We study dipolar relaxation in both ultra-cold thermal and Bose-condensed chromium atom gases. We show three different ways to control dipolar relaxation, making use of either a static magnetic field, an oscillatory magnetic field, or an optical lattice to reduce the dimensionality of the gas from 3D to 2D. Although dipolar relaxation generally increases as a function of a static magnetic field intensity, we find a range of non-zero magnetic field intensities where dipolar relaxation is strongly reduced. We use this resonant reduction to accurately determine the S = 6 scattering length of chromium atoms: a6 = 103 ± 4a0. We compare this new measurement to another new determination of a6, which we perform by analysing the precise spectroscopy of a Feshbach resonance in d-wave collisions, yielding a6 = 102.5 ± 0.4a0. These two measurements provide by far the most precise determination of a6 to date. We then show that, although dipolar interactions are long-range interactions, dipolar relaxation only involves the incoming partial wave l = 0 for large enough magnetic field intensities, which has interesting consequences on the stability of dipolar Fermi gases. We then study ultra-cold chromium gases in a 1D optical lattice resulting in a collection of independent 2D gases. We show that dipolar relaxation is modified when the atoms collide in reduced dimensionality at low magnetic field intensities, and that the corresponding dipolar relaxation rate parameter is reduced by a factor up to 7 compared to the 3D case. Finally, we study dipolar relaxation in presence of radio-frequency (rf) oscillating magnetic fields, and we show that both the output channel energy and the transition amplitude can be controlled by means of rf frequency and Rabi frequency. Strong dipole-dipole interactions arise when an atomic or molecular species carries a strong permanent magnetic or electric dipole moment. Good candidates therefore include heteronuclear molecules with large electric dipole moments (which were recently produced at large phasespace densities [7]), or atoms with large electronic spin (so far erbium [8], dysprosium [9] and chromium). Up to now, chromium is the only species with large dipole moment for which a quantum degenerate gas has been produced [10,11]. Smaller dipolar effects were also observed in a BEC of potassium for which the scattering length can be precisely tuned to zero by means of a Feshbach resonance [12].Interesting new physics comes at play when one also considers the spin degree of freedom. Spin dynamics of optically trapped multi-component Bose-Einstein Condensates (also known as spinor condensates) [13] has been observed [14]. Coherent oscillations between the spin components is driven by centrally symmetric short range exchange interactions, and the total magnetization in the system is conserved. In [15] a first dipolar effect was observed on the spin texture of a Rb spinor condensate. Dipole-dipole interaction will introduce additional new features in spin dynamics as it couples the spin degree of fre...

We study the spinor properties of S = 3 (52)Cr condensates, in which dipole-dipole interactions allow changes in magnetization. We observe a demagnetization of the Bose-Einstein condensate (BEC) when the magnetic field is quenched below a critical value corresponding to a phase transition between a ferromagnetic and a nonpolarized ground state, which occurs when spin-dependent contact interactions overwhelm the linear Zeeman effect. The critical field is increased when the density is raised by loading the BEC in a deep 2D optical lattice. The magnetization dynamics is set by dipole-dipole interactions.