E ﬀ ect of Annealing on the Structural, Magnetic, Surface Energy and Optical Properties of Co 32 Fe 30 W 38 Films Deposited by Direct-Current Magnetron Sputtering

: In this study, a 10–50 nm Co 32 Fe 30 W 38 alloy thin ﬁlm sputtered on glass substrates was annealed at di ﬀ erent temperatures for 1 h including room temperature (RT), 300, 350, and 400 ◦ C. The structure, magnetic properties, surface energy, and optical properties of the Co 32 Fe 30 W 38 alloy were studied. X-ray di ﬀ raction (XRD) patterns of the as-deposited Co 32 Fe 30 W 38 thin ﬁlms showed the amorphous structure. The apparent body-centered cubic (BCC) CoFe (110) structure was exhibited after 300 ◦ C annealing for 1 h. The 300 ◦ C annealed Co 32 Fe 30 W 38 thin ﬁlm showed the highest CoFe (110) peak compared with other temperatures. Furthermore, the thicker the Co 32 Fe 30 W 38 thin ﬁlm, the higher the CoFe (110) peak. The CoFe (110) peak revealed magneto-crystalline anisotropy, which was related to the strong low-frequency alternative-current magnetic susceptibility ( χ ac ) and induced an increasing trend of saturation magnetization ( M s ) as the thickness ( t f ) increased. Due to the thermal disturbance, the χ ac and M s for the 350 and 400 ◦ C annealed Co 32 Fe 30 W 38 thin ﬁlm decreased. The contact angles of the Co 32 Fe 30 W 38 thin ﬁlms were less than 90 ◦ . For all temperatures, the surface energy increased when the ﬁlm thickness increased from 10 to 50 nm. In addition, the surface energies for annealed samples were comparatively higher than the as-deposited samples. The higher surface energy of 28 mJ / mm 2 was obtained for the 50 nm Co 32 Fe 30 W 38 thin ﬁlm annealed at 300 ◦ C. The transmittance percentage (%) of the as-deposited Co 32 Fe 30 W 38 ﬁlm was higher than other annealed conditions. This result contributed to the fact that higher crystallization, due to perfect band structures, may inhibit the transmission of photon signals through the ﬁlm, resulting in low transmittance and high absorption.


Introduction
Recently, the density of magnetic recording has been greatly improved. CoFe films are widely used in magnetoresistance random access memory (MRAM) and magnetic head applications due to their high spin polarization, high saturation magnetization (M s ), and low coercivity (H C ) [1][2][3][4]. CoFe films are free or pinned layers in spin-valued magnetic tunnel junction (MTJ) [5][6][7][8][9][10]. MTJ has a three-layer structure including the top ferromagnetic (FM1) layer, insulated tunnel barrier layer (spacer layer), and bottom ferromagnetic (FM2) layer. Researchers have made efforts to add a third element into the CoFe material in magnetic fields [11][12][13][14][15]. Of late, the addition of a third new element to the original CoFe material has attracted extensive attention. The addition of tungsten (W) into CoFe materials to form the CoFeW alloy has rarely been studied. Few studies have added W into CoFe. In 2012, Pai et al. studied the phase transition thickness of the rare earth transition metal W, and the effect of the MTJ spin Hall angle was studied with W as the seed layer [16]. In 2016, Ghaferi et al. used a citric acid salted bath to test the CoFeW alloy, and observed variations in W content and pH value of different concentrations. The composition ratio of CoFeW alloy in citric acid borate solution, the surface morphology, structure, and magnetic properties of the films were analyzed [17]. The addition of W in the alloy has the following advantages. As a spacer or buffer layer, W can increase the benefits of the materials including durability and strong perpendicular magnetic anisotropy (PMA) at a high temperature [18][19][20]. However, the disadvantages of CoFe thin films include brittleness and reduced magnetic characteristics at high temperatures. The mechanical strength and magnetic characteristics of CoFe films can be improved by adding W, because W is a hard metal with a high melting point. The addition of W as the third element may improve the mechanical properties of the CoFe alloy [21]. CoFeW is a newly emerging and a significant soft magnetic material, which can be widely used in MRAM and gauge sensors. It can also be used as a free layer or pinning layer to combine with the magnetic process, and can be compatible with other layers in a double and multi-layer system. The performance is more sensitive to RT and high temperature at which it is operated. Therefore, the effectiveness of CoFeW films in the as-deposited and annealed states is worth studying. However, at as-deposited and annealed conditions, there are few studies on the magnetic, surface energy, and optical properties of CoFeW films. Therefore, it is valuable to study the efficiency of CoFeW films deposited by magnetron sputtering at room temperature and annealed temperature. This study also focused on the CoFeW thicknesses of various as-deposited and annealed treatments to investigate the influence of crystallinity with its magnetic properties, surface energy, and optical performance. In our previous research, the CoFeV and CoFeBV alloys were investigated for their specific properties, which is arranged in Table 1 [22][23][24][25][26]. Table 1 suggests that CoFeV and CoFeBV thin films were investigated at the as-deposited condition. As-deposited and annealed CoFeW thin films were studied in this work. Moreover, it also indicates that the low-frequency alternate-current magnetic susceptibility (χ ac ) values of CoFeW thin films are larger than other CoFeV and CoFeBV thin films.

