Observation of magnetic polarons in EuB 6 by small-angle neutron scattering

We conducted small-angle neutron scattering (SANS) experiments on the ferromagnetic semimetal EuB6, where we observed first-hand evidence for the presence of magnetic polarons. We carried out SANS experiments over a large range of scattering vectors ||⃗𝐪|, ranging from 0.006 to 0.140 Å−1 , and temperatures of 2 to 60 K. Just above 𝑇C our experiments show magnetic scattering intensity, which has a Lorentzian dependence on the wave vector, characteristic of the presence of magnetic polarons. Below 𝑇C the polarons merge, and most of the observed intensity is due to scattering from domains. We were able to extract a correlation length that 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 of a three-dimensional Heisenberg ferromagnet, demonstrating the influence of magnetic polarons on the phase transition.