Measurements of the complex susceptibility,
, as a function of the
frequency (100 Hz–18 GHz) and polarizing field (0–90 kA m−1) at room temperature
together with static magnetic measurements over the temperature range 4–300 K, are
reported for a colloidal suspension of cobalt nanoparticles.
The transition of the cobalt particles to the superparamagnetic state are supported by the
temperature dependencies of field cooling (FC) and zero field cooling (ZFC)
magnetization measurements. From these measurements, which show a typical blocking
behaviour of an assembly of superparamagnetic particles with a wide distribution of
blocking temperatures, the exponential pre-factor
of Brown's equations for Néel
relaxation, is found to be equal to 9.2 × 10−10 s.
Measurement of the complex susceptibility
over this broad frequency range, with an
upper frequency value corresponding to three times that previously reported in our
measurements on cobalt, has enabled the presence of both resonance and Néel relaxation
mechanisms to be identified. From the Néel component, a further value for
evaluated and shown to be in close agreement with that obtained from the ZFC data.
Data on the after-effect function, realised by Fourier transformation of the
component, is also presented.