Materials and Methods
CoFeW with a thickness of 10-50 nm was sputtered on the glass substrate at room temperature (RT) by the magnetron sputtering direct current (DC) method of 50 W power and under the following four conditions: (a) the deposited films were kept at RT; (b) annealed at a treatment temperature (T A ) at 300 • C for 1 h; (c) annealed at 350 • C for 1 h; and (d) annealed at 400 • C for 1 h. The power density was 1.65 W/cm 2 and the deposition rate was 1.2 nm/min. The chamber base pressure was 1.95 × 10 −9 Pa, and the Ar working pressure was 2.25 × 10 −5 Pa. The pressure in the ex situ annealed condition was 1.87 × 10 −5 Pa with a specific Ar gas. The thick of alloy target was 2 mm. The alloy target for the composition of CoFeW was 32 at% Co, 30 at% Fe, and 38 at% W with an energy dispersive spectrometer (EDS, Hitachi, Tokyo, Japan) for spectroscopy. To determine the accurate thickness, high-resolution cross-sectional field emission scanning electron microscopy (FESEM, SU 8200, Hitachi, Tokyo, Japan) was used to study the calibration thickness of the corresponding sputtering time. The elemental composition of the films was determined by FESEM with EDS for spectroscopy. The structure of CoFeW thin films was determined by using grazing incidence X-ray diffraction (GIXRD) patterns obtained with the CuK α1 (PAN analytical X'pert PRO MRD, Malvern Panalytical Ltd, Cambridge, UK) and a low angle diffraction incidence with about a two-degree angle. Surface roughness and morphology of thee CoFeW films were studied by atomic force microscopy (AFM, NT-MDT, Moscow, Russia). The in-plane low-frequency alternate-current magnetic susceptibility (χ ac ) and hysteresis loop of Co 32 Fe 30 W 38 were studied by an χ ac analyzer (XacQuan, MagQu Co., Ltd. New Tapei City, Taiwan) and alternating gradient magnetometer (AGM, PMC, OH, USA). First, the standard sample was calibrated by the χ ac analyzer with external magnetic field. Then, the sample was inserted into the χ ac analyzer. The driving frequency was between 10 and 25,000 Hz. χ ac was measured by magnetization. All test samples had the same shape and size to eliminate demagnetization. The χ ac valve is an arbitrary unit (a.u.) because the alternating current result corresponds to the reference standard sample and is a comparative value. The relationship between magnetic susceptibility and frequency was measured by means of the χ ac analyzer. The optimal resonance frequency (f res ) is measured by the χ ac analyzer, which represents the frequency of the maximum χ ac . Before measurement, the contact angle should be properly air cleaned on the surface. The contact angles of CoFeW film were measured with deionized (DI) water and glycerol. The contact angles were measured when the samples were taken out from the chamber. The surface energy was obtained from the contact angle and some calculations [27][28][29]. The transmittance of CoFeW was measured by a spectral intelligent analyzer. The wavelength of visible light was from 500 to 800 nm.

