Bloch Ferromagnetism of Composite Fermions

The magnetic properties of the ground state of a low-density, 2D electron system (2DES) have been a topic of intense theoretical and experimental speculation and controversy, because the physics here is governed by strong correlations. Bloch predicted a fully spin-polarized ground state for a dilute fermionic system in 1929. However, such a state has eluded experimental realization for the last nine decades. Here we present an experimental realization of the elusive interaction-driven spin polarization in a dilute, 2D fermionic system, namely composite fermions (CFs). CFs are exotic quasiparticles, each composed of an electron and two flux quanta, formed in the half-filled Landau level of a 2DES. We determine the spin-polarization of these CFs via direct measurements of the CFs’ Fermi wavevector. We find that at high electron densities (n_e), the CFs are fully spin-polarized, consistent with previous experiments. As we lower n_e, the CFs lose their magnetization, also as expected. Remarkably, however, as n_e is further reduced, the CFs make a sudden transition and become fully spin-polarized. This spontaneous magnetization of CFs closely resembles the Bloch ferromagnetism. We also performed theoretical calculations that provide a semi-quantitative understanding of the phenomenon.