Figure 1(a) displays the film deposited at 5 g/L of NaH₂PO₂ and annealed from 100˚C - 400˚C. The deposited film shows an amorphous structure due to the incorporation of phosphorus. Electrodeposited FeMn alloy showed an amorphous structure [22]. The incorporation of P enhances the amorphous structure [23]. The present results consent with Bassey’s results [20]. After heating the sample from 100˚C - 400˚C no apparent change was observed in the structure. However, at 300˚C - 400˚C, the sample shows a crystalline α-Fe based peak with structure orientation (110). It means that from 300˚C, the film has been crystallized due to the low diffusion of P [24]. The reduction of P diffusion promotes grain growth. Thus the structure turned from amorphous to nanocrystalline.

Figure 1(b) shows the film deposited at 30 g/L of NaH₂PO₂ and annealed at different temperatures. The deposited and annealing film shows a similar structure (amorphous) for all annealed temperatures. It could be concluded that the enhanced diffusion of phosphorus prevents grain growth during the annealing. Thus the grain growth was controlled by the diffusion of phosphorus in the films [24].

Table 2 displays the EDX results of electrodeposited films from the different

NaH₂PO₂ concentrations. The P content increased as the NaH₂PO₂ concentration increased. The Mn and Fe content decreased with the NaH₂PO₂ increases. The EDX mapping and film thickness are mentioned in the supplement file.

Hysteresis loops of electrodeposited FeMnP alloy films were measured in parallel and perpendicular to the direction of applied static magnetic films during deposition. They showed that all deposited films are isotropy (see a supplementary document). We considered parallel directions to depict the influence of P on static magnetic properties of electrodeposited FeMnP alloy films.

Figure 2(a) displays the hysteresis loop of electrodeposited FeMn films at different NaH₂PO₂ concentrations. It revealed that the saturation magnetization decreased from 586 emu/cc to 346 emu/cc with the sodium hypophosphite. Magnetization is an intrinsic property that depends on the composition and thickness of the films [25]. The distribution of non-magnetic and magnetic elements plays a key role in controlling saturation magnetization, and the high net magnetic moment is obtained for the magnetic-magnetic interaction. In contrast, non-magnetic-non-magnetic interactions produce low magnetic moments [26] [27]. This work revealed that saturation magnetization is strongly dependent on the magnetic moment. The magnetic moment is proportional to the iron content in the FeMnP alloy films. P content in the films increased with the NaH₂PO₂ increases. It reduces the iron content and increases the ratio of manganese and phosphorus in the FeMnP films. The magnetic moment decreased as the P increased due to the reduction of d-orbit electrons. Thus, the magnetic moment of Fe-based amorphous decreased with an increase in metalloid content [28]. That could be explained by atomic interactions and s-d hybridization between Fe and P elements. The incorporation of P decreases the d-orbit electrons,

leading to decreases in saturation magnetization [29] [30]. Similar results were shown in FeBSiP amorphous alloy [31].

Figure 2(b) shows linear decreases of the coercivity field from 52 Oe to 18 Oe with the NaH₂PO₂ increases. The linear decreases of Hc were also shown in the FeCOP films [32]. The coercivity change could be due to internal stress, film thickness, substrate roughness, composition, and plating pretreatment [33]. The rise of P content increases the grain refining ability (reducing grain size). Therefore the amorphous forming ability increased [23]. In the present work, the film thickness is (about 1.5 μm), substrate roughness and plating pretreatment are similar. Subsequently, the trend of coercivity is related to internal stress and film composition.

Figure 3 shows the M-H loop of the annealed film (5 g/L) at different temperatures. The influence of magnetic annealing on hysteresis is minimal. The film is still isotropy. In magnetic annealing, the uniaxial anisotropy could be due to the directional arrangements of vacancy or impurities, the presence of a directional strain, and the directional of atomic pairs [34]. However, the results show consistency with the kinetic reorientation theory [35], which predicts the uniaxial anisotropy (Ku) is zero for mono magnetic atom films, as shown in Fe₇₅P₁₅C₁₀

[36]. To induce uniaxial anisotropy by applying an external magnetic field during deposition and magnetic annealing is not suitable for inducing uniaxial anisotropy in FeMnP amorphous. However, other magnetic properties were improved. The improvements in magnetic properties are due to the relaxation of structure below the crystallization temperature and the change of internal stress [37]. The coercivity field decreased at 200˚C and then increased in parallel and perpendicular directions. The change in coercivity depends on many factors such as grain size, internal stress, and crystal structure. The crystallization temperature of FeMnP amorphous alloy was related to the P composition in the films. The higher the P content, the more diffusion rate occurs and inhibits grain growth.

Anisotropy constant is essential to discuss the dynamics of magnetic materials; however, our samples are isotropy as the VSM results shown. In this part, we will discuss the influence of phosphorus and magnetic annealing on the natural resonance frequency and resonance linewidth of the magnetic spectra.

Figure 4 shows the magnetic spectra of electrodeposited FeMnP films from various concentrations of NaH₂PO₂. The complex permeability ( μ r = μ ′ r − j μ ″ r ), where μ ′ stand for the real part and μ ″ imaginary part. The ferromagnetic resonance peak becomes broad as the NaH₂PO₂ concentration increases. The broadness of the resonance peak means that the damping factor-α increased with the P content increases in the films due to reducing the exchange interaction force between ferromagnetic metals [38]. The exchange coupling interactions are affected by doping elements and cause fluctuations such as local magnetic anisotropies, internal field, and increased damping [39].

According to the Landau-Lifshitz-Gilbert Equation (LLG)

d M d t = − γ ( M × H ) + α M M × d M d t (1)

Δ f r = α γ 2 π × ( 2 H k + 4 π M s ) (2)

where, Δ f r stand for full width half maximum, α damping constant. γ gyromagnetic ratio, H_k anisotropy constant, and M_s saturation magnetization. From Equation (2), the full width at the half maximum of the resonance curve increase with damping factor-α [40]. Subsequently, spin-orbital coupling causes a spin-mixing state (the spin states of the electrons are mixtures of pure “up” and “down”, relative to the magnetic field). Shaw and colleagues calculated the spin-mixing parameter by combining FMR With XCMD, and the results obtained reveal that any small mixing parameter influences ferromagnetic resonance (FMR) spectra [41]. The extrinsic magnetic damping is often affected by

doping or lattice effect, which changes the crystal structure [42]. From EDX results shown in Table 2, the films obtained at 30 g/L of NaH₂PO₂ has a high content of P. Thus, Figure 4(c) shows a high magnetic damping factor that could be related to the high composition of P in the films, the more P incorporated in the films, it could enhance the change of ferromagnetic to paramagnetic behavior [33].

Figure 4(d) shows the decreases of ferromagnetic resonance frequency with the NaH₂PO₂ concentration. It could be due to the reduction of saturation magnetization. According to the Kittel equation [43] [44]

f r = γ 2 π H k ( H k + 4 π M s ) (3)

where f_r, γ, M_s. and H_k stand for resonance frequency, gyromagnetic ratio, saturation magnetization, and Uniaxial anisotropy field, respectively. The resonance frequency is proportional to the saturation magnetization and magnetic anisotropy field H_k. Nevertheless, the H_k in the present work is negligible. Hence the natural resonance frequency depends on saturation magnetization.