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1 Introduction▲
2 Experimental▲
3 Results and discussion▲
4 Conclusion▲
图表▲

Rare Metals2016年第10期

Preparation and high-frequency soft magnetic property of FeCo-based thin films

Xiao-Long Liu Lai-Sen Wang Qin Luo Lei Xu Bei-Bei Yuan Dong-Liang Peng

Fujian Key Laboratory of Advanced Materials,College of Materials,Xiamen University

收稿日期：25 July 2014

基金：financially supported by the National Basic Research Program of China(No.2012CB933103);the National Natural Science Foundation of China(Nos.51171158, 51371154 and 51301145);the Natural Science Foundation of Fujian Province of China(No.2014J05009);

Preparation and high-frequency soft magnetic property of FeCo-based thin films

Xiao-Long Liu Lai-Sen Wang Qin Luo Lei Xu Bei-Bei Yuan Dong-Liang Peng

Fujian Key Laboratory of Advanced Materials,College of Materials,Xiamen University

Abstract：

A series of FeCo-based thin films were prepared by magnetron sputtering without applying an induced magnetic field.The microstructure,electrical properties,magnetic properties and thermal stability of FeCo,FeCoSiN monolayer thin film and[FeCoSiN/SiN_x]_n multilayer thin film were investigated systematically.When FeCo thin film was doped with Si and N,the resistivity and soft magnetic properties of the obtained FeCoSiN thin film can be improved effectively.The coercivity(H_c),resistivity(ρ) and ferromagnetic resonance frequency(f_r) can be further optimized for the[FeCoSiN/SiN_x]_n multilayer thin film.When the thickness of FeCoSiN layer and SiN_x layer is maintained at 7 and 2 nm,the H_c,p and f_r for[FeCoSiN/SiN_x]_n multilayer thin film are 225 A·m^-1392 μΩ·cm^-1 and 4.29 GHz,respectively.In addition,the low coercivity of easy axis(H_ce ≈ 506 A·m^-1) of[FeCoSiN/SiN_x]_n multilayer thin film can be maintained after annealing at 300 ℃ in air for 2 h.

Keyword：

Magnetic films; High-frequency; Soft magnetic; Thermal stability;

Author： Lai-Sen Wang,e-mail:wangls@xmu.edu.cn;

Received： 25 July 2014

1 Introduction

With the increasing need for miniaturization and integration of electronic devices,great efforts have been invested in the research of soft magnetic thin-film materials applied in GHz frequency range [ 1, 2, 3, 4] .In order to achieve the practical applications of soft magnetic thin films in highfrequency radio-frequency micro-electromechanical system (RF-MEMS),high saturation magnetization (M_s) and appropriate in-plane uniaxial magnetic anisotropy field(H_k) are needed to adjust the ferromagnetic resonance frequency and the static permeability (μ_s,μ_s=1+4πM_s/H_k).FeCo-based alloys,possessing high M_s,seem to be one of the most promising materials on the choice of soft magnetic materials [ 5, 6] .As many granular films show,the soft magnetic properties of FeCo-based metal-insulator granular film depend on the metal volume in the vicinity of percolation threshold [ 7] .Besides,the low H_K of granular film is not suitable for the application in high-frequency systems even though the resistivity (p) is high.In order to adjust the H_k with a large range on the basis of the high M_s,one possible way is constructing the multilayer thin films by inserting nonmagnetic insulating phase (such as SiO₂,A1₂O₃) during deposition process to form oxide interlayer [ 8, 9] .The inserted nonmagnetic insulating phase can not only interrupt the continuous growth of magnetic layer,but also increase the resistance and thus reduce the eddy current loss.Si₃N₄ is a very good insulating material with a high resistance and a large dielectric constant,which is studied extensively in recent years [ 10, 11, 12] .However,up to now,there are few researches concerning the high-frequency soft magnetic properties of multilayer thin films using Si₃N₄ as insulating interlayer.

In this paper,the microstructure,electrical properties,thermal stability,oxidation resistance and high-frequency soft magnetic properties of FeCo,FeCoSiN monolayer thin film and[FeCoSiN/SiN_x]_n multilayer thin film were studied systematically.The change in the coercivity was measured under different annealing temperatures (100,200,300,400,500 and 750℃) in air atmosphere for 2 h.Compared with the FeCo and FeCoSiN monolayer thin film,the[FeCoSiN/SiN_x]_n multilayer thin films exhibit better soft magnetic properties and oxidation resistance.Our results demonstrate that the[FeCoSiN/SiN_x]_n multilayer thin films havegreat promise in practical applications in GHz frequency range.

