Mesoscopic quantum tunneling in small ferromagnetic particles

The magnetic relaxation of a set of ferromagnetic small particles of Tb0.5Ce0.5Fe2 of mean diameter 150 angstrom has been measured at low temperatures (down to 50 mK) and with a field up to 6 kOe. A crossover temperature has been observed at T(c) = 1.2 K between thermal activation and quantum tunneling regimes. The field dependence of the crossover temperature has been determined, T(c) is-proportional-to square-root H. The behavior is in accordance with the theory of single Bloch wall tunneling in the grains, involving almost-equal-to 10(5) magnetic moments.