Thermal stability of the half-metallic band gap at the Fermi energy and high spin polarization of Co2FeAl0.5Si0.5 (CFAS) Heusler alloys (1,2) have made these systems attractive for spintronic applications. However, a stringent requirement is a controllable growth of highly ordered CFAS Heusler alloys. Moreover, giant magnetoresistance (GMR) (3,4) and the anomalous Hall effect (5,6) crucially depend on the magnitude of electrical resistivity (ρxx) and its temperature dependence. Thus, a complete understanding of the different contributions to ρxx is needed. To investigate the effect of film thickness on the structural- and electrical- properties, CFAS thin films of thickness, t, in the range 12 - 75 nm, were deposited by ultrahigh vacuum dc magnetron sputtering on Si(100) substrates with SiO2 buffer layer (300 nm), at the optimum substrate temperature of 500°C. The GIXRD patterns, shown in figure 1, reveal that B2 structural order grows with increasing t and peaks at t = 50 nm. The film with t = 75 nm showed A2 disorder. Irrespective of the magnitude of t, ρxx(T) goes through a minimum at Tmin. An elaborate quantitative analysis (6-8) of the ρxx(T, H=0) data, taken over the temperature range 5 K to 300 K, demonstrates that the electron-diffuson (e – d) and weak localization (WL) effects (responsible for the negative temperature coefficient of resistivity (TCR) for T < Tmin) compete with the electron-magnon (e – m) and electron-phonon (e – p) scattering (positive TCR) contributions to produce a minimum at Tmin. Figure 2 shows the normalized resistivity ρxx(T, H=0) / ρxx(T = 300 K, H=0) (open circles) along with the optimum theoretical fits (continuous lines), go through a minimum at Tmin. Consistent with the maximum B2 order at t = 50 nm, residual resistivity, ρ5K, and the e – d, wl, e - m and e - p scattering contributions to ρxx(T, H=0), ρe-d, ρwl, ρe-m and ρe-p, all go through a minimum at t = 50 nm. Regardless of t, the thermal renormalization of the spin-wave stiffness makes a significant contribution to ρe-m.

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