A series of amorphous FeCoZr composition gradient monolayer films with varying Zr contents was prepared on the Si(100) substrate using RF magnetron sputtering. The effects of the Zr sputtering power PZr on the static and dynamic magnetic properties of FeCoZr films were systematically investigated. The results demonstrate that the introduction of the Zr element as a composition gradient into FeCo films not only improves the soft magnetic properties of the films but also enhances their in-plane uniaxial magnetic anisotropy. In particular, the doping of Zr elements leads to the destruction of FeCo lattice, inducing a transformation of the films from polycrystalline to amorphous state, resulting in a significant decrease in coercivity (Hc reduced by 82%) and surface roughness (Ra reduced by 78%). In addition, as PZr increases from 30 to 70 W, the anisotropy fields Hk of the films increase from 128 to 340 Oe, and the resonance frequency fr increases from 4.24 to 5.23 GHz. By fitting the permeability spectrum using the LLG equation, it is found that FeCoZr composition gradient films exhibit a lower damping coefficient α of around 0.011–0.014, indicating the reduction of energy loss during magnetization dynamics. These findings highlight the potential applications of FeCoZr composition gradient films in the field of high-frequency microwaves.

Soft magnetic film materials are widely utilized in various fields, including communication, information, military applications, and others, due to their excellent magnetic properties.1,2 To meet the development requirements of high-frequency and miniaturization of electronic components, it is particularly crucial to prepare films with large saturation magnetization 4πMs and appropriate in-plane uniaxial magnetic anisotropy field Hk.3–6 FeCo-based alloy films have been the subject of extensive research because of their high 4πMs and high resistivity ρ and have broad application prospects in high-frequency magnetic devices,7–9 such as magnetic recording write heads,10 inductors,11,12 transformers,13,14 sensors,15 and antennas.16 However, as-deposited FeCo films exhibit higher coercivity Hc and lower Hk, which is primarily due to the large saturation magnetostriction coefficient λs (4–6.5 × 10−5) and the magnetocrystalline anisotropy constant K1 (10 kJ/m3).17–20 These factors will largely restrict the application of film materials in high-frequency magnetic devices. Therefore, optimizing the deposition conditions, such as deposition power, deposition pressure, and film thickness, can change the microstructures of the films and enhance their soft magnetic properties.21–23 However, there exists an upper limit to the range of optimization conditions, making it challenging to obtain films with excellent soft magnetic properties and high-frequency performance. To reduce Hc of the films, one effective method is to add a third element24–27 (B, N, Hf, Al2O3) to FeCo films, as doping with appropriate elements can reduce the grain size and thus reduce the effective magnetic anisotropy of the films. Meanwhile, the nanocrystalline structure (even the disorder state of amorphous structure) and the precipitation of doping elements can enhance the scattering in the process of electron transport, resulting in higher resistivity of the films.28 This can reduce the energy loss of film materials in high-frequency applications. In addition, much literature has confirmed that oblique sputtering,27,29 magnetic field induction,30 stress induction,17 and magnetic field annealing31 are simple and effective methods to improve the high-frequency characteristics of the films.

In this paper, FeCoZr films were prepared by composition gradient sputtering technique. Zr element was introduced into FeCo films in the form of a composition gradient, and the Zr content was regulated by the Zr deposition power PZr. The effects of Zr doping on the microstructure and magnetic properties of FeCoZr films have been systematically studied. The key role of the Zr composition gradient in improving the high-frequency characteristics of FeCoZr films has been revealed, which can achieve controllable dynamic magnetic properties.

Amorphous FeCoZr monolayer films with a thickness of 34 nm were deposited on 5 × 25 mm2 Si(100) substrates using a composition gradient co-sputtering technique at room temperature. The composition gradient sputtering device was designed as illustrated in Fig. 1(a). The main Fe65Co35 (FeCo) target faced the geometric center of the sample, while the Zr target was located at a certain angle with the sample and in the same line with the FeCo target. The substrate holder was kept stationary. A composition gradient along the connecting line between the two targets was achieved by this strategy. The 5 × 25 mm2 sample could be divided into five 5 × 5 mm2 samples [marked S1–S5, which are shown in Fig. 1(a)]. Radio frequency (RF) sputtering was adopted for the FeCo target with a fixed deposition power PFeCo of 100 W. The Zr target was used for doping Zr element and deposited by direct current (DC) sputtering. The deposition power PZr of the Zr target was varied from 10 to 70 W. The based pressure was better than 5 × 10−8 Torr (1 Torr = 133.322 Pa). An Ar flow was controlled at 24 sccm to maintain a working pressure of 4 mTorr. During deposition, to further induce in-plane uniaxial magnetic anisotropy, a DC magnetic field of about 600 Oe (1 Oe = 79.6 A/m) was applied to the film plane. As shown in [Fig. 1(b), inset], the easy axis (EA) and hard axis (HA) of the film were both located within the film plane and perpendicular to each other. The direction of EA was parallel to the direction of the applied magnetic field. The direction of HA was consistent with the direction of the composition gradient (long side direction of Si substrate) and perpendicular to the direction of EA.

