Macroscopic quantum tunneling in molecular magnets

Our present understanding of the phenomenon of quantum tunneling of magneti zation (QTM), is reviewed in the light of the experiments performed on the molecular complex Mn-12-ac. This system, in which QTM was clearly shown for the first time, consists of molecules with mesoscopic spins in dipolar int eractions. Both single and many-molecule effects are essential for the obse rvation of QTM (crystal field, hyperfine and dipolar interactions), which a llows one to make a link between mesoscopic physics and magnetism. (C) 1999 Published by Elsevier Science B.V. All rights reserved.