We have combined the results of magnetization and Hall effect measurements to conclude that the ferromagnetic moments of lightly doped ${\mathrm{CaB}}_6$ samples display no systematic variation with electron doping level. Removal of the surface with acid etching substantially reduces the measured moment, although the Hall constant and resistivity are unaffected, indicating that the ferromagnetism largely resides on and near the sample surface. Electron microprobe experiments reveal that Fe and Ni are found at the edges of facets and growth steps, and on other surface features introduced during growth. Our results indicate that the weak ferromagnetism previously reported in undoped ${\mathrm{CaB}}_6$ is most likely of extrinsic origin.

• Received 22 August 2003

DOI:https://doi.org/10.1103/PhysRevB.69.132407