coatings Effect of Annealing on the Characteristics of CoFeBY Thin Films

: In this study, the addition of Y to CoFeB alloy can reﬁne the grain size to study the magnetic, adhesion and optical properties of as-deposited and annealed CoFeB alloy. XRD analysis shows that CoFeB(110) has a BCC CoFeB (110) nanocrystalline structure with a thickness of 10–50 nm under four heat-treatment conditions, and a CoFeB(110) peak at 44 ◦ (2 θ ). The measurements of saturation magnetization (M S ) and low frequency alternate-current magnetic susceptibility ( χ ac ) revealed a thickness effect owed to exchange coupling. The maximum M S of the 300 ◦ C annealed CoFeBY ﬁlm with a thickness of 50 nm was 925 emu/cm 3 (9.25 × 10 5 A/m). The maximum χ ac value of the 300 ◦ C annealed CoFeBY nanoﬁlms with a thickness of 50 nm was 0.165 at 50 Hz. After annealing at 300 ◦ C, CoFeBY nanoﬁlms exhibited the highest surface energy of 31.07 mJ/mm 2 , where the thickness of the nanoﬁlms was 40 nm. Compared with the as-deposited CoFeBY nanoﬁlms, due to the smaller average grain size after annealing, the transmittance of the annealed nanoﬁlms increased. Importantly, when a CoFeB seed or buffer layer was replaced by a CoFeBY nanoﬁlm, the thermal stability of the CoFeBY nanoﬁlms was improved, promoting themselves on the practical MTJ applications.


Introduction
Since the discovery of Co 50 Fe 50 in CoFe systems by Ellis in 1927 and Elmen in 1929, it has shown good soft magnetic properties [1]. These characteristics include high saturation magnetization (M S ) and high Curie temperature (T C ). Due to its excellent performance, Co-Fe system has attracted extensive attention. According to the above characteristics, Co-Fe alloys have the advantages of low coercivity (Hc) and good mechanical properties. However, with increasing annealing temperature, CoFe alloy has the disadvantage of serious degradation of magnetic anisotropy, which makes the effective use of these magnetic devices at high temperature difficult. Adding a third element to the CoFe matrix can help to overcome this problem [2][3][4][5][6][7]. Therefore, to find a kind of component that can improve the magnetothermal stability has become a primary research goal. For example, CoFeB thin films are widely used as free or pinned layers of spin valued magnetic tunnel junctions (MTJ) due to their high spin polarization and high tunneling magnetoresistance (TMR). They can be used in magnetoresistance random access memory (MRAM) and recording head devices [8][9][10][11][12][13]. Recently, more and more scholars have become interested in high-abundance rare earth magnetic materials. Due to the unique properties of rare earth  Figure 1a shows the XRD patterns for the nanofilms at RT, while those of samples annealed at 100, 200 and 300 • C are shown in Figure 1b,c, respectively. The results of XRD are shown at different diffracted angles (2θ) between 35 and 60 degrees. The (110) body-centered cubic (BCC) structure for the CoFeB nanofilms was revealed to be around 2θ = 44 • , indicating that the CoFeBY nanofilms belonged to a crystallized state [29]. Moreover, the YFeO 3 oxide peaks found at 2θ = 37.3 • may be attributed to the Y doping [30].  Figure 2a shows the full width at half maximum (FWHM, B) of the CoFeB (110) peak obtained in four conditions. The average crystallite sizes could be calculated according to the FWHM parameters obtained from the XRD patterns. Using the Scherrer formula (1), the average grain size of CoFeBY films under the studied conditions was calculated with CoFeB (110) peak.
Scherrer formula is [31]: where k (0.89) is the Scherrer's constant, λ is the X-ray wavelength of Cu Kα1 line, B is the FWHM of the CoFeB (110) diffraction peaks, and θ is the half-angle of the diffraction peak. Figure 2b shows the average grain sizes, which were estimated from the half maximum (FWHM, B) of the CoFeB (110) peak under four conditions. The results demonstrated that the grain sizes of the films were related to the thickness, and the crystallinity of the films increased with the increase of thickness. The grain sizes of CoFeBY after annealing were smaller than those at RT, because the grain can be refined by adding an appropriate amount of Y [22].  Figure 2a shows the full width at half maximum (FWHM, B) of the CoFeB (110) peak obtained in four conditions. The average crystallite sizes could be calculated according to the FWHM parameters obtained from the XRD patterns. Using the Scherrer formula (1), the average grain size of CoFeBY films under the studied conditions was calculated with CoFeB (110) peak.
