MAGNETIZATION REVERSAL IN INDIVIDUAL NANOPARTICLES - MACROSCOPIC QUANTUM TUNNELING OF MAGNETIZATION

Superconducting Quantum Interference Device (SQUID) with high quality nanoparticles allowed us to check for the first time simple classical models proposed nearly 50 years ago by Neel, Stoner and Wohlfarth. For nanoparticles containing about 10(5) to 10(6) mu(B) the quantitative agreement with the Neel-Brown theory of thermal activated magnetizatio n reversal allowed us to identify unambiguously the magnetization reve rsal mechanism as uniform rotation. In the case of insulating barium f errite nanoparticles containing about 10(5) mu(B) and below 0.4 K, str ong deviations from this model are found which are quantitatively in a greement with the predictions of the theory of macroscopic quantum tun neling of magnetization in the low dissipation regime.