Evidence for Magnetic Polarons in EuB₆ from Small Angle Neutron Scattering

We present results of small-angle neutron scattering (SANS) experiments on the rare-earth, magnetic semiconductor EuB₆. This compound exhibits two phase transitions: Upon cooling from an insulating state at high temperature, it first becomes metallic with a drop at a T_M of 14.5 K, after which it orders ferromagnetically with a T_C of 11.8 K. We carried out SANS experiments over a large range of scattering wave vectors q from 0.006 to 0.140 Å^-1 and temperatures from 2 to 60 K. The experiments show a Lorentzian dependence on the wave vector in the magnetic scattering intensity for temperatures just above T_C , which demonstrates the presence of magnetic polarons. Below T_C, the polarons merge together, and most of the observed intensity is, as indicated by a q dependence, from scattering of ferromagnetic wall domains. We calculated a correlation length from this Lorentzian fit and obtained a range of 10 to 100 Å for the size of the magnetic polarons. We also applied magnetic fields from zero to 500 mT to the system. This pushes the formation of magnetic polarons to higher temperatures: 22 K for 250 mT, and 24 K for 500 mT, as measured by the integrated magnetic intensity averaged over the q-range from 0.006 to 0.140 Å^-1.