Relaxation of the collective magnetization of a dense 3D array of interacting dipolar S=3 atoms.

We report on measurements of the dynamics of the collective spin length (total magnetization) and spin populations in an almost unit filled lattice system comprising about 10^4 spin S=3 chromium atoms, under the effect of dipolar interactions. The observed spin population dynamics is unaffected by the use of a spin echo, and fully consistent with numerical simulations of the S=3 XXZ spin model. On the contrary, the observed spin length decays slower than in simulations, and surprisingly reaches a small but nonzero asymptotic value within the longest timescale. Our findings show that spin coherences are sensitive probes to systematic effects affecting quantum many-body behavior that cannot be diagnosed by merely measuring spin populations.