*3.4. Surface Energy*

The surface energy of films is significant as it relates to the adhesion of thin films. When CoFeW thin films are used as a seed layer or buffer layer, strong adhesion of thin films is essential. The data of contact angles are used to calculate the surface energy using the Young equation [29,30].

The Young Equation (2) is

$$
\sigma\_{s\theta} = \sigma\_{s\mathsf{l}} + \sigma\_{\mathsf{l}\mathsf{g}} \cos \Theta \tag{2}
$$

where σsg is the surface free energy of the solid, and σsl denotes the interfacial tension between liquid and solid. σlg is the surface tension of the liquid, and θ is contact angle. Figure 4 exhibits the surface energy of the Co40Fe40W20 thin films. These data are further shown in Table 1. It can be observed that the surface energy of annealed CoFeW films was higher than that of the deposited films. As the post-annealing temperature increased, the surface energy also increased, and the surface energy of as-deposited CoFeW thin films reached 25.1 mJ/mm2 at 40 nm, which was the highest value. When the post-annealing temperature was at 250 ◦C, the highest surface energy at 20 nm was 31.79 mJ/mm2. When the post-annealing temperature reached 350 ◦C, the surface energy was 36.02 mJ/mm<sup>2</sup> at 30 nm. According to the XRD results [22], the formation of oxide layers on the thin films' surface resulted in decreased contact angles and increased surface energy [31]. These CoFeW thin film data are shown in Table 1. According to the grain size result of Figure 1b and Table 1, the annealed treatment produces larger grain size distribution than as-deposited treatment. From the result of Figure 4, it suggests that the surface energy of annealed 350 ◦C is larger about 1.5 times than as-deposited condition. It can be reasonably concluded that when the grains are arranged in the material, the large grains of annealed material show large gaps between adjacent grains. The gaps become larger, and the support between the crystal grains will be reduced, so the crystal grain size increases and leads to a contact angle reduction trend. When water drops on the surface, it is easy for water drops to flow into the gap, resulting in a small contact angle, strong adhesion, and high surface energy. It shows the same behavior as PTFE films [32]. In contrast, the small grains

of as-deposited material show small gaps and the grains are tightly arranged. When the water droplets fall on the surface, the water droplets have difficulty flowing to the gaps, resulting in a large contact angle, weak adhesion, and a low surface energy.

**Figure 4.** The surface energy of CoFeW thin films.

Figure 5a–e shows the relationship between contact angle, surface energy, and postannealing temperature. It can be found that the surface energy of the CoFeW films increased as their contact angles decreased. It results from the Young equation [29,30]. After the post-annealing, the surface energy of CoFeW films tends to increase, suggesting that the post-annealing is significant for the surface energy of the films. When CoFeW thin films have high surface energy, the adhesion is the strongest. The adhesion depends on surface energy, but about adhesion force decides the so-called weak boundary layer. It means there can be strong adhesion from the top layer with high surface energy what decreases cohesion to the layer below, and total adhesion between layers is very low due to the presence of created weak boundary layers. The outcomes represent that a thin film is easier to combine with other layers in forming MTJs. The CoFeW film can be a free or pinned layer in MTJ. To detect the surface energy and adhesion of CoFeW performance, Table 2 is compared with other specific CoFeBY and CoFeW materials under various Si and Glass substrates. Table 2 demonstrates that the surface energy of current research is larger than other CoFeBY and CoFeW materials, which indicates that the CoFeW film is more compatible with other layers for MTJ.


**Table 2.** Comparing surface energy for specific CoFeW and CoFeBY thin films from different substrates.

**Figure 5.** The relationship between contact angle, surface energy, and post-annealing temperature. (**a**) 10 nm, (**b**) 20 nm, (**c**) 30 nm, (**d**) 40 nm, (**e**) 50 nm.
