Ferromagnetism vs paramagnetism and false quantum oscillations in lanthanum-doped CaB₆

Magnetization and resistivity were measured as functions of temperature and magnetic field on two single crystals of CaB₆ and three of (Ca_0.995La_0.005)B₆. One of the doped crystals exhibited ferromagnetism with the magnetic moment of 0.32 μ_B/La at 10kOe and 5 K, while the other two were paramagnetic, displaying the Curie law below 120 K with the effective magnetic moment of 1.3 or 2.0 μ_B/La. We propose that the sample-dependent magnetic behaviors of (Ca_0.995La_0.005)B₆ may be explained in terms of superparamagnetism, assuming that the ferromagnetic state is spatially inhomogeneous. All the samples exhibited the metallic behavior of electrical conduction. Quantum oscillations were found in the magnetoresistance of one of the doped crystals. The angular dependence of the oscillations, however, indicates that their origin is not the sample bulk.