FIG. 1.

(a) Diagram of composition gradient sputtering device and sample structure. Composition profile of FeCoZr films deposited with (b) PZr = 70 W at different positions and (c) different PZr values at position 3.

FIG. 1.

(a) Diagram of composition gradient sputtering device and sample structure. Composition profile of FeCoZr films deposited with (b) PZr = 70 W at different positions and (c) different PZr values at position 3.

Close modal

The composition of the films was measured using x-ray photoelectron spectroscopy (XPS, Thermo Scientific, K-Alpha). The structure of the films was analyzed by x-ray diffraction (XRD, RigaKu, SmartLab SE). The thickness and surface morphology of the films were measured by atomic force microscopy (AFM, BenYuan, CSPM 5500). Vibrating sample magnetometry (VSM, MicroSense, EZ9) was used to characterize the static magnetic properties of the films at room temperature. The film resistivity was measured using the traditional four-probe method (4Probes Tech Ltd., RTS-9). A vector network analyzer (VNA, Agilent, E8363B) was used to measure the permeability spectrum of the films by the shorted microstrip method at zero magnetic fields.

Figure 1(b) shows the composition distribution of FeCoZr films at different positions when PZr is 70 W. The content of Zr in the films exhibits almost a linear increase from position 1 (S1) to position 5 (S5), which monotonously changes from 31.5 at. % at position 1 to 41.2 at. % at position 5. It is evident that as the deposition position approaches the Zr target, the Zr composition gradually increases, indicating that a Zr composition gradient is formed in the films. This result is consistent with the experimental expectation of the composition gradient film preparation. In the subsequent studies, all tested films (5 × 5 mm2 size) are selected at position 3 (S3). Figure 1(c) shows the composition distribution of FeCoZr films deposited with varying PZr. As PZr increases, the Zr content shows almost a linear increase from 9.0 at. % for PZr = 10 W to 36.1 at. % for PZr = 70 W. Meanwhile, Fe and Co contents exhibit the same downward trend. The atomic ratio of Fe and Co remains close to 1.85 and is consistent with the composition of the FeCo target. Therefore, Zr doping content is altered under different PZr, while the Fe to Co atomic ratio remains constant.

Figure 2 shows XRD patterns and AFM images of FeCoZr films deposited under different PZr (0, 10, 40, 70 W). The α-Fe(Co) (110) diffraction peak can be observed in FeCo film deposited without Zr doping. In Zr-doped films, the diffraction peak (110) in XRD patterns disappears. The results indicate that the precipitation of α-Fe(Co) grains is significantly inhibited in FeCoZr films by Zr doping, and the crystal structure of films is changed. Therefore, the introduction of Zr can promote the formation of amorphous FeCoZr films.

FIG. 2.

XRD patterns and AFM images of FeCoZr films deposited with PZr of (a) 0, (b) 10, (c) 40, and (d) 70 W.

FIG. 2.

XRD patterns and AFM images of FeCoZr films deposited with PZr of (a) 0, (b) 10, (c) 40, and (d) 70 W.

Close modal

The AFM images on the right of Fig. 2 reflect the surface morphology of the films, and the FeCoZr films display a nanoparticle structure. Noticeably, for the films deposited with Zr doping, the particle size of the films is significantly smaller than that of the film deposited without Zr doping, and the particle morphology becomes relatively regular and uniform. In addition, by doping Zr, the surface roughness Ra (arithmetic mean of the absolute value of the height deviation measured relative to the central plane in the investigated area) of the films is greatly improved from 3.24 nm for PZr = 0 W to 0.72 nm for PZr = 40 W. The films deposited with moderate Zr doping show excellent Ra and uniformity, which are conducive to the enhancement of magnetization ability and the smooth progress of magnetization reversal. However, with the excessive doping of Zr, Ra shows an upward trend, and it rises to 1.96 nm for FeCoZr film deposited with PZr = 70 W. Ra of all FeCoZr films is lower than that of pure FeCo film. According to the random anisotropy model,32 when the grain size of the film is lower than the ferromagnetic exchange length Lex (about 35 nm for iron-based alloys33), the magnetocrystalline anisotropy and magnetoelastic anisotropy are reduced by the strong exchange coupling between grains, so they show good soft magnetic properties. As can be seen from XRD patterns and AFM images, doping Zr can inhibit the precipitation of α-Fe(Co) grains and the growth of FeCo particles. The films deposited with Zr doping show amorphous structure and lower Ra, which are important reasons for the soft magnetic property optimization of films.

