  We calculate the magnetisation relaxation in the short-time regime for an
ensemble of nanomagnets in the presence of a low-frequency external AC biasing
field, at temperatures lower than the magnetic anisotropy energy of the
individual nanomagnets. It is then found that the relaxation is strongly
affected by AC fields with amplitude larger than that of the $T_2$ fluctuations
in the nuclear field. This will allow experimental probing of the nuclear spin
relaxation mechanism.
