Fermi-surface measurements on the low-carrier density ferromagnet ${\mathrm{Ca}}_{1-x}{\mathrm{La}}_x{\mathrm{B}}_6$ and ${\mathrm{SrB}}_6$

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–van Alphen experiments on two concentrations of La-doped ${\mathrm{CaB}}_6$ as well as Ca-deficient ${\mathrm{Ca}}_{1-\delta }{\mathrm{B}}_6$ and Sr-deficient ${\mathrm{Sr}}_{1-\delta }{\mathrm{B}}_6.$ 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.