Spin Fluctuation and Coherence in Concentrated systems*

In materials with a relatively high density of electron spins without direct exchange pathways, the spin decoherence tends to be dominated by dipolar-interaction mediated spin-exchange/diffusion processes. These spin exchange processes will significantly be reduced at high magnetic fields and low temperatures when the spin polarization approaches the saturation limit. We will show some examples of single crystals of molecular magnetic complexes in which the decoherence is measured experimentally at high frequencies, and which form a reference for direct theoretical models that predict the spin decoherence in these systems, and their dependence on orientation, temperature and field.