Unusual magnetism of hexaborides

Various recent experimental investigations have revealed unusual magnetic properties of hexaborides with divalent cations M2+. EuB6 is ferromagnetic below 16 K and its low-temperature properties show remarkable similarities to those of manganese oxides, exhibiting the phenomenon of colossal magnetoresistance. Close to the phase transition as well as far below the ordering temperature, EuB6 exhibits anomalous features, that are briefly discussed. Alkaline-earth hexaborides are close to a metal-insulator transition and it has been found that, in a narrow range of electron doping, an itinerant-type of ferromagnetic order, stable up to temperatures of the order of 600-900 K, is established. This remarkable phenomenon is suspected to be due to the peculiar electronic band structure of these materials. (C) 2000 Elsevier Science B.V. All rights reserved.