Theory of severe slowdown in the relaxation of rings and clusters with antiferromagnetic interactions

We show that in the severe slowing-down temperature regime the relaxation of antiferromagnetic rings and similar magnetic nanoclusters is governed by the quasicontinuum portion of their quadrupolar fluctuation spectrum and not by the lowest excitation lines. This is at the heart of the intriguing near-universal power-law temperature dependence of the electronic correlation frequency ωc with an exponent close to 4. The onset of this behavior is defined by an energy scale which is fixed by the lowest spin gap Δ0. This explains why the experimental curves of ωc for different cluster sizes and spins nearly coincide when T is rescaled by Δ0.