Impact of Annealing on Magnetic Properties and Structure of Co40Fe40W20 Thin Films on Si(100) Substrate

Co40Fe40W20 monolayers of different thicknesses were deposited on Si(100) substrates by DC magnetron sputtering, with Co40Fe40W20 thicknesses from 10 to 50 nm. Co40Fe40W20 thin films were annealed at three conditions (as-deposited, 250 °C, and 350 °C) for 1 h. The structural and magnetic properties were then examined by X-ray diffraction (XRD), low-frequency alternative-current magnetic susceptibility (χac), and an alternating-gradient magnetometer (AGM). The XRD results showed that the CoFe (110) peak was located at 2θ = 44.6°, but the metal oxide peaks appeared at 2θ = 38.3, 47.6, 54.5, and 56.3°, corresponding to Fe2O3 (320), WO3 (002), Co2O3 (422), and Co2O3 (511), respectively. The saturation magnetization (Ms) was calculated from the slope of the magnetization (M) versus the CoFeW thickness. The Ms values calculated in this manner were 648, 876, 874, and 801 emu/cm3 at the as-deposited condition and post-annealing conditions at 250, 350, and 400 °C, respectively. The maximum MS was about 874 emu/cm3 at a thickness of 50 nm following annealing at 350 °C. It indicated that the MS and the χac values rose as the CoFeW thin films’ thickness increased. Owing to the thermal disturbance, the MS and χac values of CoFeW thin films after annealing at 350 °C were comparatively higher than at other annealing temperatures. More importantly, the Co40Fe40W20 films exhibited a good thermal stability. Therefore, replacing the magnetic layer with a CoFeW film improves thermal stability and is beneficial for electrode and strain gauge applications.


Introduction
Ever since the discovery of Co 50 Fe 50 by Ellis in 1927 and Elmen in 1929, it has been shown that it has good soft magnetic properties [1]. The soft magnetic material of the Co-Fe system has been extensively applied in read heads of hard disks and magnetoresistive random access memories (MRAMs) because of their good mechanical properties, high spin polarization, high Curie temperature (T c ), high saturation magnetization (M s ), low coercivity (H c ), and high spin polarization [2][3][4][5][6][7][8]. The CoFeB layer was utilized to combine with the MgO layer to form magnetic tunnel junctions (MTJs). For an MTJ system, being perpendicular magnetic anisotropic (PMA) is indispensable. However, one obstacle is the degradation of PMA properties at high temperatures. Recently, researchers have focused on increasing perpendicular magnetic anisotropic properties and thermal stability and improving the mechanical properties of MTJs' structure. To insert other metal spacer Materials 2021, 14, 3017 2 of 12 layers into an MTJ system is common and effective at improving its thermal stability [9][10][11]. Another way to enhance PMA properties is to use multilayer structures, but this method causes the multilayer films' whole thickness to increase. Transition metal (TM) alloys have a unique capability that can be used to improve some properties of magnetic films, including their thermal stability, mechanical strength, and chemical and physical properties [12][13][14][15][16][17][18]. Ghaferi et al. used a citrate bath to test a CoFeW alloy. Subsequent researchers found a variation of the W content with a pH value of different concentrations in 2016 [19]. Pai et al. studied the phase transition thickness of rare earth transition metal W. The results showed that the effect of the spin Hall angle is critical when W is employed as seed layer [20]. The addition of W in CoFe materials has some benefits such as durability, corrosion resistance, and thermal stability [21,22]. For magnetic research, CoFeW is a novel material. It is therefore attractive to research the characteristics of thin CoFeW films deposited by DC magnetron sputtering at as-deposited and annealed conditions. The various thicknesses (t f ) of as-deposited and annealed CoFeW films and, therefore, the effect of crystallinity on the magnetic properties of CoFeW films were also studied. In our previous research, the as-deposited and post-annealing glass/CoFeW were examined for their magnetic and crystallinity properties, as mentioned in Table 1 [23]. In this study, it was found that the thermal stability of Co 40 Fe 40 W 20 was about 350 • C. However, in order to apply a magnetic film in MRAM, the 400 • C of thermal tolerance is required for complementary metal-oxide-semiconductor (CMOS) and back-end-of-line (BEOL) process compatibility [22,24]. The thermal tolerance of Co 40 Fe 40 W 20 is less than 400 • C. CoFeW film can be used as an electrode and is compatible with the whole device on the substrate after annealing at 300 • C [25]. Moreover, the Co 40 Fe 40 W 20 film also offers the potential for high sensitivity for strain gauge application [26].

