168 8 Magnetic Nanomaterials, Superparamagnetism
However, these values must be interpreted with extreme care. Yes, the value at
the lowest temperature is correct; this is the magnetic crystal field. At higher
temperatures, due to the thermal fluctuations of the magnetization vector, the
average value of the magnetic field that the nucleus “feels” during one revolution
gets smaller. At the blocking temperature, this average field is nil; therefore, above
80 K, this material is superparamagnetic. Now, the Mößbauer spectrum shows only
the doublet.
The definition of the blocking temperature given above is applicable for specimens with narrow particlesize distributions. In all other cases, one defines
the temperature, where half of the material is superparamagnetic as blocking
temperature.
On putting a superparamagnetic specimen into an external magnetic field, a
certain part of the magnetic moments of the particles becomes aligned in the
direction of the field. Increasing the external field up to a value where saturation
is observed, the Mößbauer spectrum no longer shows the doublet but the sextet,
as thermal fluctuations are thwarted.
8.5
Applications of Superparamagnetic Materials
The best chances for economically successful applications are found in the utilization of particulate composites and ferrofluids. The properties making the application of superparamagnetic nanocomposites attractive are the avoidance of the
formation of magnetic clusters, the relatively high magnetic moment per particle
(as compared to a paramagnetic molecule), and the possibility to manipulate these
particles with an external magnetic field.
Figure 8.21 Magnetic crystal field of a
γ-Fe 2 O 3 specimen. The particles were coated
with zirconia, ZrO 2 , as a distance holder. The
magnetic crystal field was calculated from the
splitting of the energy levels represented by
the sextet in the Mößbauer spectrum A
crystal field zero characterizes
superparamagnetic material.
0
50
100
150
200
temperature [K]
0
20
40
60
crystal
field
[T]
Blocking
temperature
However, these values must be interpreted with extreme care. Yes, the value at
the lowest temperature is correct; this is the magnetic crystal field. At higher
temperatures, due to the thermal fluctuations of the magnetization vector, the
average value of the magnetic field that the nucleus “feels” during one revolution
gets smaller. At the blocking temperature, this average field is nil; therefore, above
80 K, this material is superparamagnetic. Now, the Mößbauer spectrum shows only
the doublet.
The definition of the blocking temperature given above is applicable for specimens with narrow particlesize distributions. In all other cases, one defines
the temperature, where half of the material is superparamagnetic as blocking
temperature.
On putting a superparamagnetic specimen into an external magnetic field, a
certain part of the magnetic moments of the particles becomes aligned in the
direction of the field. Increasing the external field up to a value where saturation
is observed, the Mößbauer spectrum no longer shows the doublet but the sextet,
as thermal fluctuations are thwarted.
8.5
Applications of Superparamagnetic Materials
The best chances for economically successful applications are found in the utilization of particulate composites and ferrofluids. The properties making the application of superparamagnetic nanocomposites attractive are the avoidance of the
formation of magnetic clusters, the relatively high magnetic moment per particle
(as compared to a paramagnetic molecule), and the possibility to manipulate these
particles with an external magnetic field.
Figure 8.21 Magnetic crystal field of a
γ-Fe 2 O 3 specimen. The particles were coated
with zirconia, ZrO 2 , as a distance holder. The
magnetic crystal field was calculated from the
splitting of the energy levels represented by
the sextet in the Mößbauer spectrum A
crystal field zero characterizes
superparamagnetic material.
0
50
100
150
200
temperature [K]
0
20
40
60
crystal
field
[T]
Blocking
temperature