This thesis investigates the growth of intermetallic compounds by co-sputtering from singleelemental targets. The preliminary work involved constructing a sputtering set-up to grow abinary alloy (Sm-Co) and investigating how to control the composition spread that was obtained.Having achieved this, a larger sputtering flange was built up to grow the ternary Heusler alloy,Co2MnSi.Co2MnSi has been predicted to be a half-metallic ferromagnet, which means that there is an energygap in the minority spin band at the Fermi energy. This leads to 100 % spin polarised conductionelectrons, which would enable ideal spin-device performance to be obtained. Co2MnSi isparticularly promising because it is predicted to have a large energy gap in the minority band of~0.4 eV and has the highest Curie temperature among the known Heuslers of 985 K.Initially, Co2MnSi was grown on a-plane sapphire and stoichiometric films were single phase andhighly (110) textured, without the use of a seed layer. They exhibited the bulk value of thesaturation magnetisation, Ms and films grown at the highest deposition temperature (715 K)showed the lowest resistivity (47 µΩcm at 4.2 K) and the lowest room temperature coercivity(18 Oe). The spin polarisation of the transport current, Pt of a 400 nm film grown at thisdeposition temperature was 54 %, consistent with measurements on bulk single crystals. Msdecreased with decreasing film thickness indicating a graded disorder. By growing on GaAs(001), which has a similar lattice parameter to Co2MnSi it was expected that this disordered regionwould be confined to the first few atomic layers. However, this was not the case becauseinterfacial reactions resulted in the formation of an epitaxial Mn-As region, and a thin interfaciallayer that was Co-Ga rich. This prevented the lattice matching of the Co2MnSi to the GaAs(001)hence hindering epitaxial growth of the Heusler. The reaction zone also meant that films exhibiteda Ms slightly below the bulk value. The expected fourfold anisotropy was not obtained for thiscubic material, which is most likely due to the anisotropy of the reconstructed GaAs surface. Inspite of this anomalous behaviour, Pt was 55 %, similar to the result obtained on sapphire,indicating that either Pt is independent of orientation or that the Heusler surface reconstructs in thesame way. Films showed some improved properties to films grown on a-plane sapphire,indicating the potential of growing on this technologically important substrate. With this in mind,pseudo spin valves involving Co2MnSi as one of the ferromagnetic electrodes were fabricated inboth the CIP and CPP configurations. Clear low-field spin-valve contributions were observed at15 K but the MR values are much lower than that expected from a PSV with a predicted 100 %spin polarised electrode.
Materials Science and Metallurgy, University of Cambridge 2005
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