Muon Spin Relaxation Investigation of Tetranuclear Iron(III) Fe4(OCH3)6(dpm)6 Molecular Cluster

We present a study of the spin dynamics of Fe4(OCH3)6(dpm)6 single molecule magnet by means of SQUIDmagnetization and muon relaxation (mu+SR) measurements. In longitudinal field mu+SR experiments performedat magnetic fields H = 200, 1000 Oe, the muon asymmetry P(t) could be fitted by means of three components,the first constant, the second fast relaxing through a quasiexponential decay, and the third, the slowest relaxing,showing an exponential decay. The slowest muon relaxation rate Lambda studied as a function of temperature Tdisplayed two structures, a broad peak at T ca. 15/20 K and a shoulder at T < 5 K, both decreasing in amplitudeand displacing toward higher temperatures as the field is increased. To mimic qualitatively the temperaturebehavior Lambda(T) at the investigated fields, we used a function expressed as the sum of two Bloembergen-Purcell-Pound (BPP)-like laws, reproducing the mechanism of relaxation. The exponential data resulted well fitted bymeans of a heuristic model which takes into account two correlation times tau' and tau, related to the ground-statemultiplet barrier (Delta'/kB=7.25 K) and to the intermultiplet separation (Delta/kB=86.4 K) between S=5 andS=4