First direct observation of magnetic polarons by small-angle neutron scattering

Abstract

High temperature superconductors start out as two-dimensional antiferromagnetic Mott insulators. These are then lightly doped, where the motion of these free charges distorts the surrounding Néel order and forms a string of distorted spins in the form of a magnetic polaron. Thus, magnetic polarons are thought to be central to the high-$T_c$ problem. The shape and size of such dynamic magnetic domains can be determined from small angle neutron scattering (SANS). In order to show that magnetic polarons can be detected by SANS we studied the three-dimensional ferromagnetic model system \eu, where the results of other measurements suggest the presence of magnetic polarons. In \eu, just above $T_{\mathrm{C}}$, our experiments show magnetic scattering intensity, which has a Lorentzian dependence on the wave vector, 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~\AA\ for the size of the magnetic polarons. This size is much larger than one would expect for magnetic fluctuations of a 3D Heisenberg ferromagnet, demonstrating the usefulness of SANS for detecting magnetic polarons.