I will first describe our experiments where magnetic chromium atoms loaded in a 3D optical lattice undergo spin-exchange processes due to long-range dipole-dipole interactions. A ChromiumBose-Einstein condensate is loaded into a 3D optical lattice. At large lattice depth, atoms are pinned in each of the lattice sites, forming a Mott insulating state; we then observe a non-equilibrium spinor dynamics resulting from inter-site Heisenberg-like spin-spin interactions provided by non-local dipole-dipole interactions. This spin dynamics is inherently many-body, as each atom is coupled to its many neighbors. At low lattice depth, when tunneling is not suppressed, we observe an intricate spin dynamics with interplay between spin-exchange due to dipolar interactions and spin exchange due to contact interactions, which is well reproduced by a mean-field Gross-Pitaevskii simulation.
We have also studied the thermodynamics of chromium atoms at low magnetic field. Due to the anisotropy of dipolar interactions, magnetization is free and adapts to temperature. We observe that the BEC always forms in the lowest energy Zeeman state. By applying a magnetic field gradient, we introduce a selective loss of atoms in spin-excited states, which provides a specific loss channel for thermal atoms. This new cooling mechanism based on spin filtering results in purification of the BEC and an increased phase-space density.