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

TL;DR

This study uses small-angle neutron scattering to observe magnetic polarons in EuB$_6$, revealing their significant size and role in the phase transition, which may inform understanding of high-temperature superconductors.

Contribution

First direct evidence of magnetic polarons in EuB$_6$ over a wide temperature and wave vector range, linking magnetic polarons to phase transition behavior.

Findings

Magnetic polarons have sizes of 100-300 Å.

Magnetic polarons influence the phase transition.

Below $T_C$, polarons merge into domain walls.

Abstract

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 $|\vec{q}|$ from 0.006 to 0.140~\AA$^{-1}$ 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 $300$~\AA\ 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 $300$~\AA\ 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.