147
Magnetic Nanomaterials, Superparamagnetism
8
8.1
Magnetic Materials
On putting any material into a magnetic field, one observes two, more or less
distinct, reactions: the material is pulled into the magnetic field – paramagnetic
behavior – or it is repelled from the magnetic field – diamagnetic behavior. Both
reactions are connected to the electronic structure of the atoms, molecules, or
solids. Any material is diamagnetic; however, in many cases, this diamagnetism
is superimposed by paramagnetism, which is stronger; therefore, these materials
belong to the group of the paramagnetic materials.
Diamagnetism is caused by the movement of the electrons around the atomic
nucleus. According to Faraday’s law of magnetism (more specifically known as
Lenz’s rule) the magnetic field caused by the circular motion of the electrons is
oriented opposite to the external field. The electrons move not only around the
nucleus, they also rotate around their axis. This spin of the electrons also causes a
magnetic moment. In cases, where the electrons are paired, the spins are directed
opposite. Therefore, in an atom with an even number of electrons, the magnetic
moments of the spins compensate each other; these atoms are diamagnetic. All the
other atoms are paramagnetic. The same rules are valid in the case of compounds.
For the further discussion in connection to nanoparticles and nanomaterials, only
paramagnetic materials in their different varieties are of importance.
Figure 8.1 displays the situation for a crystallized solid without an external
magnetic field. This figure shows the cases where the orientation of the elementary dipoles are disordered, paramagnetism, and the one, with ordered magnetic dipoles, ferromagnetism. In ferromagnetic materials, the dipoles interact; a
process leading to a longrange ordering, lining up the dipoles parallel, or in the
case of antiferromagnetic materials, antiparallel. A ferromagnetic body, as depicted
in Figure 8.1b, also shows in the absence of an external magnetic field a magnetic
dipole moment. In the case of paramagnetic materials, an external magnetic field
results in a partial ordering of the elementary dipoles, which, in detail, depends
on the temperature and the strength of the external magnetic field. The scatter of
the dipole orientation is due to the distribution of thermal energy among the
individual atoms.
Nanoparticles – Nanocomposites – Nanomaterials: An Introduction for Beginners, First Edition. Dieter Vollath.
© 2013 WileyVCH Verlag GmbH & Co. KGaA. Published 2013 by WileyVCH Verlag GmbH & Co. KGaA.
Magnetic Nanomaterials, Superparamagnetism
8
8.1
Magnetic Materials
On putting any material into a magnetic field, one observes two, more or less
distinct, reactions: the material is pulled into the magnetic field – paramagnetic
behavior – or it is repelled from the magnetic field – diamagnetic behavior. Both
reactions are connected to the electronic structure of the atoms, molecules, or
solids. Any material is diamagnetic; however, in many cases, this diamagnetism
is superimposed by paramagnetism, which is stronger; therefore, these materials
belong to the group of the paramagnetic materials.
Diamagnetism is caused by the movement of the electrons around the atomic
nucleus. According to Faraday’s law of magnetism (more specifically known as
Lenz’s rule) the magnetic field caused by the circular motion of the electrons is
oriented opposite to the external field. The electrons move not only around the
nucleus, they also rotate around their axis. This spin of the electrons also causes a
magnetic moment. In cases, where the electrons are paired, the spins are directed
opposite. Therefore, in an atom with an even number of electrons, the magnetic
moments of the spins compensate each other; these atoms are diamagnetic. All the
other atoms are paramagnetic. The same rules are valid in the case of compounds.
For the further discussion in connection to nanoparticles and nanomaterials, only
paramagnetic materials in their different varieties are of importance.
Figure 8.1 displays the situation for a crystallized solid without an external
magnetic field. This figure shows the cases where the orientation of the elementary dipoles are disordered, paramagnetism, and the one, with ordered magnetic dipoles, ferromagnetism. In ferromagnetic materials, the dipoles interact; a
process leading to a longrange ordering, lining up the dipoles parallel, or in the
case of antiferromagnetic materials, antiparallel. A ferromagnetic body, as depicted
in Figure 8.1b, also shows in the absence of an external magnetic field a magnetic
dipole moment. In the case of paramagnetic materials, an external magnetic field
results in a partial ordering of the elementary dipoles, which, in detail, depends
on the temperature and the strength of the external magnetic field. The scatter of
the dipole orientation is due to the distribution of thermal energy among the
individual atoms.
Nanoparticles – Nanocomposites – Nanomaterials: An Introduction for Beginners, First Edition. Dieter Vollath.
© 2013 WileyVCH Verlag GmbH & Co. KGaA. Published 2013 by WileyVCH Verlag GmbH & Co. KGaA.