Short Time Quantum AC response of a system of Nanomagnets

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.