Figure 3 shows the in-plane hysteresis loops of FeCoZr films deposited with different PZr. Note that the easy axis is parallel to the direction of the external magnetic field during deposition, and the hard axis is parallel to the direction of the Zr composition gradient. The pure FeCo film shows relatively large coercivity Hc of ∼169 Oe and lower remanent magnetization ratio Mr/Ms of 0.94, and the in-plane uniaxial magnetic anisotropy in the film is not obvious, which limits their practical application. With the increase in PZr, the films exhibit good soft magnetic properties and well-defined in-plane uniaxial magnetic anisotropy. The rectangular ratio of the loops of the easy axis is improved, and the remanent magnetization ratio Mr/Ms is ∼1, indicating that the magnetic moment orientation of the films is basically along the easy axis. The anisotropy fields Hk of the films significantly increase from 128 to 340 Oe with PZr increases from 30 to 70 W. The high Hk is the result of the combined action of the magnetic field-induced and compositional gradient. Hk induced by the external magnetic field is usually tens of Oersted. Previous literature has reported that the composition gradient can generate additional uniaxial stress within the films,34–36 which leads to high Hk. According to the theory of ferromagnetism, the magnetoelastic energy can be expressed as E = −3λsσ cos2θ/2. Here, λs and σ are the saturation magnetostriction coefficient and stress, respectively. σ > 0 indicates a tensile stress, and vice versa for compressive stress. θ is the angle between the stress applied to the film and the magnetization strength. Based on the above equation, for a film with a positive λs, the compressive stress results in the arrangement of magnetic moments perpendicular to the compressive stress direction, and vice versa.36 The FeCoZr films in this paper have a positive λs and show compressive stress along the direction of the composition gradient, which results in the magnetic moments perpendicular to the direction of the composition gradient. Therefore, the composition gradient is the main reason for generating such high Hk.

FIG. 3.

Hysteresis loops of FeCoZr films deposited with PZr of (a) 0, (b) 10, (c) 20, (d) 30, (e) 40, (f) 50, (g) 60, and (h) 70 W. The blue dashed lines represent the method for determining the anisotropy field Hk.

FIG. 3.

Hysteresis loops of FeCoZr films deposited with PZr of (a) 0, (b) 10, (c) 20, (d) 30, (e) 40, (f) 50, (g) 60, and (h) 70 W. The blue dashed lines represent the method for determining the anisotropy field Hk.

Close modal

Figure 4 shows the variation trend of coercivity Hc and saturation magnetization 4πMs of the films with PZr. Hc of the films exhibits a trend of first decreasing and then increasing as PZr increases, reaching the minimum value at PZr = 40 W. The coercivities along easy (Hce) and hard (Hch) axes for the film deposited with PZr of 40 W are 30.5 and 31.5 Oe, respectively. Hc shows a decreasing trend when PZr is below 40 W, which can be attributed to the amorphous structure of the films caused by Zr doping, thereby eliminating the magnetocrystalline anisotropy. When PZr exceeds 40 W, Hc shows a slight increase. This may be attributed to the uneven film thickness and composition caused by gradient deposition, which generates higher internal stress. Furthermore, the trend in coercivity variation is closely associated with the surface roughness of the films. Therefore, under the gradient deposition of Zr, the soft magnetic properties of the films in all aspects are optimized, which is accorded with the inference made from the structure of the films in Fig. 2. In addition, the increase in non-magnetic Zr content inevitably leads to a decrease in 4πMs. 4πMs decreases from 24.5 kG for PZr = 0 W to 9.7 kG for PZr = 70 W. According to Fig. 1(c), when PZr = 70 W, the Fe content is 40.5 at. %, which is only 4.4 at. % more than the Zr content. As PZr continues to increase, the Zr content in films will surpass that of Fe and occupy a dominant position. Excessive doping of non-magnetic Zr elements results in further degradation of 4πMs, which will hinder the improvement of high-frequency properties of the films. At the same time, the coercivity of the films will tend to deteriorate, which damages their good soft magnetic properties.

