Itinerant ferromagnetism represents one of the most spectacular manifestations of interactions within many-body fermion systems. In contrast to weak-coupling phenomena, it requires strong repulsion to develop, making aquantitative description of ferromagnetic materials notoriously difficult. In this frame, it is still debated whether the simplest case envisioned by Stoner of a homogeneous Fermi gas with short-range repulsive interactions can exhibit ferromagnetism at all.
Here, we will discuss our experimental route to tackle such an intriguing problem with ultracold Fermi gases.
We will describe a recent experiment in which we create an artificial ferromagnetic state by segregating degenerate spin mixtures of ^6 Li atoms into two initially disconnected reservoirs. Starting from such a configuration, we characterize the behavior of the gas as a function of the interspecies interactions and temperature through two distinct but interconnected measurements of spin dynamics. First, we observe the softening of the spin-dipole mode, unambiguously connected to the diverging magnetic susceptibility approaching the ferromagnetic transition. Second, we investigate the diffusion dynamics of the two spin domains, whose temporary halt outlines the metastable character of the ferromagnetic state. Notwithstanding the limited lifetime of the repulsive Fermi gas due to the competing pairing instability, our findings allow us to draw in the temperature-interaction plane the critical boundaries for the ferromagnetic phase to exist, at least in a metastable sense.