Comment on Tunneling in Mesoscopic Magnetic Molecules

VOLUME 76, NUMBER 16 PHYSICAL REVIEW LETTERS 15A PRIL 19962 3Comment on “Tunneling in Mesoscopic Magnetic flip-flops with V , Sm yr (here DS › 61), or elsed zvia multimolecule (higher than binary) processes to gen-Molecules”erate transitions with DS › 62 (e.g., a four-moleculezIn a very interesting recent Letter [1] Politi et al. process could involve two neighboring molecules forstudied magnetic relaxation in Mn O acetate [2], which DS › 0, and two others having DS › 62).12 12 z zassuming a spin-phonon coupling mechanism. They Applying a magnetic field suppresses the flip-flop21 3predicted t , H for the relaxation rate in a high resonances. Increasing the field to hyA › 1 brings one21magnetic field, neglecting the dipolar interactions be- back to resonance, so t sHd will have a minimum [2]21tween molecules. We find the calculations in [1] to [there will also be a slow monotonic increase in t sHd21be entirely correct; however, the experimental t sHd due to barrier height reduction]. One would also expect21 3clearly disagrees with t , H , and shows nonmono- an increased propensity toward “avalanche” relaxation attonic behavior with a minimum at H › 0.2 T. In this this resonance field—this provides a natural explanation21Comment we argue that the minimum in t in the for the peak observed in such relaxation [2]. In any case21activation regime is due to the dipole-dipole interactions our picture clearly predicts maxima in t at resonantneglected in [1], which cause ...