Materials and Methods
CoFeW with a thickness of 10-50 nm was sputtered onto an Si(100) substrate at room temperature (RT) by the magnetron DC sputtering direct method with 50 W power and under the subsequent four conditions: (a) the deposited films were kept at RT, (b) annealed at a treatment temperature (T A ) at 250 • C for 1h, and (c) annealed at 350 • C for 1 h. The chamber base pressure was 2 × 10 −7 Torr, and, therefore, the Ar working pressure was 3 × 10 −3 Torr. The pressure in the ex-situ annealed condition was 3 × 10 −3 Torr with a selected Ar gas. The alloy target of the CoFeW composition was 40 at% Co, 40 at% Fe, and 20 at% W. Co 40 Fe 40 B 20 thin films are often used as free layers or pinned layers to make MTJ with an MgO layer [27,28]. The most common purpose of selecting the CoFeW composition is to explore the comparison of the roles of B and W. The structure of CoFeW thin films was detected by grazing incidence X-ray diffraction (GIXRD) patterns obtained with CuKα1 (PAN analytical X'pert PRO MRD) and a low-angle diffraction incidence (about a two-degree angle). The in-plane low-frequency alternate-current magnetic susceptibility (χ ac ) and the hysteresis loop of Co 40 Fe 40 W 20 were studied by an χ ac analyzer (XacQuan) and an alternating-gradient magnetometer (AGM). To research the thermal tolerance, the saturation magnetization (M s ) of the post-annealing 400 • C condition was calculated from the slope of the magnetization (M) versus the CoFeW thickness. Moreover, in χ ac measurement, the χ ac analyzer was used to calibrate the standard sample under the action of an 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 an equivalent shape and size to eliminate demagnetization. The χ ac valve was an arbitrary unit (a.u.) because the AC result corresponded to the reference standard sample and may be a comparison value. The connection between magnetic susceptibility and frequency was measured by an χ ac analyzer. The best resonance frequency (f res ) was measured by an χ ac analyzer and represents the frequency of the maximum χ ac .  Figure 1a shows the patterns of thin films that were formed as-deposited, whereas those that were formed post-annealing at 250 and 350 • C are displayed in Figure 1b,c. The XRD is presented at diffracted angles (2θ) between 35 and 60 degrees. The CoFe (110) peaks are exhibited, which could be clearly detected at around 2θ = 44 • , indicating that the CoFeW thin films belonged to a crystallized state. It was generally observed in CoFeW thin films that the intensity of the CoFe(110) peaks increased with greater thickness. The specific oxide peaks appeared at 2θ = 38.3, 47.6, 54.5, and 56.3 • in all CoFeW samples. They corresponded to Fe 2 O 3 (320), WO 3 (002), Co 2 O 3 (422), and Co 2 O 3 (511). Although the chamber was pumped to 10 −7 Torr within the sputtering system, oxygen may still have been present. Both natural oxides on the Si(100) substrate and oxygen contamination on the sputtering targets contributed to the formation of oxidation peaks [29]. As CoFeW thicknesses increased, we found that the intensity of all oxide peaks decreased. It was speculated that the thickness of oxide was substantially the same in CoFeW thin films. Figure 1a-c demonstrates that the intensity of CoFe (110) peaks increased as the post-annealing temperature increased. According to [23], it was suggested that the crystallization of as-deposited CoFeW films on Si(100) substrate was better than ones on glass substrate. FOR PEER REVIEW 3 of 13 measurement, the χac analyzer was used to calibrate the standard sample under the action of an external magnetic field. Then, the sample was inserted into the χac analyzer. The driving frequency was between 10 and 25,000 Hz. χac was measured by magnetization. All test samples had an equivalent shape and size to eliminate demagnetization. The χac valve was an arbitrary unit (a.u.) because the AC result corresponded to the reference standard sample and may be a comparison value. The connection between magnetic susceptibility and frequency was measured by an χac analyzer. The best resonance frequency (fres) was measured by an χac analyzer and represents the frequency of the maximum χac. Figure 1 displays the XRD patterns of as-deposited and annealed Si(100)/Co40Fe40W20 thin films with a thickness of 10 to 50 nm. Figure 1a shows the patterns of thin films that were formed as-deposited, whereas those that were formed post-annealing at 250 and 