Scherrer formula is [31]: where k (0.89) is the Scherrer's constant, λ is the X-ray wavelength of Cu Kα1 line, B is the FWHM of the CoFeB (110) diffraction peaks, and θ is the half-angle of the diffraction peak. Figure 2b shows the average grain sizes, which were estimated from the half maximum (FWHM, B) of the CoFeB (110) peak under four conditions. The results demonstrated that the grain sizes of the films were related to the thickness, and the crystallinity of the films increased with the increase of thickness. The grain sizes of CoFeBY after annealing were smaller than those at RT, because the grain can be refined by adding an appropriate amount of Y [22].

Magnetic Properties
Figure 3a-d plot the magnetic hysteresis loops of CoFeBY films under four conditions, in which the thicknesses were measured between 10 and 50 nm by AGM measurement, and the magnetic field is parallel to the film surface. The in-plane external field (Hext) field of 500 Oe (3.98 × 10 4 A/m) was sufficient to observe the saturated magnetic spin status. The enlarged image illustrates low coercivity (HC), which suggests that the CoFeBY films were soft magnetic. These data are summarized in Table 1, which shows the magnetic properties of the nanofilms that were obtained by AGM. Figure 4 shows the corresponding saturation magnetization (MS) of the CoFeBY film under four conditions. The results revealed that the observed MS increased with the increase of thickness, indicating the thickness effect of MS on CoFeBY films. The MS value of CoFeBY films increased with the increase of annealing temperature. However, Ms was the highest after annealing at 300 °C, which was the best heat-resistant temperature in this study. The results show that the size of CoFeBY nanofilms is affected by the grain refinement, which improves the ferromagnetic spin exchange coupling and thus increases the Ms [32]. In addition, iron oxide was also detected in the XRD results, which may be the characteristic of antiferromagnetism, so the magnetization of iron oxide was deduced. In this study, the addition of Y and annealed treatment can increase the magnetization.
The corresponding HC is displayed in Table 1. HC increased from 1.5 Oe to 15.1 Oe when tf ranged from 10 to 50 nm at room temperature; HC increased from 3.5 Oe to 18.8

Magnetic Properties
Figure 3a-d plot the magnetic hysteresis loops of CoFeBY films under four conditions, in which the thicknesses were measured between 10 and 50 nm by AGM measurement, and the magnetic field is parallel to the film surface. The in-plane external field (H ext ) field of 500 Oe (3.98 × 10 4 A/m) was sufficient to observe the saturated magnetic spin status. The enlarged image illustrates low coercivity (H C ), which suggests that the CoFeBY films were soft magnetic. These data are summarized in Table 1, which shows the magnetic properties of the nanofilms that were obtained by AGM. Figure 4 shows the corresponding saturation magnetization (M S ) of the CoFeBY film under four conditions. The results revealed that the observed M S increased with the increase of thickness, indicating the thickness effect of M S on CoFeBY films. The M S value of CoFeBY films increased with the increase of annealing temperature. However, Ms was the highest after annealing at 300 • C, which was the best heat-resistant temperature in this study. The results show that the size of CoFeBY nanofilms is affected by the grain refinement, which improves the ferromagnetic spin exchange coupling and thus increases the Ms [32]. In addition, iron oxide was also detected in the XRD results, which may be the characteristic of antiferromagnetism, so the magnetization of iron oxide was deduced. In this study, the addition of Y and annealed treatment can increase the magnetization. Adding Y and heat treatment can increase the amount of magnetization. creased from 12.5 Oe to 23.1 Oe when tf ranged from 10 to 50 nm following annealing at 300 °C. In this study, the addition of Y led to the increase of HC, because the addition of Y refines the grain size, leading to an increase of HC. However, if the HC of CoFeBY films is between 10 and 20 Oe, and if they have high MS, the CoFeBY films will have soft magnetism, thus making them suitable for MRAM and recording head applications. The HC of nanofilm is usually enhanced by decreasing the grain size, which is an effect related to the transition from magnetic multi-domains to single domains [33,34].        