FIG. 4.

Variation trend of coercivity and saturation magnetization with PZr.

FIG. 4.

Variation trend of coercivity and saturation magnetization with PZr.

Close modal

In high-frequency applications of soft magnetic film materials, energy loss is also one of the key issues we are concerned about. At gigahertz frequency, the main energy loss is caused by eddy currents and domain motion. In an alternating electric field, eddy currents will form within the ferromagnetic film, causing losses. The loss power of eddy current can be expressed as Pef2t2/ρ, where f is the frequency, t is the film thickness, and ρ is the resistivity. It can be seen that in the film materials, theoretically reducing t and increasing ρ can effectively reduce eddy current losses in high-frequency applications. The FeCoZr film in this study has a smaller film thickness of 34 nm, which can better reduce eddy current loss and improve the high-frequency characteristics of the films compared to the films with hundreds of nanometers. In addition, according to Fig. 2, the films deposited with Zr doping show an amorphous structure. The amorphous structure will cause strong additional scattering of electrons during transport, thereby increasing ρ of the films.

Figure 5 shows the variation trend of ρ of the films with different PZr. ρ of the films increases from 27.5 µΩ cm for PZr = 0 W to 286 µΩ cm for PZr = 50 W, which is greatly improved by the doping of the Zr element. As PZr continues to increase, ρ shows a slight decrease. The initial increase of ρ can be attributed to the transition of the film structure to amorphous. However, as a metal element, Zr has little effect on the increase in film resistivity. The Zr content of the films increases with the increase in PZr from 50 to 70 W, but ρ does not further increase. Therefore, it is considered that the main reason for the increase in ρ is the change in the film structure. The electron scattering effect of amorphous structure leads to the increase in ρ. However, according to the theory of ferromagnetism, the domain wall motion is hindered by both doping and internal stress. The domain motion of the film will be more difficult with the addition of Zr, which leads to an increase in the loss due to domain motion.

FIG. 5.

Resistivity of FeCoZr films deposited with different PZr.

FIG. 5.

Resistivity of FeCoZr films deposited with different PZr.

Close modal
The permeability spectra of the films deposited with different PZr are illustrated in Figs. 6(a)6(c). There is no resonance peak (not shown here) for the undoped FeCo film because of the pure FeCo film almost exhibits in-plane isotropy. For the FeCoZr films deposited with PZr < 30 W, the permeability spectra show a relaxation characteristic, which can be attributed to the high coercivity of the films. As PZr increases, the permeability spectra show a typical resonance characteristic and have a good high-frequency response. These facts indicate that the resonance characteristic of the films can be controlled by PZr. The variation of resonant frequency fr and Hk with PZr is shown in Fig. 6(d). fr increases from 4.24 GHz for PZr = 30 W to 5.23 GHz for PZr = 70 W. Therefore, the FeCoZr films exhibit adjustable dynamic magnetic properties. In addition, fr exhibits the same increasing trend as Hk. fr of the films is mainly determined by 4πMs and Hk, which can be expressed as fr=γ2π(4πMs+Hk)Hk, where γ is the rotatory magnetic ratio (γ = 1.76 × 107 Hz/Oe).37, Hk increases from 128 to 340 Oe with the increase in PZr changes from 30 to 70 W, which is the main reason for the increase in fr. Landau–Lifshitz–Gilbert (LLG) equation can be used to analyze the dynamic magnetization behavior of FeCo-based films,38,
dMdt=γ(M×H)+αMM×dMdt,
(1)
where M is the magnetization intensity, H is the magnetic field intensity, and α is the dimensionless damping coefficient. Based on this formula, the real (μ′) and imaginary (μ′′) parts of the microwave permeability of the films can be expressed as39 
μ=1+4πMsγ2×(4πMs+Hk)(1+α2)[ωr2(1+α2)ω2]+(4πMs+2Hk)(ωα)2[ωr2(1+α2)ω2]2+[γωα(4πMs+2Hk)]2,
(2)
μ=4πMsγωαγ2(4πMs+Hk)2(1+α2)+ω2[ωr2(1+α2)ω2]2+[γαω(4πMs+2Hk)]2,
(3)
where ωr is the resonant angular frequency, and ωr=2πfr=γ(4πMs+Hk)Hk. According to Eqs. (2) and (3), μ-f curves were fitted, and the fitting results are shown in blue and green curves in Figs. 6(b) and 6(c). The fitting curves are consistent with the experimental results, and the difference between the theoretical values and the experimental values of fr is less than 0.1 GHz. This fact indicates that Gilbert damping is still dominant or the damping mechanism can be included in the form of Gilbert damping. α of the films can be obtained by fitting, and α maintains a small value between 0.011 and 0.014. The films show a small α, which is beneficial for the rapid switching of magnetic moments and the reduction of energy loss in high-frequency applications.
FIG. 6.