350 °C are displayed in Figure 1b,c. The XRD is presented at diffracted angles (2θ) between 35 and 60 degrees. The CoFe (110) peaks are exhibited, which could be clearly detected at around 2θ = 44°, indicating that the CoFeW thin films belonged to a crystallized state. It was generally observed in CoFeW thin films that the intensity of the CoFe(110) peaks increased with greater thickness. The specific oxide peaks appeared at 2θ = 38.3, 47.6, 54.5, and 56.3° in all CoFeW samples. They corresponded to Fe2O3 (320), WO3 (002), Co2O3 (422), and Co2O3 (511). Although the chamber was pumped to 10 −7 Torr within the sputtering system, oxygen may still have been present. Both natural oxides on the Si(100) substrate and oxygen contamination on the sputtering targets contributed to the formation of oxidation peaks [29]. As CoFeW thicknesses increased, we found that the intensity of all oxide peaks decreased. It was speculated that the thickness of oxide was substantially the same in CoFeW thin films. Figure 1a-c demonstrates that the intensity of CoFe (110) peaks increased as the post-annealing temperature increased. According to [23], it was suggested that the crystallization of as-deposited CoFeW films on Si(100) substrate was better than ones on glass substrate.

Magnetic Analysis
Figure 2a-c displays the magnetic hysteresis loops of the CoFeW thin films under the three annealed conditions with thicknesses from 10 to 50 nm. The external magnetic field of 500 Oe in the plane was enough to observe the saturation magnetic spin state. The enlarged figure shows low coercivity (HC), which indicates that he CoFeW films were soft magnetic. The saturation magnetization (MS) of the CoFeW thin films under the three post-annealing conditions illustrates the magnetic properties of the CoFeW thin film, which were measured by the AGM, as shown in Figure 2. The MS of the CoFeW thin films is summarized in Table 2. CoFeW films showed an in-plane magnetization in this study because the CoFeW film was too thick and was deposited on the Si substrate, which was due to the perpendicular magnetic anisotropy properties that originated from the Fe-O bond and the in-plane demagnetization field, which was too large owing to the thick CoFeW [30,31].   Table 2. CoFeW films showed an in-plane magnetization in this study because the CoFeW film was too thick and was deposited on the Si substrate, which was due to the perpendicular magnetic anisotropy properties that originated from the Fe-O bond and the in-plane demagnetization field, which was too large owing to the thick CoFeW [30,31].

Thickness (nm) As-Deposited (emu/cm 3 ) Post-Annealing at 250 °C (emu/cm 3 )
Post-Annealing at 350 °C (emu/cm 3 )  10  546  714  568  20  588  729  604  30  647  779  691  40  691  807  726  50 754 842 818 Figure 3 shows the saturation magnetization (MS) of the CoFeW film with the asdeposited state and two post-annealing conditions. The results display that the MS increased as a function of the thicknesses and indicate the thickness effect of MS in CoFeW thin films. The magnetic dead layer (MDL) thickness of as-deposited and post-annealing conditions was obtained from thickness fitting the intercept in the magnetization versus thickness plot, which is shown in Figure 4a-d. From the result, the MDL thickness of postannealing CoFeW thin films was thicker than the as-deposited films owing to the annealing effect. The MDL thicknesses, obtained from Figure 4, were 3.09, 4.08, 6.47, and 6.91 nm at the as-deposited condition and the post-annealing conditions at 250, 350, and 400 °C, respectively. The natural oxidation layer of the Si surface was not removed before depositing the CoFeW film. However, the thickness of the native oxidation layer was 2 nm. The magnetic dead layer was detected in the interface. It can be reasonably concluded that there was a magnetic dead layer at the bottom Si(100) surface [32]. The influence of the natural oxidation layer on the magnetic properties was large enough because the magnetic dead layer thickness was comparable to the natural oxidation layer on the Si substrate as mentioned above. It can be reasonably concluded that antiferromagnetic oxidation was formed in the CoFeW film. To obtain the Ms of CoFeW, the MDL must be considered because the MDL thickness is not negligible compared to the CoFeW thickness. In this case, the Ms should be calculated from the slope of the M versus the CoFeW thickness. The magnetic dead layer (MDL) thickness of as-deposited and post-annealing conditions was obtained from thickness fitting the intercept in the magnetization versus thickness plot, which is shown in Figure 4a-d. From the result, the MDL thickness of post-annealing CoFeW thin films was thicker than the as-deposited films owing to the annealing effect. The MDL thicknesses, obtained from Figure 4, were 3.09, 4.08, 6.47, and 6.91 nm at the as-deposited condition and the post-annealing conditions at 250, 350, and 400 • C, respectively. The natural oxidation layer of the Si surface was not removed before depositing the CoFeW film. However, the thickness of the native oxidation layer was 2 nm. The magnetic dead layer was detected in the interface. It can be reasonably concluded that there was a magnetic dead layer at the bottom Si(100) surface [32]. The influence of the natural oxidation layer on the magnetic properties was large enough because the magnetic dead layer thickness was comparable to the natural oxidation layer on the Si substrate as mentioned above. It can be reasonably concluded that antiferromagnetic oxidation was formed in the CoFeW film. To obtain the Ms of CoFeW, the MDL must be considered because the MDL thickness is not negligible compared to the CoFeW thickness. In this case, the Ms should be calculated from the slope of the M versus the CoFeW thickness. The Ms calculated from this manner was 648, 876, 874, and 801 emu/cm 3 at the as-deposited condition and the post-annealing conditions at 250, 350, and 400 • C, respectively. These results indicate that the Ms value was almost the same at the annealed conditions at 250 and 350 • C. Therefore, the thermal tolerance of CoFeW was not 250 • C but more than or equal to 350 • C. Furthermore, it suggested that the M S of the CoFeW thin films increased when raising the post-annealing temperature. There was an important correlation between all conditions of the M S , the temperature, and the thickness. These results indicate that the highest M S after post-annealing was at 350 • C, which was the best heat-resistant temperature in this research. The oxidation of all CoFeW thin films on Si(100) substrate was better than those on glass substrate [24]. The results showed that the Ms of CoFeW films decreases obviously under various conditions. This means that the oxide was unfavorable for the M S of the CoFeW thin films. According to the fitting result of Figure 4, the M S value of Co 40 Fe 40 W 20 thin films was increased to 350 • C, then decreased to 400 • C, which shows that the thermal stability of Co 40 Fe 40 W 20 thin films is better than that found in other research [14]. The thermal tolerance of Co 40 Fe 40 W 20 was less than 400 • C. The results show that Co 40 Fe 40 W 20 film is suitable for electrode and strain gauge applications.
The Ms calculated from this manner was 648, 876, 874, and 801 emu/cm 3 at the as-deposited condition and the post-annealing conditions at 250, 350, and 400 °C, respectively. These results indicate that the Ms value was almost the same at the annealed conditions at 250 and 350 °C. Therefore, the thermal tolerance of CoFeW was not 250 °C but more than or equal to 350 °C. Furthermore, it suggested that the MS of the CoFeW thin films increased when raising the post-annealing temperature. There was an important correlation between all conditions of the MS, the temperature, and the thickness. These results indicate that the highest MS after post-annealing was at 350 °C, which was the best heatresistant temperature in this research. The oxidation of all CoFeW thin films on Si(100) substrate was better than those on glass substrate [24]. The results showed that the Ms of CoFeW films decreases obviously under various conditions. This means that the oxide was unfavorable for the MS of the CoFeW thin films. According to the fitting result of Figure 4, the MS value of Co40Fe40W20 thin films was increased to 350 °C, then decreased to 400 °C, which shows that the thermal stability of Co40Fe40W20 thin films is better than that found in other research [14]. The thermal tolerance of Co40Fe40W20 was less than 400 °C. The results show that Co40Fe40W20 film is suitable for electrode and strain gauge applications.      