  The relaxation rate and temperature-dependent switching field curve of a
spherical magnetic nanocluster are calculated by taking into account the effect
of surface anisotropy via an effective anisotropy model. In particular, it is
shown that surface anisotropy may change the thermally activated magnetization
reversal by more than an order of magnitude, and that temperature-dependent
switching field curves noticeably deviate from the Stoner-Wohlfarth astroid.
With recent and future $\mu$-SQUID measurements in mind, we indicate how
comparison of our results with experimental data on isolated clusters may allow
one to obtain valuable information on surface anisotropy.
