Growth and magnetic properties of single crystal Co2MnX (X = Si, Ge) Heusler alloys

Half-metallic (HM) ferromagnetic materials have recently drawn intense interests due to their potential use in magnetoelectronic devices. Co-based Heusler alloys of the type Co2MnX (X = Si, Ge), predicted to be EM by first principles band structure calculations, are of particular interest since they alone possess Curie temperatures in excess of 900 K. Since the spin polarization (P) is believed to be sensitive to antisite defects that are likely to occur in vapor-quenched thin film synthesis, single crystals of Co-2 MnX (X = Si, Ge) were prepared using the tri-arc Czochralski method. X-ray diffraction, including Laue backscattering, was employed to determine the high crystalline quality of these crystals. SQUID magnetometry measured a magnetic moment per formular unit that is close to the calculated value indicating that these alloys may in fact be EM. However, point contact Andreev reflection, a technique that has reliably measured high P in CrO2, measures P values for these crystals of 50-60%, well below their theoretical values. The reduced spin polarization may be due to the effects of crystal symmetry breaking at the surface or the presence of antisite defects, or that these materials are not truly half-metallic.