Permeability spectra (scatter plot) and fitting curve (solid line plot) of the films deposited with PZr of (a) 10, (b) 40, and (c) 70 W, and (d) variation trend of anisotropy field and resonance frequency with PZr.

FIG. 6.

Permeability spectra (scatter plot) and fitting curve (solid line plot) of the films deposited with PZr of (a) 10, (b) 40, and (c) 70 W, and (d) variation trend of anisotropy field and resonance frequency with PZr.

Close modal

Table I summarizes the magnetic properties of FeCoZr films studied in this paper and FeCo-based films discussed in the literature.31,40–44 FeCoZr films prepared by composition gradient sputtering technique in this paper exhibit obvious advantages in anisotropy field and resonance frequency, and have great research value and application prospects in the field of high-frequency microwaves.

TABLE I.

Summary of the magnetic properties of FeCo-based films.

MaterialThickness/nmHce/OeHch/OeMs/kGHk/Oefr/GHz
FeCoZra 34 60.0 68.5 9.7 340 5.23 
FeCoZr31  100 10 20 ⋯ 305 >3 
FeCoZr40  1000 15 18 64 3.4 
FeCo–SiO241  500 2.5 4.5 11.6 108 4.05 
FeCoSiN/SiN42  120 2.81 ∼3 11.8 163 4.29 
FeCo–TiO243  350 5.3 10 14 95 3.2 
FeCoAlON44  ⋯ 2.6 3.1 12.5 44 2.1 
MaterialThickness/nmHce/OeHch/OeMs/kGHk/Oefr/GHz
FeCoZra 34 60.0 68.5 9.7 340 5.23 
FeCoZr31  100 10 20 ⋯ 305 >3 
FeCoZr40  1000 15 18 64 3.4 
FeCo–SiO241  500 2.5 4.5 11.6 108 4.05 
FeCoSiN/SiN42  120 2.81 ∼3 11.8 163 4.29 
FeCo–TiO243  350 5.3 10 14 95 3.2 
FeCoAlON44  ⋯ 2.6 3.1 12.5 44 2.1 
a

FeCoZr film deposited with PZr = 70 W in this paper.

The static and dynamic magnetic properties of amorphous FeCoZr composition gradient films by controlling Zr deposition power PZr were investigated in this paper. The results show that the soft magnetic properties of the films can be effectively optimized by Zr doping. The coercivity Hc is reduced by an order of magnitude. The surface roughness Ra of the films decreases greatly from 3.24 nm for pure FeCo film to 0.72 nm for PZr = 40 W, which is one of the reasons for obtaining good soft magnetic properties. The large in-plane uniaxial anisotropy field Hk of 340 Oe is generated for the films deposited with PZr = 70 W. High-frequency studies show that the resonance frequency fr increases from 4.24 to 5.23 GHz with increasing PZr from 30 to 70 W. The damping coefficient α maintains a small value of around 0.011–0.014. Therefore, the FeCoZr films prepared in this paper are potential candidate materials for high-frequency microwave devices.

This work was supported by the Natural Science Foundation of Ningxia (Grant Nos. 2023AAC03079 and 2023AAC03006) and the National Natural Science Foundation of China (Grant Nos. 11964027 and 52261036).

The authors have no conflicts to disclose.

Chengji Song: Data curation (lead); Investigation (lead); Writing – original draft (lead). Zeyu Han: Data curation (equal); Investigation (equal). Jie Zhou: Data curation (equal); Investigation (equal). Xuan Wang: Data curation (equal); Investigation (equal). Luran Zhang: Data curation (equal). Zhi Ma: Data curation (equal). Li Ma: Data curation (equal); Investigation (equal); Writing – review & editing (equal). Fu Zheng: Conceptualization (lead); Data curation (lead); Funding acquisition (lead); Investigation (lead); Writing – review & editing (lead).

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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