Biomedical Engineering Reference
In-Depth Information
2.2
Schematic diagram of zero-field-coded (ZFC) and field-coded (FC)
magnetization curves as a function of temperature taken in an applied
field
H
. Arrow indicates blocking temperature,
T
B .
2.3.2 Superparamagnetism
Very fine ferromagnetic particles have very short relaxation times, even at
room temperature, and behave superparamagnetically [57-69]. Their
behavior is paramagnetic but their magnetization values are typical of
ferromagnetic substances. The individual particles have normal ferromag-
netic movements but very short relaxation times, enabling them to rapidly
follow directional changes in an applied field. Superparamagnetism is
characterized by two significant features. Firstly, there is no hysteresis,
which means both retentivity and coercivity are zero. Secondly, magnetiza-
tion curves measured at different temperatures superimpose when magne-
tization (M) is plotted as a function of field (H)/temperature (T), as is shown
in Fig. 2.2. This also demonstrates that superparamagnetism can be
destroyed by cooling. The temperature at which this occurs is called the
blocking temperature (T B ) and is dependent linearly on volume and the
magnitude of the crystal field anisotropy.
For a particle of constant size below the blocking temperature T B , the
magnetization will be stable and shows hysteresis for those particles which
have a relaxation time for demagnetization of longer than 100 s. For
uniaxial particles (using the same criterion for stability):
￿ ￿ ￿ ￿ ￿ ￿
KV
25k B
T B ¼
½
2
:
1
where K=anisotropy
constant, V=volume of
the particle
and
6
k B =Boltzmann's constant (1.38
10.23 J K).
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