Structure Property and Composition Analysis
Figure 1a-d shows the grazing incidence X-ray diffraction (GIXRD) of the as-deposited and annealed Co 32 Fe 30 W 38 thin films. No apparent crystalline peaks were obvious in Figure 1a, indicating that the as-deposited samples were amorphous structures. Therefore, it can be reasonably inferred that the thickness of the Co 32 Fe 30 W 38 thin films had a discontinuous growth model and random atomic arrangement, which led to the amorphous state [24]. Through the 300 • C annealing process, the thin film gained enough thermal energy as a driving force to transform the structure from an amorphous state to a crystalline state. The characteristic peak located at 2θ = 44.7 • belongs to the body-centered cubic (BCC) CoFe (110), as displayed in Figure 1b. The intensity of CoFe (110) was enhanced as the thickness of the Co 32 Fe 30 W 38 thin films increased from 10 to 50 nm. The diffraction intensity in Figure 1c,d showed weaker crystallize phenomena than that in Figure 1b. The diffraction intensity of 300 • C annealed Co 32 Fe 30 W 38 thin films was higher than the as-deposited ones. After 300 • C annealing, the structure of the Co 32 Fe 30 W 38 thin films was improved from an amorphous state to a crystalline state. Figure 1e is the GIXRD pattern of the glass substrate, which also indicates an amorphous state.  Figure 2 shows the full width of half maximum (FWHM, B) of the CoFe (110) peak obtained after 300 °C annealing. According to the XRS results in Figure 1b, the B of the CoFe (110) peak decreased when the thickness of Co32Fe30W38 thin films increased from 10 to 50 nm. Therefore, the thinner Co32Fe30W38 thin film possessed larger FWHM. The thicker Co32Fe30W38 thin films with annealing treatment had a smaller B.   High-resolution cross-sectional field emission scanning electron microscopy (SEM) images of the as-deposited and 300 • C annealed samples are displayed in Figure 3a,b. From the SEM results, the annealed Co 32 Fe 30 W 38 thin films became denser than the as-deposited ones. The thickness of the annealed Co 32 Fe 30 W 38 thin film decreased slightly. In particular, the XRD results of the Co 32 Fe 30 W 38 thin films showed that the films were amorphous. Through the annealing treatment, enough thermal energy was used as the driving force to change the structure from amorphous to crystalline. High density of the Co 32 Fe 30 W 38 thin films was obtained by using an annealing treatment. The plane-view images of the as-deposited and 300 • C annealed samples are shown in Figure 3c,d. From the SEM results, the as-deposited Co 32 Fe 30 W 38 thin film displayed the loose surface morphology. Figure 2 shows the full width of half maximum (FWHM, B) of the CoFe (110) peak obtained after 300 °C annealing. According to the XRS results in Figure 1b, the B of the CoFe (110) peak decreased when the thickness of Co32Fe30W38 thin films increased from 10 to 50 nm. Therefore, the thinner Co32Fe30W38 thin film possessed larger FWHM. The thicker Co32Fe30W38 thin films with annealing treatment had a smaller B.   High-resolution cross-sectional field emission scanning electron microscopy (SEM) images of the as-deposited and 300 °C annealed samples are displayed in Figure 3a,b. From the SEM results, the annealed Co32Fe30W38 thin films became denser than the as-deposited ones. The thickness of the annealed Co32Fe30W38 thin film decreased slightly. In particular, the XRD results of the Co32Fe30W38 thin films showed that the films were amorphous. Through the annealing treatment, enough thermal energy was used as the driving force to change the structure from amorphous to crystalline. High density of the Co32Fe30W38 thin films was obtained by using an annealing