*3.1. Full-Width at Half Maximum (FWHM) and Grain Size Distribution*

The corresponding X-ray diffraction patterns (XRD) of the CoFeW films have been proved in our previous literature [22]. From a previous study, it was indicated that the CoFe(110) peak and specific Fe2O3(320), WO3(002), Co2O3(422), and Co2O3(511) oxide peaks are displayed in XRD. The corresponding joint committee on powder diffraction standards (JCPDS) cards are #JCPDS 49-1567, #JCPDS 24-0081, #JCPDS 05-0363, and #JCPDS 02-0770. The reason for the formation of the oxide peak can be reasonably inferred to be that there may still be oxygen in the chamber system of the sputtering system. In addition, both the adventitious oxide on the Si(100) substrate and the oxygen contamination on the sputtering target contribute to the formation of oxidation peaks. According to the above reason, the main peak of CoFeW film is CoFe(110), and the FWHM of CoFe(110) were analyzed. Moreover, it can be calculated grain size and FWHM of CoFe(110) by XRD data. Figure 1a shows the FWHM of the as-deposited and post-annealed CoFeW thin films. The results indicate that the FWHM decreased as CoFeW thickness increased. In addition, Figure 1a shows that the FWHM decreased as the post-annealing temperature raised. Finally, we calculated the grain size of CoFe(110) by using the FWHM determined by XRD and the Scherrer equation.

**Figure 1.** (**a**) FWHM of CoFeW thin films. (**b**) Grain size of CoFeW thin films.

The Scherrer equation is [23,24]:

$$\mathbf{D} = \mathsf{K}\lambda / \beta \circledast \mathbf{e} \mathbf{e} \mathbf{e} \tag{1}$$

where the quantity D is grain size, K (0.89) is the Scherrer's constant, λ is the X-ray wavelength of the Cu Kα1 line, β is the FWHM diffraction of CoFe(110) peak, and θ is the half-angle of the diffraction peak. The FWHM of the CoFe(110) peak was used to estimate the grain sizes at the as-deposited and the two annealing temperatures. Figure 1b shows the grain size of CoFeW films after deposition and annealing. The as-deposited CoFeW thin films possessed the smallest grain size. However, as post-annealing temperature increased, we found that the grain size increased, and was affected by the thicknesses of CoFeW thin films, as shown in Figure 1b. Moreover, the post-annealing of CoFeW thin films at 350 ◦C when the thickness was 50 nm possessed the highest observed grain size value—this is because post-annealing provided energy to CoFeW thin films, resulting in an increased grain size of CoFeW thin films.
