Anomalous Magnetism in Eu(Ca)B6

The transport and magnetic properties of the Eu1-xCaxB6 (0 <= x <= 0.4) single crystals were studied in the wide ranges of temperatures (1.8-300 K) and magnetic fields (up to 8 T). The experimental data allow to identify a metal-insulator transition (MIT) at the critical Ca concentration x(MIT)approximate to 0.3, which agrees well with the predictions of double exchange model (V.M. Pereira et al. Phys. Rev. Lett., 93 (2004) 147202). A significant enhancement of magnetoresistive effect is observed below 100K for Eu1-xCaxB6 compounds corresponding to the metallic side of the MIT (x <= x(MIT)). The drastic decrease of paramagnetic Curie temperature evaluated from the magnetic and magnetotransport data is discussed in terms of quantum MIT scenario recently proposed for this low carrier density system.