treatment. The plane-view images of the as-deposited and 300 °C annealed samples are shown in Figure 3c,d. From the SEM results, the as-deposited Co32Fe30W38 thin film displayed the loose surface morphology. The composition analysis for the as-deposited and annealed Co32Fe30W38 thin films by using EDS spectroscopy is presented in Figure 4a-d. The ratio of Co, Fe, and W in the as-deposited Co32Fe30W38 thin films was 32% for Co, 30% for Fe, and 38% for W. After 350 and 400 °C annealing, the Co32Fe30W38 thin films possessed the oxygen composition shown in the EDS spectroscopy, which indicates that the film may have reduced magnetic properties due to the presence of oxygen after annealing. This result suggests that the composition was not the same because the multi-directional scattering and multi angle scattering of sputtered atoms were the main reasons for various compositions [30,31]. Moreover, as the target was magnetic and the thickness was 2 mm, it could certainly perturb the magnetic field of the magnetron gun, and thus induce serious film uniformity issues in both terms of thickness and composition. The composition analysis for the as-deposited and annealed Co 32 Fe 30 W 38 thin films by using EDS spectroscopy is presented in Figure 4a-d. The ratio of Co, Fe, and W in the as-deposited Co 32 Fe 30 W 38 thin films was 32% for Co, 30% for Fe, and 38% for W. After 350 and 400 • C annealing, the Co 32 Fe 30 W 38 thin films possessed the oxygen composition shown in the EDS spectroscopy, which indicates that the film may have reduced magnetic properties due to the presence of oxygen after annealing. This result suggests that the composition was not the same because the multi-directional scattering and multi angle scattering of sputtered atoms were the main reasons for various compositions [30,31]. Moreover, as the target was magnetic and the thickness was 2 mm, it could certainly perturb the magnetic field of the magnetron gun, and thus induce serious film uniformity issues in both terms of thickness and composition.

Surface Roughness
The root-mean-square roughness (Rq) values of all Co32Fe30W38 thin films were detected by AFM, as shown in Figure 5. The surface roughness of the Co32Fe30W38 thin film decreased when the thickness increased. Surface roughness of the 300 °C annealed sample was smoother than other samples due to its dense structure. With the increase in surface roughness, the domain wall pinning effect is not easy to move, resulting in the decrease in the χac value [32].

Magnetic Properties
Figure 6a-d presents the in-plane low-frequency alternative-current magnetic susceptibility (χac) at RT, 300, 350, and 400 °C, with thicknesses ranging from 10 to 50 nm. The low frequency measured range was from 10 to 25,000 Hz and the χac value was the highest at low frequency. The χac values of all samples decreased with the increase in thickness. At high frequency, the χac of the CoFeW film drops sharply, as shown in Figure 6. When the magneto-crystalline anisotropy of the CoFe (110) crystallization effect is maximized, χac is maximized [33,34]. When annealed at 300 °C , the optimal frequency (fres) of 50 Hz had the strongest χac value 0.52, and the spin sensitivity was the strongest.

Surface Roughness
The root-mean-square roughness (R q ) values of all Co 32 Fe 30 W 38 thin films were detected by AFM, as shown in Figure 5. The surface roughness of the Co 32 Fe 30 W 38 thin film decreased when the thickness increased. Surface roughness of the 300 • C annealed sample was smoother than other samples due to its dense structure. With the increase in surface roughness, the domain wall pinning effect is not easy to move, resulting in the decrease in the χ ac value [32].