The corresponding H C is displayed in Table 1. H C increased from 1.5 Oe to 15.1 Oe when t f ranged from 10 to 50 nm at room temperature; H C increased from 3.5 Oe to 18.8 Oe when t f ranged from 10 to 50 nm following annealing at 100 • C; H C increased from 4.5 Oe to 24.8 Oe when t f ranged from 10 to 50 nm following annealing at 200 • C; H C increased from 12.5 Oe to 23.1 Oe when t f ranged from 10 to 50 nm following annealing at 300 • C. In this study, the addition of Y led to the increase of H C , because the addition of Y refines the grain size, leading to an increase of H C . However, if the H C of CoFeBY films is between 10 and 20 Oe, and if they have high M S , the CoFeBY films will have soft magnetism, thus making them suitable for MRAM and recording head applications. The H C of nanofilm is usually enhanced by decreasing the grain size, which is an effect related to the transition from magnetic multi-domains to single domains [33,34]. Figure 5a-d show the results of CoFeBY films with thicknesses from 10 to 50 nm at four conditions (RT, 100, 200 and 300 • C) under which the low-frequency alternating-current magnetic susceptibility (χ ac ) was studied. The low frequencies were in the range of 50 to 25,000 Hz. The results showed that the thickness of CoFeBY was between 10 and 50 nm, and the χ ac values of t f decreased with the increase of frequency (Hz). The maximum χ ac corresponding to various CoFeBY thicknesses under four conditions is shown in Figure 6. It could be found that the maximum χ ac value was 0.053 when t f was 50 nm at RT; the maximum χ ac value was 0.074 when t f was 50 nm at 100 • C; the maximum χ ac value was 0.152 when t f was 50 nm at 200 • C; and the maximum χ ac value was 0.165 when t f was 50 nm at 300 • C. Obviously, these results revealed the thickness effect of χ ac in CoFeBY films. With the increase of t f , the increase of χ ac was due to the thickness effect. The maximum χ ac value of annealed CoFeBY films was larger than that at RT. This is because CoFeBY films are affected by grain refinement, which improves the ferromagnetic spin exchange coupling and increases the χ ac value. Table 2 shows the optimal resonance frequency (ƒ res ) of CoFeBY. The maximum χ ac indicated that the spin sensitivity was the highest at the optimal resonant frequency. The peak of χ ac reflected the spin exchange-coupling interaction and dipole moment of the domain at frequency [35]. Additionally, the ƒ res value of nanofilm is below 500 Hz, which allows CoFeBY nanofilm to be applied in soft magnetic devices. It was found that the ƒ res values of all CoFeBY thicknesses were in the range from 50 to 500 Hz, indicating the maximum χ ac had the strongest spin sensitivity at this frequency [36].        Figure 6. Maximum alternate-current magnetic susceptibility for the CoFeBY films.

Surface Morphology
To study the surface morphology and magnetic results, SEM images of CoFeBY at 40 nm were observed under four conditions, as shown in Figure 7. Figure 7a shows a looser surface morphology in the as-deposited state. The surface morphology after annealing at 100 • C is shown in Figure 7b, which shows the loose surface phenomenon. In Figure 7c, the surface morphology after annealing at 200 • C shows a dense distribution. The surface morphology of annealed at 300 • C was more densely distributed, as shown in Figure 7d. In addition, the surface morphology w very close to magnetism. In the cases of Figure 7c,d, some defects can be found, which lead to the enhancement of domain wall pinning effect. This can induce high coercivity and improve the spin coupling strength [37][38][39][40]. The susceptibility is also related to magnetic noise and exchange coupling. High susceptibility can enhance the strong dipole interaction effect [41].

