8
Magnetic Properties of Nanoparticles
8.1
Magnetic Materials
Materials are classified by their response to an external magnetic field as diamagnetic, paramagnetic, or ferromagnetic. Although, in general, all materials show
inherently diamagnetic properties, only those materials not showing predominantly
paramagnetic or ferromagnetic behavior in addition are known as diamagnetic.
The origin of diamagnetism is found in the orbital motion of electrons of the
atoms acting like tiny electric current loops, producing magnetic fields. In an
external magnetic field, these current loops align in a way so as to oppose the applied
field and, therefore, diamagnetic materials are exposed to a force pushing them out
of the magnetic field.
Paramagnetism is, in most cases, significantly stronger than diamagnetism and
produces magnetization in the direction of the applied field. In a paramagnetic
material, the atoms act as tiny magnetic dipoles that may be oriented by an external
magnetic field. This situation for a paramagnetic material in absence of an external
magnetic field is shown in Figure 8.1a.
In ferromagnetic materials, the dipoles, represented by the unpaired electron spins
of an atom, are interacting. This leads to a long-range ordering phenomenon
causing the dipoles to line up parallel. This is depicted in Figure 8.1b.
For energetic reasons, the size of the ranges where this parallel orientation occurs
in ferromagnetic materials is limited; these ranges, known as “magnetic domains,”
are usually smaller than the grain size. Within a grain, magnetic domains are
separated by Bloch walls.
This situation is shown, in drastically simplified form, in Figure 8.2. Here, magnetic
domains 1 and 2 with antiparallel orientation are separated by a 180
Bloch wall. In
reality, the thickness of a Bloch wall is around 100 lattice constants or even more. Bloch
walls may connect magnetic domains with orientation differences of 90
or 180
and
the size of the domains and width of the Bloch walls are determined by thermodynamics. The direction of magnetization within a grain is changed by moving the
Bloch walls. It is important to note that the existence of magnetic domains and Bloch
walls makes it easier to change the direction of magnetization. For most ferromagnetic
materials, a specimen will remain magnetized to some extent after the removal of an
Nanomaterials: An Introduction to Synthesis, Properties and Applications, Second Edition. Dieter Vollath.
Ó 2013 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2013 by Wiley-VCH Verlag GmbH & Co. KGaA.
j167
Magnetic Properties of Nanoparticles
8.1
Magnetic Materials
Materials are classified by their response to an external magnetic field as diamagnetic, paramagnetic, or ferromagnetic. Although, in general, all materials show
inherently diamagnetic properties, only those materials not showing predominantly
paramagnetic or ferromagnetic behavior in addition are known as diamagnetic.
The origin of diamagnetism is found in the orbital motion of electrons of the
atoms acting like tiny electric current loops, producing magnetic fields. In an
external magnetic field, these current loops align in a way so as to oppose the applied
field and, therefore, diamagnetic materials are exposed to a force pushing them out
of the magnetic field.
Paramagnetism is, in most cases, significantly stronger than diamagnetism and
produces magnetization in the direction of the applied field. In a paramagnetic
material, the atoms act as tiny magnetic dipoles that may be oriented by an external
magnetic field. This situation for a paramagnetic material in absence of an external
magnetic field is shown in Figure 8.1a.
In ferromagnetic materials, the dipoles, represented by the unpaired electron spins
of an atom, are interacting. This leads to a long-range ordering phenomenon
causing the dipoles to line up parallel. This is depicted in Figure 8.1b.
For energetic reasons, the size of the ranges where this parallel orientation occurs
in ferromagnetic materials is limited; these ranges, known as “magnetic domains,”
are usually smaller than the grain size. Within a grain, magnetic domains are
separated by Bloch walls.
This situation is shown, in drastically simplified form, in Figure 8.2. Here, magnetic
domains 1 and 2 with antiparallel orientation are separated by a 180
Bloch wall. In
reality, the thickness of a Bloch wall is around 100 lattice constants or even more. Bloch
walls may connect magnetic domains with orientation differences of 90
or 180
and
the size of the domains and width of the Bloch walls are determined by thermodynamics. The direction of magnetization within a grain is changed by moving the
Bloch walls. It is important to note that the existence of magnetic domains and Bloch
walls makes it easier to change the direction of magnetization. For most ferromagnetic
materials, a specimen will remain magnetized to some extent after the removal of an
Nanomaterials: An Introduction to Synthesis, Properties and Applications, Second Edition. Dieter Vollath.
Ó 2013 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2013 by Wiley-VCH Verlag GmbH & Co. KGaA.
j167