Surface Roughness
The root-mean-square roughness (Rq) values of all Co32Fe30W38 thin films were detected by AFM, as shown in Figure 5. The surface roughness of the Co32Fe30W38 thin film decreased when the thickness increased. Surface roughness of the 300 °C annealed sample was smoother than other samples due to its dense structure. With the increase in surface roughness, the domain wall pinning effect is not easy to move, resulting in the decrease in the χac value [32].  Figure 6a-d presents the in-plane low-frequency alternative-current magnetic susceptibility (χac) at RT, 300, 350, and 400 °C, with thicknesses ranging from 10 to 50 nm. The low frequency measured range was from 10 to 25,000 Hz and the χac value was the highest at low frequency. The χac values of all samples decreased with the increase in thickness. At high frequency, the χac of the CoFeW film drops sharply, as shown in Figure 6. When the magneto-crystalline anisotropy of the CoFe (110) crystallization effect is maximized, χac is maximized [33,34]. When annealed at 300 °C , the optimal frequency (fres) of 50 Hz had the strongest χac value 0.52, and the spin sensitivity was the strongest.  Figure 6a-d presents the in-plane low-frequency alternative-current magnetic susceptibility (χ ac ) at RT, 300, 350, and 400 • C, with thicknesses ranging from 10 to 50 nm. The low frequency measured range was from 10 to 25,000 Hz and the χ ac value was the highest at low frequency. The χ ac values of all samples decreased with the increase in thickness. At high frequency, the χ ac of the CoFeW film drops sharply, as shown in Figure 6. When the magneto-crystalline anisotropy of the CoFe (110) Coatings 2020, 10, 1028 7 of 13 crystallization effect is maximized, χ ac is maximized [33,34]. When annealed at 300 • C, the optimal frequency (f res ) of 50 Hz had the strongest χ ac value 0.52, and the spin sensitivity was the strongest.  The results indicate that maximum value of χac increased with the increase in thickness (tf) from 10 to 50 nm. Moreover, the film annealed at 300 °C had the highest value of χac. In particular, the χac value of annealed films at 300 °C was larger than that of TA = 350 and 400 °C due to the thermal disturbance, which reduced the magnetic spin coupling. At the maximum frequency, χac had the best physical meaning. At low frequency, the oscillation of the volume dipole moment in each region is the contribution to the alternate-current (AC) dipole moment. The magnetic interaction between the domains seems to be restored. Frequency is the driving force of the system. Therefore, the peak value of low frequency susceptibility corresponds to the oscillation frequency of the magnetic dipole moment in the domain. In frequency, the peak of χac depicts the spin exchange coupling and dipole moment [35]. Therefore, the physical significance of the low frequency magnetic susceptibility peak indicates the coupling of the magnetic exchange. Moreover, the maximum χac of thee Co32Fe30W38 films was compared to the Co40Fe40V20 films. The highest χac value of the 50-mm-thick Co32Fe30W38 thin film was larger than the Co40Fe40V20 thin film [24]. Figure 7b displays the maximum χac corresponding to the optimal resonance frequency (fres) under four specific conditions. At this frequency, the maximum χac is measured with the strongest spin sensitivity [35,36]. The optimal resonance frequency was less than 500 Hz and can be used in low frequency applications and magnetic junctions, except for the 10 nm range deposited at 1000 Hz. The fres values of all Co32Fe30W38 thin films were less than 1000 Hz, which indicates that the film is favorable for the application of sensors, transformers, and low-frequency magnetic recording media. Figure 7c depicts the in-plane saturation magnetization (Ms) of Co32Fe30W38 films measured by AGM under four different conditions. The results suggest that there is a significant relationship between Ms and thickness. When the film thickness increased from 10 to 50 nm, Ms tended to be saturated, indicating the thickness effect of MS on the Co32Fe30W38 thin films. Moreover, the Ms value of the film annealed at 300 °C was higher than the other conditions. The maximum Ms at 50 nm was about 1133 emu/cm 3 at annealed 300 °C . Due to the strong magneto-crystalline anisotropy, it suggests that it has high spin coupling strength and can induce large Ms. It was also found that the Ms and χac of annealed  The results indicate that maximum value of χ ac increased with the increase in thickness (t f ) from 10 to 50 nm. Moreover, the film annealed at 300 • C had the highest value of χ ac . In particular, the χ ac value of annealed films at 300 • C was larger than that of T A = 350 and 400 • C due to the thermal disturbance, which reduced the magnetic spin coupling. At the maximum frequency, χ ac had the best physical meaning. At low frequency, the oscillation of the volume dipole moment in each region is the contribution to the alternate-current (AC) dipole moment. The magnetic interaction between the domains seems to be restored. Frequency is the driving force of the system. Therefore, the