2 Experimental

The structures and thickness of FeCo-based thin films deposited on (100)-oriented silicon wafers and glass substrates are listed in Table 1.The base pressure of sputtering chamber was evacuated to less than 5×10^-4 Pa before deposition.The sputtering process was carried out in Ar(10 ml·min^-1) or Ar (10 ml·min^-1)+N₂ (1.11 ml·min^-1)atmosphere,and the working pressure was kept at 0.4 Pa.The deposition process of the films was conducted using a power of 75 W for Fe₆₅Co₃₅ alloy target (7.62 cm,99.99%) and 200 W for Si target (5.08 cm,99.99%).The substrate holder with a rotation speed of 24 r·min^-1 was mounted in the upper part of the chamber.The thickness of the thin films was regulated at about 120 nm by adjusting the sputtering time and monitored by the surface profiler(Alpha-Step D-100).The thin films were annealed under different temperatures ranging from 100 to 750℃in air for 2 h.

X-ray diffraction (XRD,PANalytical B.V.) was used to analyze the crystal structure of thin films.The surface morphologies were obtained by the field emission scanning electron microscopy (FESEM,SU 70) with an accelerating voltage of 15 kV.The chemical components were identified by X-ray photoelectron spectroscopy (XPS,PHI Quantum 2000).The static magnetic properties of thin films with different annealing temperatures were characterized by vibrating sample magnetometer (VSM,LakeShore 7404).The microwave permeability was measured by vector network analyzer with frequency ranging from500 MHz to 5 GHz.

3 Results and discussion

The XRD patterns of Samples L1,L2 and L3 sputtered at room temperature are shown in Fig.1.It can be noted that all the thin films exhibit an obvious diffraction peak nearby44°,indicating that the films have a bcc (110) phase.The half-width of the (110) diffraction peak gets wider from Samples L1 to L3,while the diffraction intensity of (110)peak accordingly decreases.Based on the Scherrer's equation,the average grain sizes are estimated to be 18.1,13.8 and 8.8 nm for Samples L1,L2 and L3,respectively.Therefore,the addition of Si and N elements and the adoption of the multilayer technology could decrease the grain size effectively.Additionally,there are no diffraction peaks originated from SiN_x phase in the XRD patterns of Samples L2 and L3,suggesting that SiN_x may exist as an amorphous phase.

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Table 1 Essential information of FeCo-based thin films sputtered at room temperature

Figure 2 displays the FESEM images of Samples L1,L2and L3.The surface of the thin films becomes smooth,and the particle size decreases after the doping of Si and N elements into the FeCo alloy (Fig.2a,b).Moreover,the particle of Sample L3 is deeply refined,which means that SiN_x interlayer could interrupt the growth of the FeCoSiN.

Figure 3 presents the high-resolution XPS spectra of Fe2p,Co 2p,Si 2p and N 1s of Samples L1,L2 and L3.The obvious characteristic peaks appeared in the vicinity of706.5,719.8,778.0 and 793.2 eV indicate zero-valent Fe and zero-valent Co in the samples (Fig.3a,b).The small satellite peaks (binding energy at around 711.4 and782.3 eV) are vested in the Fe-N bonds and Co-N bonds,respectively (the detailed analysis was well established in other publication [ 13] ).In addition,it can be clearly observed that the positions of Fe 2p and Co 2p spectra slightly shift to lower binding energy from FeCo to[FeCoSiN/SiN_x]_n multilayer thin film.The intensity of each peak after background subtraction shows a decreasing trend,which can be ascribed to the decrease in Fe and Co concentrations.The main peaks centered at 99.3 and101.4 eV are related to the zero-valent Si and Si⁴⁺,respectively (Fig.3c).Figure 3d shows typical N 1s spectra;the characteristic peak located at 396.9 eV is related to the N^3-.Therefore,Si₃N₄ is formed in both the FeCoSiN monolayer film and the[FeCoSiN/SiN_x]_n multilayer thin film.When observing the Si 2p spectra again,the relative intensity of zero-valent Si decreases and that of Si⁴⁺significantly increases with the formation of multilayer thin film,indicating that the interlayer is mainly composed of Si₃N₄.

