  Using the highly sensitive anomalous Hall effect (AHE) we have been able to
measure the reversal of a single magnetic island, of diameter 220nm, in an
array consisting of more than 80 of those islands. By repeatedly traversing the
hysteresis loop, we measured the thermally actuated fluctuation of the
switching field of the islands at the lower and higher ends of the switching
field distribution. Based on a novel easy-to-use model, we determined the
switching field in the absence of thermal activation, and the energy barrier in
the absence of an external field from these fluctuations. By measuring the
reversal of individual dots in the array as a function of temperature, we
extrapolated the switching field and energy barrier down to 0K. The
extrapolated values are not identical to those obtained from the fluctation of
the switching field at room temperature, because the properties of the magnetic
material are temperature dependent. As a result, extrapolating from temperature
dependent measurements overestimates the energy barrier by more than a factor
of two. To determine fundamental parameters of the energy barrier between
magnetisation states, measuring the fluctuation of the reversal field at the
temperature of application is therefore to be preferred. This is of primary
importance to applications in data storagea and magnetic logic. For instance in
fast switching, where the switching field in the absence of thermal activation
plays a major role, or in long term data stability, which is determined by the
energy barrier in the absence of an external field.
