ALBERTO BOLLERO REAL

3.2 Wohlfarth’s relation and Henkel plot
can help to clarify the nature of the magnetisation processes. J d r (H m ) is the remanence after
application of a reverse field, -H m , to the saturated sample; J r is the remanence of the
material, measured after saturation (H m → ∞); J m r (H m ) is the remanence after application
and removal of the maximum field H m to a thermally demagnetised sample. Measurements
of these quantities are shown in Fig. 3.4 for a typical exchange-coupled two-phase
nanocomposite magnet.
J r
J rd
(H m
)
J rm
(H m
)
Fig. 3.4: First magnetisation and demagnetisation curves for an exchange-coupled twophase
composite magnet (Fe 3 B / Nd 2 Fe 14 B); (adapted from Schneider et al [45]).
However, some important requirements must be considered for the validity of
Wohlfarth’s relation: the system must consist of an assembly of non-interacting
magnetically uniaxial single-domain particles and the sample must be initially in the
thermally demagnetised state. Thus, a plot of J m r (H m )/J r vs. J d r (H m )/J r would result in a
straight line as that shown in Fig. 3.5 for a system where the validity requirements are
verified. This is the so-called “Henkel-plot” [47] which is a simple and elegant way of
presenting Wohlfarth’s relation independent of the applied magnetic field. However,
exchange-coupling implies interactions between particles and, as a consequence, a
deviation from that linear behaviour as it can be seen in Fig. 3.5.
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