Surface Morphology
To study the surface morphology and magnetic results, SEM images of CoFeBY at 40 nm were observed under four conditions, as shown in Figure 7. Figure 7a shows a looser surface morphology in the as-deposited state. The surface morphology after annealing at 100 °C is shown in Figure 7b, which shows the loose surface phenomenon. In Figure 7c, the surface morphology after annealing at 200 °C shows a dense distribution. The surface morphology of annealed at 300 °C was more densely distributed, as shown in Figure 7d. In addition, the surface morphology w very close to magnetism. In the cases of Figure 7c,d, some defects can be found, which lead to the enhancement of domain  Table 3 displays the contact angle (θ) of the CoFeBY at RT. The contact angle of the films was investigated using DI water and glycerol. Table 3 shows the result of the contact angles (θ) of the CoFeBY using DI water, which were 81.7°, 81.1°, 80.9°, 81.3°, and 81.0°, as well as the contact angles (θ) with glycerol, which were 78.4°, 78.2°, 78.8°, 80.8°, and 79.5°, respectively.  Table 3 displays the contact angle (θ) of the CoFeBY at RT. The contact angle of the films was investigated using DI water and glycerol. Table 3 shows the result of the contact angles (θ) of the CoFeBY using DI water, which were 81.     Table 5 displays the contact angle (θ) of the CoFeBY at 200 • C. The contact angle of the films was investigated using DI water and glycerol. Table 5 shows the result of the contact angles (θ) of the CoFeBY using DI water, which were 77.   Table 6 shows the annealed 300 • C result of the contact angles (θ) of the CoFeBY using DI water, which were 78.1 • , 78.0 • , 76.8 • , 72.9 • , and 71.7 • , as well as the contact angles (θ) with glycerol, which were 75.9 • , 77.0 • , 75.8 • , 72.7 • , and 71.6 • , respectively. From the above results, the contact angles of all CoFeBY films under RT, 100, 200 and 300 • C were less than 90 • , indicating that the films were hydrophilic. When films are hydrophilic, they will have good wetting effect. In addition, the contact angle was closely related to the surface energy. When the surface free energy is high, the liquid absorption is large and the liquid absorption area is large, which leads to the decrease of the contact angle [42]. The surface energy was calculated according to the contact angle and Young's equation [26][27][28].  Figure 8 shows the surface energy of the CoFeBY films. As a consequence, it indicates that the surface energy of as-deposited CoFeBY films was 24.4 mJ/mm 2 at 40 nm, which was the highest value. When the annealing temperature was 100 and 200 • C, the best surface energy of 50 nm was 25.59 mJ/mm 2 and 26.93 mJ/mm 2 , respectively. After annealing at 300 • C, the best surface energy was 31.07 mJ/mm 2 at 40 nm, as shown in Figure 8. This result is consistent with SEM. When annealed at 300 • C for 40 nm, some defects can be observed, which are caused by high surface energy. After heat treatment, the surface energy of the film tended to become higher. With the increase of oxide content, the contact angle decreases. Low contact angle corresponded to higher surface energy. When the film has high surface energy, the adhesion is the strongest. These results show that it is easier to combine with free layer and pinning layer of layered magnetic tunnel junctions.  Figure 8 shows the surface energy of the CoFeBY films. As a consequence, it indicates that the surface energy of as-deposited CoFeBY films was 24.4 mJ/mm 2 at 40 nm, which was the highest value. When the annealing temperature was 100 and 200 °C, the best surface energy of 50 nm was 25.59 mJ/mm 2 and 26.93 mJ/mm 2 , respectively. After annealing at 300 °C, the best surface energy was 31.07 mJ/mm 2 at 40 nm, as shown in Figure 8. This result is consistent with SEM. When annealed at 300 °C for 40 nm, some defects can be observed, which are caused by high surface energy. After heat treatment, the surface energy of the film tended to become higher. With the increase of oxide content, the contact angle decreases. Low contact angle corresponded to higher surface energy. When the film has high surface energy, the adhesion is the strongest. These results show that it is easier to combine with free layer and pinning layer of layered magnetic tunnel junctions.  