Fig.1 XRD patterns of Samples L1,L2 and L3 sputtered at room temperature

Fig.2 FESEM images of surface morphology of a Sample L1,b Sample L2 and c Sample L3

Fig.3 High-resolution XPS spectra of a Fe 2p,b Co 2p,c Si 2p and d N 1s peaks of Samples L1,L2 and L3

The classical in-plane hysteresis loops of Samples L1,L2 and L3 are shown in Fig.4a-c.All films exhibit an excellent squareness ratio and an evident in-plane uniaxial magnetic anisotropy.By forming the multilayered structure thin film by adding the nonmagnetic elements N and Si,M_smonotonously decreases from 2.10 (Sample L1) to 1.18 T(Sample L3),while H_k significantly increases from 2924 to13,051 A·m^-1 without applying an induced magnetic field.The decrease in M_s is associated with the decrease in magnetic FeCo phase content.Meanwhile,the coercivity of easy axis (H_ce) presents a descending trend from Samples L1 to L3 (here the easy axis is along the tangential direction of the rotation direction).This is mainly due to the decrease in grain size as seen from XRD results.The grain refinement could strengthen the exchange coupling effect between the grains,which reduces the effective magnetocrystalline anisotropy of a single grain and improves the soft magnetic properties of thin film [ 14] .The p of thin films measured by four-point probe is 48,166 and392μΩ·cm for Samples L1,L2 and L3,respectively.The increase in p stems from the increase in nonmagnetic insulating phase content.The high-frequency permeability(real (μ') and imaginary (μ")spectra of the three samples are shown in the right column of Fig.4d-f.Being similar to the changed trend of H_k,the resonance frequency (f_r)monotonously increases from 2.48 to 4.29 GHz,while the static permeability (μ_s) decreases from 500.00 to 70.13.The results of the simulations based on the Landau-Lifshitz-Gilbert (LLG) equation are also displayed in the high-frequency permeability spectra (the black dashed line in Fig.4),which are in good consistent with the experimental results.

Fig.4 In-plane hysteresis loops (a,c,e) and frequency dependencies of real (μ') and imaginary(μ") parts of permeability spectra (b,d,f) at room temperature:a,b FeCo;c,d FeCoSiN;e,f[FeCoSiN/SiN_x]_n

Figure 5 depicts the coercivity of easy magnetizing axis(H_ce) for Samples L1,L2 and L3 as a function of the annealing temperature.With the annealing temperature increasing,H_ce of FeCo and FeCoSiN monolayer thin film increases initially and then decreases.The increase in H_ce mainly results from the increase in grain size.However,the decrease in H_ce after further annealing is mainly attributed to the remov al of internal stresses and the formation of magnetic oxide with smaller coercivity [ 15] .The FeCo and FeCoSiN monolayer thin film have very poor oxidation resistance,H_ce(FeCo)=8800 A·m^-1,and H_ce(FeCoSiN)=2940 A·m^-1 when the annealing temperature increases to 300℃,but the H_ce of[FeCoSiN/SiN_x]_nmultilayer thin film is still low (H_ce≈509 A·m^-1).As is well known,the insertion of Si and N and the constructing of Si₃N₄insulating interlayer can effectively prevent the formation of FeCo oxides and restrain the growth of grain,which improves the soft magnetic properties of[FeCoSiN/SiN_x]_n multilayer thin film even after annealing treatment.Therefore,comparing with the FeCo and FeCoSiN monolayer thin film,the multilayer technology can effectively improve the thermal stability and the oxidation resistance.

Fig.5 Coercivity of Samples L1,L2 and L3 annealed at different temperatures

4 Conclusion

The microstructure,electrical properties,soft magnetic properties and thermal stability of FeCo,FeCoSiN monolayer thin film and[FeCoSiN/SiN_x]_n multilayer thin film were investigated systematically.All the films show the high 4πM_sand an evident in-plane uniaxial magnetic anisotropy (H_k).Compared with FeCo and FeCoSiN monolayer thin film sputtered at room temperature,[FeCoSiN/SiN_x]_n multilayer thin film exhibits a relatively low H_ce(225 A·m^-1),high H_k(13,051 A·m^-1),highp (392μΩ·cm)andhighf_r (4.29 GHz).In addition,the thermal stability and oxidation resistance of[FeCoSiN/SiN_x]_n multilayer thin film are proved to be far stronger than those of FeCo thin film and FeCoSiN monolayer thin film.Therefore,the[FeCoSiN/SiN_x]_n multilayer thin film will be a more suitable candidate for high-frequency application in electromagnetic devices.

Acknowledgments This study was financially supported by theNational Basic Research Program of China (No.2012CB933103),theNational Natural Science Foundation of China (Nos.51171158,51371154 and 51301145),and the Natural Science Foundation ofFujian Province of China (No.2014J05009).