Resistivity minima in disordered Co2FeAl0.5Si0.5 Heusler alloy thin films

Abstract

An exhaustive study of the effect of anti-site disorder on the ‘zero-field’, ρ(T, H = 0), and ‘in-field’, ρ(T, H = 80 kOe), electrical resistivity in 50 nm thick Co2FeAl0.5Si0.5 (CFAS) Heusler alloy thin films (deposited on the Si(100) or SiO2/Si(100) substrates at fixed substrate temperatures in the range 27°C≤TS≤550°C), has been carried out. Irrespective of the strength of disorder, resistivity goes through a minimum as a function of temperature at T = Tmin. With increasing substrate temperature, the crystalline order of the CFAS thin films improves so much so that the films deposited at 500 °C have the lowest anti-site disorder within the B2 structure and the least residual resistivity. A quantitative comparison of our results with the predictions of the existing theoretical models permits us to unambiguously identify the diffusive and ballistic transport mechanisms, responsible for ρ(T, H = 0) and ρ(T, H = 80 kOe) in different temperature ranges and accurately determine their relative magnitudes. The electron-diffuson (e – d) scattering and weak localization (WL) mechanisms, responsible for negative temperature coefficient of resistivity (TCR) for T < Tmin, compete with the positive TCR mechanisms, electron-magnon (e – m) and electron–phonon (e – p) scattering, to produce the resistivity minimum at Tmin. The e – d and WL contributions to ρ, ρe-d and ρwl, dominate over the e – m and e – p contributions, ρe-m and ρe-p, for T<Tmin whereas the reverse is true for T>Tmin. At any given temperature, ρe-d, ρwl and ρe-m decrease while ρe-p increases as the atomic order improves with increasing substrate temperature, TS. ρe-p and ρe-m originate from the phonon-induced non-spin-flip two-band (s↑↓- d↑↓) scattering and magnon - induced spin-flip s - d interband (s↑↓- d↓↑) transitions, respectively. In all the CFAS films, except for the one deposited at TS = 27°C, the thermal renormalization of the spin-wave stiffness, due to the electron - magnon interaction, contributes significantly to ρe-m(T). Furthermore, we demonstrate that the negative magnetoresistance (MR) is a consequence of a progressive suppression of the WL effect and e – m scattering by external magnetic field. However, the WL contribution to MR turns out to be negligibly small as the e – m contribution almost entirely accounts for MR over the temperature range 5K≤T≤300K.