  Collective behavior in many-body systems is the origin of many fascinating
phenomena in nature ranging from swarms of birds and modeling of human behavior
to fundamental magnetic properties of solids. We report on the first
observation of collective spin dynamics in an ultracold Fermi sea with large
spin: We observe long-lived and large-amplitude coherent spin oscillations,
driven by local spin interactions. At ultralow temperatures, Pauli blocking
stabilizes the collective behavior and the Fermi sea behaves as a single entity
in spin space. With increasing temperature, we observe a stronger damping
associated with particle-hole excitations. As a striking feature, we find a
high-density regime where excited spin configurations are collisionally
stabilized.
