We report the results of a 11B nuclear magnetic resonance (NMR) study of SrB6 at temperatures between 0.1 and 30 K and in a magnetic field of 4.74 T. Below 30 K the NMR spectrum is temperature independent but the spin-lattice relaxation rate T1-1 exhibits different features in two different temperature regimes. At high temperatures, between 30 K and a field-dependent crossover temperature TB between 0.5 and 2 K, T1-1 is almost temperature independent. We point out that for T in the crossover temperature range the magnitude of T1l-1 of SrB6 is distinctly larger than for LaB6, a metal with a charge carrier concentration at least two orders of magnitude higher than that of SrB6. A possible cause for this behavior maybe the very weak itinerant ferromagnetism that has subsequently been established to occur in nominally pure SrB6. At low temperatures, below TB, T1-1 decreases substantially with decreasing temperature confirming a cross-over or phase transition phenomenon as observed by measurements of thermal and transport properties. © 2000 Elsevier Science B.V. All rights reserved.

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.