Strong polaron–spin fluctuations drive the insulator–semimetal transition in low-density electron gases

One of the outstanding questions in physics is the formation of magnetic moments in metallic systems of low carrier density. For the design of materials for spintronic applications, we need to understand the physics in a regime where mobile charge carriers couple strongly to the magnetic moments. One way to realise such a strong interaction is to work with materials with a low carrier density where two competing electronic ground states are close in energy. Switching between these different electronic ground states then leads to large effects in the electronic transport. What makes the physics in these colossal magnetoresistance (CMR) materials fascinating is that the CMR effects stem from an electronic phase separation at the nanometre scale, despite the fact that these materials are chemically homogeneous. ``Magnetic polarons'' are thought to be responsible for the CMR effects in some materials. Here we present results of a small angle neutron scattering (SANS) study showing the dramatic influence of magnetic polarons on ferromagnetic fluctuations in EuB₆, giving clear evidence for their involvement in the insulator-to-metal transition as the result of large scale magnetic fluctuations.