Superparamagnetic clusters in antiferromagnetic Eu_0.52Ca_0.48B₆ seen by Small Angle Neutron Scattering (SANS)

Since the term was introduced by Pekar in 1946, polarons have been thought to underpin a range of prominent phenomena from high-T_c superconductivity to colossal magnetoresistance. Polarons are quasiparticles consisting of charge carriers ``dressed'' by lattice distortions. Similarly, magnetic polarons are charge carriers dressed with spins. In the europium chalcogenides, magnetic polarons are thought to be formed as a spin polarization of the mobile charge carriers by the localized 4f moments. This leads to the formation of magnetic ``bubbles'' and the localization of charge carriers within them. Here we report on experiment which extends an earlier experiment on EuB6 where SANS indicated the presence of magnetic polarons. The experiment was carried out at the Institut Laue Langevin (ILL) in Grenoble. It shows the presence of superparamagnetic clusters well above the critical temperature in 48% Ca doped-EuB6. Unlike EuB6, Eu_0.52Ca_0.48B₆ does not undergo an insulator to metal phase transition, and its resistivity grows rapidly below 10 K. A metallic state can be obtained through the application of a magnetic field greater than 2 T. In Eu_0.52Ca_0.48B₆, the charge carrier stay trapped inside the magnetic polarons, leading to an increasing resistivity at lower temperature.