The corresponding maximum χ ac values of different CoFeW thicknesses under three conditions are shown in Figure 6. The results reveal that the χ ac values of the CoFeW films decreased with the increase in thickness. The χ ac values of all CoFeW films dropped sharply at high frequency, as shown in Figure 6. The as-deposited CoFeW thin films showed that the maximum χ ac value was 0.095 when the thickness was 50 nm. The post-annealing condition at 250 • C for the CoFeW thin films showed that the maximum χ ac value was 0.583 when the thickness was 50 nm. The post-annealing condition of 350 • C for the CoFeW thin films showed that the maximum χ ac value was 0.725 when the thickness was 50 nm. The results clearly reveal the thickness effect of χ ac in all CoFeW samples. As the thickness of the CoFeW films increased, the maximum χ ac value was augmented because of the thickness effect. The maximum χ ac value of the CoFeW thin films after annealing was larger than the as-deposited samples. When the magneto-crystalline anisotropy of the CoFe (110) crystallization effect was maximized, the χ ac value of CoFeW was maximized [33,34]. The corresponding maximum χac values of different CoFeW thicknesses under three conditions are shown in Figure 6. The results reveal that the χac values of the CoFeW films for the CoFeW thin films showed that the maximum χac value was 0.725 when the thickness was 50 nm. The results clearly reveal the thickness effect of χac in all CoFeW samples. As the thickness of the CoFeW films increased, the maximum χac value was augmented because of the thickness effect. The maximum χac value of the CoFeW thin films after annealing was larger than the as-deposited samples. When the magneto-crystalline anisotropy of the CoFe (110) crystallization effect was maximized, the χac value of CoFeW was maximized [33,34]. Figure 6. Maximum alternate-current magnetic susceptibility for the CoFeW thin films. Table 3 represents the optimal resonance frequency (ƒres) of the CoFeW thin films. When the maximum χac value was presented with the ƒres value, the spin sensitivity was exceptional. Furthermore, when the ƒres value was below 1000 Hz for the thin films, it created CoFeW magnetic thin films for applications in the field of soft magnetism devices. Table 3 presents that the ƒres values of all CoFeW thicknesses, which were from 50 Hz to 1000 Hz. This means that the maximum χac had the strongest spin sensitivity at this frequency [35,36].  10  500  50  100  20  1000  250  50  30  250  50  250  40  1000  50  50  50 100 100 100 Figure 6. Maximum alternate-current magnetic susceptibility for the CoFeW thin films. Table 3 represents the optimal resonance frequency (ƒ res ) of the CoFeW thin films. When the maximum χ ac value was presented with the ƒ res value, the spin sensitivity was exceptional. Furthermore, when the ƒ res value was below 1000 Hz for the thin films, it created CoFeW magnetic thin films for applications in the field of soft magnetism devices. Table 3 presents that the ƒ res values of all CoFeW thicknesses, which were from 50 Hz to 1000 Hz. This means that the maximum χ ac had the strongest spin sensitivity at this frequency [35,36].  10  500  50  100  20  1000  250  50  30  250  50  250  40  1000  50  50  50 100 100 100

Conclusions
The XRD patterns revealed that the CoFeW thin films are composed of a nanocrystalline body-centered cubic (BCC) CoFe phase and that they contain several Fe-, W-, and Co-based oxides. The intensity of CoFe (110) peaks generally increased with increasing film thicknesses, indicating a development of the crystallographic texture. Moreover, increasing the film thickness was accompanied by a reduced intensity in the diffraction peaks originating from oxides. An oxidation caused an increase in film thickness after annealing. The CoFeW films exhibited soft magnetism owing to the low H c and in-plane magnetization; thicker CoFeW thicknesses showed a large in-plane demagnetization field and a weak Fe-O bonding effect. The MDL thickness of the post-annealing CoFeW thin films was thicker than the as-deposited films because of oxidation and the annealing effect. The Ms should be fitted from the slope of the M versus the CoFeW thickness and obtained 648, 876, 874, and 801 emu/cm 3 at the as-deposited condition and the post-annealing conditions at 250, 350, and 400 • C, respectively. Alloying additions of W improved the thermal stability of the CoFe films. The maximum Ms and the maximum χ ac value of 0.725 were achieved for CoFeW films with a thickness of 50 nm after annealing at 350 • C. The f res values of all films being less than 1000 Hz confirmed that the CoFeW magnetic films are suitable for magnetic component applications.