## 3 Results
### Evaporation Optimization
#### 3.1.2 Optimization of the Ball-Milled Cu-Fe  
A common description of the mechanochemical reaction mechanism for ductile-ductile alloying is given by [PERSON] [37]. As the milling starts, the ductile materials are subjected to plastic deformation and form bigger agglomerates. With milling time, the grains undergo cold hardening and eventually break, exposing fresh active surfaces. This breaking reduces the grain size and favors the exchange of electrons and ions between the reactants. A further increase in milling time leads to the formation and growth of  
Figure 3: Ternary contour plot of the phase diagram of Cu, Fe, and S obtained through DFT calculations. short-range crystal order. A complete reaction is only possible if one material successfully diffuses into the other. For CuFe alloys, [PERSON] et al. [38] used a shaker mill and suggested that the dissolution of Fe into the Cu requires longer milling times (20 h) compared to those employed here (3 h) for a 20% Fe proportion. However, metastable _fc_ CuFe alloy with 1:1 proportion (desired for CuFeS\({}_{2}\)) is only achieved by even longer milling times (20-360 h) for both low- and high-energy ball-milling [39; 40] or by high-pressure synthesis [41; 42]. Even upon completion, when thermally treated, CuFe alloys remain metastable and tend to decompose back into separate Cu and Fe phases [43; 44]. However, the mechanisms of milling, such as plastic deformation, agglomeration, and nucleation, can still be used to better intermix Cu and Fe for evaporation, even if the formation of the CuFe alloy is not complete. Milling can also tackle another problem observed during the evaporation of the metal precursors. As discussed earlier (Samples 3-7), the evaporation of non-milled power results in bilayer films, where the ferromagnetic properties of _fc_ Fe cause significant damage to the film surface (see Figure S23). Therefore, three different milling conditions were tested, as described in Section 2.1 and Table S1. The first (FeCu-01) and second (Fe\({}_{1.76}\)Cu-02) samples were milled for 2 h, with Fe:Cu ratios of 1:1 and 1.76:1, respectively. The third sample (with Fe:Cu = 1:1) was milled for 3 h using ethanol as a control process agent. The XRD patterns of the three as-milled Fe-Cu powders are shown in Figure 4. The Rietveld refinements (see Figures S18-S20 of the Supplementary Materials) show that the Cu\({}_{0.8}\)Fe\({}_{0.2}\) (PDF4+ database entry #40031453) gives a good fit for the diffractograms for the three different samples. However, the Cu:Fe proportion might not be 0.8:0.2 for all of them. Since Cu and Fe have a similar number of electrons, this ratio is hard to correctly retrieve using XRD; however, the incorporation of Fe into the structure can be seen by the decrease in the Fe relative intensities of samples FeCu-01 and FeCu-03. This is also observed in an increase in the lattice parameters from FeCu-01 to Fe1.76 Cu-02, and to FeCu-03 of 3.6197(3) A, 3.628(4) A, and 3.640(4) A, respectively. In order to investigate the effect of milling in evaporation, the first tests using the FeCu-01 milled powders were performed for Samples 9-11. All the strategies use a stepwise increase in current (see Table S2 for reference). The diffractograms for these samples are shown in Figure 5a, and the main parameters of synthesis are found in Figure 5b. The strategy now consists of optimizing the source-to-target distance. Figure 4: Diffractograms of the FeCu-01, Fe\({}_{1.76}\)Cu-02, and FeCu-03 ball-milled powders. Rietveld refinements are available in Figures S18–S20. The tick markers indicate the Bragg 20 positions of the Fe\({}_{x}\)Cu\({}_{100-x}\) fcc phase (black, PDF4+ database entry #40031453), Fe bcc (red, PDF4+ database entry #40079753), and WC (green, PDF4+ database entry #40045823). and Sample 10 (7.5 cm), the crystallinity of the as-evaporated films is improved, although the adhesion is still poor. Good adhesion is only achieved when the vacuum is improved, down to 10\({}^{-6}\) mbar for Sample 11.