peak value of low frequency susceptibility corresponds to the oscillation frequency of the magnetic dipole moment in the domain. In frequency, the peak of χ ac depicts the spin exchange coupling and dipole moment [35]. Therefore, the physical significance of the low frequency magnetic susceptibility peak indicates the coupling of the magnetic exchange. Moreover, the maximum χ ac of thee Co 32 Fe 30 W 38 films was compared to the Co 40 Fe 40 V 20 films. The highest χ ac value of the 50-mm-thick Co 32 Fe 30 W 38 thin film was larger than the Co 40 Fe 40 V 20 thin film [24]. Figure 7b displays the maximum χ ac corresponding to the optimal resonance frequency (f res ) under four specific conditions. At this frequency, the maximum χ ac is measured with the strongest spin sensitivity [35,36]. The optimal resonance frequency was less than 500 Hz and can be used in low frequency applications and magnetic junctions, except for the 10 nm range deposited at 1000 Hz. The f res values of all Co 32 Fe 30 W 38 thin films were less than 1000 Hz, which indicates that the film is favorable for the application of sensors, transformers, and low-frequency magnetic recording media. Figure 7c depicts the in-plane saturation magnetization (M s ) of Co 32 Fe 30 W 38 films measured by AGM under four different conditions. The results suggest that there is a significant relationship between M s and thickness. When the film thickness increased from 10 to 50 nm, M s tended to be saturated, indicating the thickness effect of M s on the Co 32 Fe 30 W 38 thin films. Moreover, the M s value of the film annealed at 300 • C was higher than the other conditions. The maximum M s at 50 nm was about 1133 emu/cm 3 at annealed 300 • C. Due to the strong magneto-crystalline anisotropy, it suggests that it has high spin coupling strength and can induce large M s . It was also found that the M s and χ ac of annealed Co 32 Fe 30 W 38 thin films were higher than those of the films before the as-deposited films due to the magneto-crystalline anisotropy, except those annealed at T A = 350 and 400 • C, because the thermal interference reduces the magnetic spin coupling. From the magnetic results of Figure 7a,c, it can be reasonably inferred that the Co 32 Fe 30 W 38 thin films exhibited stronger magneto-crystalline anisotropy and better magnetic properties at 300 • C, except for annealing at 350 and 400 • C for 1 h. Due to the magneto-crystalline anisotropy, the M S and χ ac values of the retreated Co 32 Fe 30 W 38 thin films were higher than those of the as-deposited samples. The M S value of the Co 32 Fe 30 W 38 thin films increased to 300 • C, then decreased to 350 • C, which indicates that the thermal stability of the Co 32 Fe 30 W 38 thin films was better than that in the other literature [37,38]. Co32Fe30W38 thin films were higher than those of the films before the as-deposited films due to the magneto-crystalline anisotropy, except those annealed at TA = 350 and 400 °C , because the thermal interference reduces the magnetic spin coupling. From the magnetic results of Figure 7a,c, it can be reasonably inferred that the Co32Fe30W38 thin films exhibited stronger magneto-crystalline anisotropy and better magnetic properties at 300 °C , except for annealing at 350 and 400 °C for 1 h. Due to the magneto-crystalline anisotropy, the MS and χac values of the retreated Co32Fe30W38 thin films were higher than those of the as-deposited samples. The MS value of the Co32Fe30W38 thin films increased to 300 °C , then decreased to 350 °C , which indicates that the thermal stability of the Co32Fe30W38 thin films was better than that in the other literature [37,38].  Figure 8a,b illustrates the contact angles of the Co32Fe30W38 thin films with deionized (DI) water and glycerol under two different temperature treatments. Before measurement, the contact angle should be properly air cleaned on the surface. An air gun may only remove dust, but no other contamination and adsorb species. The results of all samples showed that the water droplets on the Co32Fe30W38 thin films were nearly spherical and the contact angle was less than 90°. This result showed that the thin film had flat water droplets. An oxide layer formed on the surface of the Co32Fe30W38 thin films when the sample was exposed to an atmospheric environment, where the native oxide was in the nm-scale in thickness. The surface energy of the native oxide layers was presented on the surface of the Co32Fe30W38 thin film. Surface tension is also called surface free energy. The surface tension of an oxide layer on the surface of the Co32Fe30W38 thin film is the important factor for initial deposition of a seed layer. In this work, the surface tension was measured based on an oxide layer on the surface of the Co32Fe30W38 thin films. The surface energy was obtained from the calculation of contact angles [27][28][29], as shown in Figure 8c. For different temperature-treated samples, the results showed that the surface energy increased when the film thickness increased from 10 to 50 nm. The surface energy of the heat-treated sample was higher than that