Figure 9 shows the optical transmittance spectra of CoFeBY at visible wavelengths of 500 to 800 nm. In Figure 9a (CoFeBY at RT), the transmittance (%) decreased from 37 to 10.5%, as tf changed from 10 to 50 nm. In Figure 9b (CoFeBY after annealing at 100 °C), the transmittance (%) decreased from 39.7 to 17%, when tf changed from 10 to 50 nm. In Figure 9c (CoFeBY after annealing at 200 °C), the transmittance (%) decreased from 40 to 16.7%, as tf changed from 10 to 50 nm. In Figure 9d (CoFeBY after annealing at 300 °C), the transmittance (%) decreased from 39.9 to 16%, when tf changed from 10 to 50 nm. The transmittance of annealed samples was higher than that of RT samples. The addition of Y and annealing could cause grain refinement [22]. With the increase of annealing temperature, only a slight change in the transmittance could be observed, because the crystal grain size did not change much, and the trend was not obvious. This result is in good agreement with the average grain size measured by XRD. The result indicated that thinner CoFeBY films have a higher transmission rate, because thicker films impeded the  Figure 9 shows the optical transmittance spectra of CoFeBY at visible wavelengths of 500 to 800 nm. In Figure 9a (CoFeBY at RT), the transmittance (%) decreased from 37 to 10.5%, as t f changed from 10 to 50 nm. In Figure 9b (CoFeBY after annealing at 100 • C), the transmittance (%) decreased from 39.7 to 17%, when t f changed from 10 to 50 nm. In Figure 9c (CoFeBY after annealing at 200 • C), the transmittance (%) decreased from 40 to 16.7%, as t f changed from 10 to 50 nm. In Figure 9d (CoFeBY after annealing at 300 • C), the transmittance (%) decreased from 39.9 to 16%, when t f changed from 10 to 50 nm. The transmittance of annealed samples was higher than that of RT samples. The addition of Y and annealing could cause grain refinement [22]. With the increase of annealing temperature, only a slight change in the transmittance could be observed, because the crystal grain size did not change much, and the trend was not obvious. This result is in good agreement with the average grain size measured by XRD. The result indicated that thinner CoFeBY films have a higher transmission rate, because thicker films impeded the signal of the incident light and result in decreased transmittance. It is confirmed that the transmission of photons through the film is reduced due to the thickness effect and interface effect of the film [43,44]. signal of the incident light and result in decreased transmittance. It is confirmed that the transmission of photons through the film is reduced due to the thickness effect and interface effect of the film [43,44].

Conclusions
Because of the addition of a suitable amount of Y in the alloys, the crystalline grain size was reduced after the post-annealing treatment. When the thickness of CoFeBY nanofilms increases from 10 to 50 nm, Ms tends to saturate, indicating the thickness effect of MS on CoFeBY nanofilm. The MS was improved when the temperature in the post-annealing process increased. The CoFeBY nanofilms were affected by size, and the refinement of grains facilitated the ferromagnetic spin exchange coupling, thus increasing the saturation magnetization. For the 300 °C annealed CoFeBY nanofilms, the maximum χac value was 0.165 at a 50 Hz fres, where the thickness of the films is 50 nm. The fres values of the CoFeBY nanofilms in all thickness conditions were less than 500 Hz, signifying that it could fulfill applications as the magnetic component in low-frequency sensors. The 50-nm-thick film following annealing at 300 °C had the highest surface energy in this work. It is worth mentioning that the strongest adhesion corresponded to the highest surface energy. At the same time, the samples undergoing annealing showed that the transmittance increased in comparison with the as-deposited films, because the average grain size became smaller after annealing, causing the transmittance to increase. The results show that the thermal stability of CoFeB nanofilms can be improved by adding appropriate amount of Y into CoFeB alloy, which indicates that the application of rare earth materials in soft magnetic materials needs further development.

Conclusions
Because of the addition of a suitable amount of Y in the alloys, the crystalline grain size was reduced after the post-annealing treatment. When the thickness of CoFeBY nanofilms increases from 10 to 50 nm, Ms tends to saturate, indicating the thickness effect of M S on CoFeBY nanofilm. The M S was improved when the temperature in the post-annealing process increased. The CoFeBY nanofilms were affected by size, and the refinement of grains facilitated the ferromagnetic spin exchange coupling, thus increasing the saturation magnetization. For the 300 • C annealed CoFeBY nanofilms, the maximum χ ac value was 0.165 at a 50 Hz f res , where the thickness of the films is 50 nm. The f res values of the CoFeBY nanofilms in all thickness conditions were less than 500 Hz, signifying that it could fulfill applications as the magnetic component in low-frequency sensors. The 50-nm-thick film following annealing at 300 • C had the highest surface energy in this work. It is worth mentioning that the strongest adhesion corresponded to the highest surface energy. At the same time, the samples undergoing annealing showed that the transmittance increased in comparison with the as-deposited films, because the average grain size became smaller after annealing, causing the transmittance to increase. The results show that the thermal stability of CoFeB nanofilms can be improved by adding appropriate amount of Y into CoFeB alloy, which indicates that the application of rare earth materials in soft magnetic materials needs further development.