent magnetic moments
determine the magnetic order in the material. Two classes of interactions can be
distinguished, namely direct exchange interaction and indirect exchange interac-
tion.
Direct exchange
occurs between moments close enough to have signifi cant