of the as-deposited  Figure 8a,b illustrates the contact angles of the Co 32 Fe 30 W 38 thin films with deionized (DI) water and glycerol under two different temperature treatments. Before measurement, the contact angle should be properly air cleaned on the surface. An air gun may only remove dust, but no other contamination and adsorb species. The results of all samples showed that the water droplets on the Co 32 Fe 30 W 38 thin films were nearly spherical and the contact angle was less than 90 • . This result showed that the thin film had flat water droplets. An oxide layer formed on the surface of the Co 32 Fe 30 W 38 thin films when the sample was exposed to an atmospheric environment, where the native oxide was in the nm-scale in thickness. The surface energy of the native oxide layers was presented on the surface of the Co 32 Fe 30 W 38 thin film. Surface tension is also called surface free energy. The surface tension of an oxide layer on the surface of the Co 32 Fe 30 W 38 thin film is the important factor for initial deposition of a seed layer. In this work, the surface tension was measured based on an oxide layer on the surface of the Coatings 2020, 10, 1028 9 of 13 Co 32 Fe 30 W 38 thin films. The surface energy was obtained from the calculation of contact angles [27][28][29], as shown in Figure 8c. For different temperature-treated samples, the results showed that the surface energy increased when the film thickness increased from 10 to 50 nm. The surface energy of the heat-treated sample was higher than that of the as-deposited sample. The surface energy varied from 22.5 to 28 mJ/mm 2 .

Analysis of Optical Properties
Figure 9a-d shows the optical transmission spectra of the Co32Fe30W38 thin films with different thicknesses under four specific conditions. From Figure 9a, two bumps of the entire transmittance curve at 550-680 nm were observed. Due to the nanoscale effect, the band structures would gradually be separated, resulting in the density of states changing from dense to sparse. Therefore, the highest transmittance of visible light located at 580 and 630 nm could be attributed to the sparse density of states, allowing for the passing of more light when the film becomes thinner. In addition, the optical transmission spectra can be used to judge the penetration effect of the sample for different light sources. From the transmittance spectrum, the light penetration range was from 550 to 680 nm, indicating the transmission range of yellow to orange visible light. Figure 9 shows the transmittance curve at the visible wavelength of 500 to 800 nm. With the increase in thickness from 10 to 50 nm, the transmittance of the as-deposited Co32Fe30W38 thin film decreased from 23% to 1%. The change trends of transmittance under other annealed conditions were similar. When the thickness increased from 10 to 50 nm, the transmittance decreased from 19% to 1%. Figure 9 shows that the thin and asdeposited films induced high transmittance because thicker films after the annealing treatment had more excellent crystallization and may inhibit the transmission of photon signals through the film,   Figure 9a, two bumps of the entire transmittance curve at 550-680 nm were observed. Due to the nanoscale effect, the band structures would gradually be separated, resulting in the density of states changing from dense to sparse. Therefore, the highest transmittance of visible light located at 580 and 630 nm could be attributed to the sparse density of states, allowing for the passing of more light when the film becomes thinner. In addition, the optical transmission spectra can be used to judge the penetration effect of the sample for different light sources. From the transmittance spectrum, the light penetration range was from 550 to 680 nm, indicating the transmission range of yellow to orange visible light. Figure 9 shows the transmittance curve at the visible wavelength of 500 to 800 nm. With the increase in thickness from 10 to 50 nm, the transmittance of the as-deposited Co 32 Fe 30 W 38 thin film decreased from 23% to 1%. The change trends of transmittance under other annealed conditions were similar. When the thickness increased from 10 to 50 nm, the transmittance decreased from 19% to 1%. Figure 9 shows that the thin and as-deposited films induced high transmittance because thicker films after the annealing treatment had more excellent crystallization and may inhibit the transmission of photon signals through the film, resulting in low transmittance [39]. In the optical result, the main motivation was to find the best optical properties and combine magnetic properties to apply to magnetic-optical fields. According to optical performance, the optimal thickness of the as-deposited Co 32 Fe 30 W 38 thin films was found to be 10 nm, which is suitable for magnetic-optical recording medium applications.

Analysis of Optical Properties
Coatings 2020, 10, x FOR PEER REVIEW 10 of 13 optical performance, the optimal thickness of the as-deposited Co32Fe30W38 thin films was found to be 10 nm, which is suitable for magnetic-optical recording medium applications.

