Muon spin relaxation investigation of tetranuclear iron(III) Fe_4(OCH_3)_6(dpm)_6 molecular cluster

Abstract

We present a study of the spin dynamics of Fe_4(OCH_3)_6(dpm)_6 single molecule magnet by means of SQUID magnetization and muon relaxation (µ^(+)SR) measurements. In longitudinal field µ^+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 studied as a function of temperature T displayed two structures, a broad peak at T~15/20 K and a shoulder at T<5 K, both decreasing in amplitude and displacing toward higher temperatures as the field is increased. To mimic qualitatively the temperature behavior λ(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 τ' and τ, related to the ground-state multiplet barrier (Δ'/k_B=7.25 K) and to the intermultiplet separation (Δ'/k_B=86.4 K) between S=5 and S=4.

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