Anisotropic lattice expansion and magnetism in sputter-deposited Ca(La)B₆ films

Nanocrystalline thin films of CaB₆ and Ca_0.995La _0.005B₆ were fabricated in this study using rf-magnetron sputtering. X-ray diffraction (XRD) measurements and scanning electron microscopy/energy dispersive spectrometer analyses revealed that lattice expansion of up to 6% with respect to the bulk value occurs along the direction perpendicular to the film plane. It is caused by the trapping of Ar gas into the film. On the other hand, in-plane XRD showed that spacing along the directions within the film plane is shrunk slightly. Large ferromagnetic moment of 0.01-0.02 μ_B/f.u. was observed in some La-doped films only when the lattice expansion rate was larger than 2.5%. The Curie temperature was found to be markedly higher than 400 K.