Conclusions
XRD results showed that the as-deposited Co32Fe30W38 films were amorphous structures, and after more than 300 °C annealing for 1 h, the CoFe (110) crystalline structure was revealed. When the 50 Hz fres was achieved, the maximum χac for the 300 °C annealed Co32Fe30W38 thin film was 0.52. For all Co32Fe30W38 thin films with different thicknesses, the fres was less than 1000 Hz, indicating that these alloy thin films are suitable for low frequency magnetic components. The diffraction intensity for the 300 °C annealed Co32Fe30W38 thin films was higher than that of the as-deposited and other temperature annealed thin films. In addition, the thicker Co32Fe30W38 thin films had a higher diffraction intensity. From the SEM results, the as-deposited Co32Fe30W38 thin film became denser after the annealing treatment. Furthermore, the thickness was slightly decreased after annealing. The AFM result showed that the surface roughness of the 350 °C annealed samples was smoother than other samples due to the denser structure. The peak of CoFe (110) showed magneto-crystalline anisotropy, which is related to the highest χac. When the thickness increased, the induced saturation of Ms increased. In addition, the χac and Ms of the films annealed at 350 and 400 °C decreased due to thermal interference. Furthermore, the contact angle of the Co32Fe30W38 films was less than 90°. As the thickness increased from 10 to 50 nm, the surface energy increased. The surface energy of the asdeposited Co32Fe30W38 thin film was enlarged after annealing. The transmittance decreased when the thin film became thicker. The as-deposited thin films had higher transmittance because the excellent crystallization after annealing may inhibit the transmission of photon signals through the films, resulting in lower transmittance and higher absorption. The highest transmittance of the Co32Fe30W38

Conclusions
XRD results showed that the as-deposited Co 32 Fe 30 W 38 films were amorphous structures, and after more than 300 • C annealing for 1 h, the CoFe (110) crystalline structure was revealed. When the 50 Hz f res was achieved, the maximum χ ac for the 300 • C annealed Co 32 Fe 30 W 38 thin film was 0.52. For all Co 32 Fe 30 W 38 thin films with different thicknesses, the f res was less than 1000 Hz, indicating that these alloy thin films are suitable for low frequency magnetic components. The diffraction intensity for the 300 • C annealed Co 32 Fe 30 W 38 thin films was higher than that of the as-deposited and other temperature annealed thin films. In addition, the thicker Co 32 Fe 30 W 38 thin films had a higher diffraction intensity. From the SEM results, the as-deposited Co 32 Fe 30 W 38 thin film became denser after the annealing treatment. Furthermore, the thickness was slightly decreased after annealing. The AFM result showed that the surface roughness of the 350 • C annealed samples was smoother than other samples due to the denser structure. The peak of CoFe (110) showed magneto-crystalline anisotropy, which is related to the highest χ ac . When the thickness increased, the induced saturation of M s increased. In addition, the χ ac and M s of the films annealed at 350 and 400 • C decreased due to thermal interference. Furthermore, the contact angle of the Co 32 Fe 30 W 38 films was less than 90 • .
As the thickness increased from 10 to 50 nm, the surface energy increased. The surface energy of the as-deposited Co 32 Fe 30 W 38 thin film was enlarged after annealing. The transmittance decreased when the thin film became thicker. The as-deposited thin films had higher transmittance because the excellent crystallization after annealing may inhibit the transmission of photon signals through the films, resulting in lower transmittance and higher absorption. The highest transmittance of the Co 32 Fe 30 W 38 film was 23%. Based on the results of magnetic property and surface energy, the optimal thickness of the Co 32 Fe 30 W 38 film was 50 nm after 300 • C annealing. The film is suitable for use as a free layer of MTJ and can be used in the application of magnetic recording media. The article discusses how to improve the quality of the emerging Co 32 Fe 30 W 38 material by annealing at a higher temperature. In summary, through the annealing process, the Co 32 Fe 30 W 38 thin films showed better surface energy and higher magnetic characteristics than the as-deposited films. However, the as-deposited thin films had better transmittance than the annealed films.