Abstract
The magnetic relaxation of a set of small ferromagnetic particles of Tb0.5Ce0.5Fe2 of mean diameter 150 Å has been measured at low temperature (down to 50 mK) and with a field up to 8 Tesla. A change between thermally activated and temperature-independent relaxation has been observed near Tc = 0.6 K. All the results obtained at low temperature are consistent with theoretical predictions of Macroscopic Quantum Tunnelling with weak dissipation in a ferromagnetic system.