Does a low-carrier density ferromagnet hold the key to understanding high temperature superconductors?

We conducted a small-angle neutron scattering experiments (SANS) on the ferromagnetic semi-metal EuB${}_6$, where we observed direct evidence for the presence of magnetic polarons. We carried out SANS experiments over a large range of scattering vectors $\mathrm{\mid }\vec{q}\mathrm{\mid }$ from 0.006 to 0.140~Å${}^{-1}$ and 2 to 60~K. Just above $T_{\mathrm{C}}$ our experiments show magnetic scattering intensity, which has a Lorentzian dependence on the wave vector, which is characteristic for the presence of magnetic polarons. Below $T_{\mathrm{C}}$ the polarons merge and most of the observed intensity is due to scattering from domain walls.We were able to extract a correlation length $\xi$ which ranges from $100$ to $300$~Å for the size of the magnetic polarons. This size is much larger than one would expect for magnetic fluctuations, demonstrating the influence the magnetic polarons on the phase transition. We were able to extract a correlation length $\xi$ which ranges from $100$ to $300$~Å for the size of the magnetic polarons. This size is much larger than one would expect for magnetic fluctuations, demonstrating the influence the magnetic polarons on the phase transition.