| Because the alkaline earth and rare earth hexaborides straddle the metal-insulator and magnetic-non-magnetic transitions, this class of materials has consistently been a source of interest. The discovery that CaB6, when lightly electron-doped and without any inherent magnetic constituent, exhibits long-range ferromagnetism to a Curie temperature of 600 K further enhances the reputation of the hexaborides for their unusual properties.; Experimental results are presented of electron transport studies of the novel ferromagnet CaB6 in three doping concentrations: stoichiometric CaB6, electron-doped Ca1−δLaδ B6, and Ca-deficient Ca1−δB6 . In particular, dependences of electronic properties on temperature and applied magnetic field were studied. These investigations were conducted to advance the understanding of the origin of this unusual ferromagnetism.; Upon measurement of the quantities of resistivity, magnetoresistance, Hall effect, and electron tunneling spectra, a band structure model consistent with our results has been formulated. We have discovered that CaB6 single crystals, when grown in excess Ca to counteract the tendency to form Ca vacancies, exhibit semimetallic behavior with a Fermi level marginally crossing the conduction band separated in energy from the valence band. This result contradicts theoretical expectations of a semiconductor or compensated semimetal. The La-doped counterpart is metallic, as expected. The Ca-deficient compound, however, retains a low electron concentration while exhibiting semiconducting transport properties, implying that the Fermi level resides near the bottom of the conduction band within the semiconducting gap.; Recent theoretical efforts, in addition to predictions of a polarized electron fluid and the formation of a doped excitonic insulator, have proposed that an impurity-induced magnetic moment is conceivable. The results of this work are consistent with the presence of an impurity band and may indicate the validity of a theory of impurity-driven ferromagnetism.; Owing to the combination of a high TC, low carrier density, and proximity to semiconducting behavior, this novel ferromagnet may be applicable to the field of spintronics, in which the exploitation of the spin degree of freedom aims for integration of spin-based devices into the semiconductor industry. The control of electronic spin in semiconducting devices suggests a variety of technologically important applications, including spin-based transistors, light emitting diodes, and optical sensors. |
