Effect of site disorder on the resonant microwave absorption in Co2Fe0.5Ti0.5Si Heusler alloy thin films

Abstract

Motivated by immense scientific interest in Co-based Heusler compounds [1 - 3] Co2FeSi and Co2TiSi as promising candidates for spintronics applications, 100 nm thick Co2Fe0.5Ti0.5Si (CFTS) thin films were grown on Si(100) substrates at substrate temperatures (TS) ranging from 200°C to 550°C by ultra-high vacuum magnetron sputtering. The deposited films were annealed in-situ at TS for 30 min. Energy-dispersive X-ray absorption spectroscopy yielded the actual alloy composition as Co2.01Fe0.51Ti0.24Si1.24. X-ray diffraction (XRD) patterns, shown in figure 1, reveal no Bragg peaks for TS200 film, broad fundamental peak (220) for TS350 film, and three sharp peaks (111), (220), and (422) for the remaining films. Thus, the crystalline structure gradually evolves from amorphous to ordered L21 structure as the TS increases; the TS500 film has the highest L21 order. Ferromagnetic resonance (FMR) spectra were recorded at various polar (‘out-of-plane’, OP) field angles, θH, at a fixed frequency of 9.45 GHz. A typical FMR spectrum, taken at θH = 0 for the TS500 film, shown in figure 2(a), demonstrates that besides the uniform precession q = 0 spin wave (SW) mode, the SW modes with q ≠ 0 exist. A detailed analysis [4 - 6] of the resonance field, Hres, versus θH plots (see Fig.2(a) inset) reveals a dominant OP uniaxial anisotropy. Fig.2(b) bears out that the frequency (f) vs. Hres plots, obtained using the broad-band (3–17 GHz) FMR setup, are well described by the Kittel resonance condition [7] (the curves through the data points):f=γ/2π[(Hr+Hk)×(Hr+HK+4πMeff)]1/2, where γ=(2πgµB)/h is the gyromagnetic ratio, Meff, effective magnetization while HK, g, and µB are the in-plane (IP) anisotropy field, Lande g-factor and Bohr magneton. We find that the IP anisotropy is very weak, and g has the minimum value of 2.02 for the TS500 film with the highest L21 order. The Gilbert damping constant (α) is determined from the linear fits (straight lines) to the frequency-dependent `peak-to-peak’ FMR linewidth (ΔH) data (symbols), shown in Fig.2(b) inset, based on the relation [8] ΔH=ΔHinhomo+(2/√3)2παf/γ. The TS500 film has the lowest value of α = 0.0032.  Fig.1:XRD patterns.  Fig.2(a) FMR spectrum; inset: Hres(θH). (b) f(Hres); inset: ΔH(f).