Fermi-surface measurements on the low-carrier density ferromagnet Ca 1 − x La x B 6 and SrB 6

Recently it has been discovered that weak ferromagnetism of a dilute three-dimensional electron gas develops on the energy scale of the Fermi temperature in some of the hexaborides; that is, the Curie temperature approximately equals the Fermi temperature. We report the results of de Haas\char21{}van Alphen experiments on two concentrations of La-doped ${\mathrm{CaB}}_{6}$ as well as Ca-deficient ${\mathrm{Ca}}_{1\ensuremath{-}\ensuremath{\delta}}{\mathrm{B}}_{6}$ and Sr-deficient ${\mathrm{Sr}}_{1\ensuremath{-}\ensuremath{\delta}}{\mathrm{B}}_{6}.$ The results show that a Fermi surface exists in each case and that there are significant electron-electron interactions in the low-density electron gas.