Magnetotransport in the low carrier density ferromagnet EuB_6

We present a magnetotransport study of the low–carrier density ferromagnet EuB_6. This semimetallic compound, which undergoes two ferromagnetic transitions at T_l = 15.3 K and T_c = 12.5 K, exhibits close to T_l a colossal magnetoresistivity (CMR). We quantitatively compare our data to recent theoretical work, which however fails to explain our observations. We attribute this disagreement with theory to the unique type of magnetic polaron formation in EuB_6.