Muon spin relaxation investigation of tetranuclear iron(III)Fe4(OCH3)6(dpm)6molecular cluster

Abstract

We present a study of the spin dynamics of ${\text{Fe}}_{4}{({\text{OCH}}_{3})}_{6}{(\text{dpm})}_{6}$ single molecule magnet by means of SQUID magnetization and muon relaxation $({\ensuremath{\mu}}^{+}SR)$ measurements. In longitudinal field ${\ensuremath{\mu}}^{+}SR$ experiments performed at 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 $\ensuremath{\lambda}$ studied as a function of temperature $T$ displayed two structures, a broad peak at $T\ensuremath{\sim}15/20\text{ }\text{K}$ and a shoulder at $T<5\text{ }\text{K}$, both decreasing in amplitude and displacing toward higher temperatures as the field is increased. To mimic qualitatively the temperature behavior $\ensuremath{\lambda}(T)$ 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 by means of a heuristic model which takes into account two correlation times ${\ensuremath{\tau}}^{\ensuremath{'}}$ and $\ensuremath{\tau}$, related to the ground-state multiplet barrier $({\ensuremath{\Delta}}^{\ensuremath{'}}/{k}_{B}=7.25\text{ }\text{K})$ and to the intermultiplet separation $(\ensuremath{\Delta}/{k}_{B}=86.4\text{ }\text{K})$ between $\text{S}=5$ and $\text{S}=4$.