Spin decoherence in an iron-based magnetic cluster

Continuous wave (cw) and pulsed high frequency electron paramagnetic resonance (HF-EPR) measurements were performed on an Fe-based magnetic cluster: Fe₇O₄(O₂CPh)₁₁(dmem)₂, abbreviated Fe₇. The cw EPR results show that two different molecular species exist in the crystal, with slightly different zero-field-splitting parameters. The spin decoherence time, T₂, was measured at high magnetic fields and low temperatures, which makes it possible to obtain high spin polarization and to significantly reduce decoherence due to electron spin flip-flop processes. Theoretical fitting of T₂ versus temperature shows that, for crystalline samples of this molecule, spin flip-flop fluctuations represent the main source of spin decoherence at low temperatures, as reported also for the Fe₈ single-molecule magnet [Phys. Rev. Lett. 102 (2009) 087603]. Moreover, it is found that T₂ is position dependent within the EPR line, a model for which is given. We also note that this is the third example of an Fe-based cluster that exhibits a measurable decoherence time, and only the second involving a crystal.