Macroscopic Quantum Tunneling of Magnetization of Single Ferrimagnetic Nanoparticles of Barium Ferrite

Presented are switching field measurements of individual ferrimagnetic and insulating BaFeCoTiO nanoparticles containing about 10⁵μB at very low temperatures (0.1--6 K). For temperatures higher than 0.4 K, the quantitative agreement with the N\'eel-Brown theory of thermal activated magnetization reversal allowed us to identify unambiguously the magnetization reversal of uniform rotation. Below 0.4 K, strong deviations from this model are evidenced which are quantitatively in agreement with the predictions of the theory of macroscopic quantum tunneling of magnetization in the low dissipation regime.