Spin mixing and protection of ferromagnetism in a spinor dipolar condensate

Abstract

We study spin mixing dynamics in a chromium dipolar Bose-Einstein condensate, after tilting the atomic spins by an angle $\ensuremath{\theta}$ with respect to the magnetic field. Spin mixing is triggered by dipolar coupling, but, once dynamics has started, it is mostly driven by contact interactions. For the particular case $\ensuremath{\theta}=\ensuremath{\pi}/2$, an external spin-orbit coupling term induced by a magnetic gradient is required to enable the dynamics. Then the initial ferromagnetic character of the gas is locally preserved, an unexpected feature that we attribute to large spin-dependent contact interactions.