Thickness-dependent Magnetic and Microwave Resonance Characterization of Combined Stripe Patterned FeCoBSi Films
In this paper, we fabricated a series of FeCoBSi multistoried patterned magnetic films with different thickness by traditional UV lithography method and DC sputtering deposition. Broad resonance band phenomenon was observed during high frequency property characterization, with full width half maximum (FWHM) of 4 GHz when the film thickness is 45 nm. The broad resonance band effect was contributed to the existence of multiple resonance peaks due to different stripe width of the combined stripe pattern, which induced distinguish shape anisotropic field in each stripe. Each resonance peak was independent due to the gap between the stripes, leading to a controllable method to tune the microwave properties of such structure. With thickness varied, the resonance band could be altered according to the mathematic prediction. This work presents an effective method for tuning the microwave resonance characterization in magnetization dynamic.
KeywordsSoft magnetic materials High frequency properties EMI absorbers
Full width half maximum
With the rapid development of telecommunication technology, the problems of electromagnetic inference (EMI), which deteriorate the performance of such systems in high frequency, attract public attentions significantly [1, 2, 3, 4, 5]. In order to satisfy the requirements of EMI shielding materials, broadband and controllable resonances of magnetic films are desired [6, 7]. Meanwhile, a high damping factor at designed frequency would make contribution to realize promising EMI devices [8, 9]. Due to in-plane uniaxial anisotropy of a film could lead to well soft magnetic properties at gigahertz frequency, hence, better absorption properties, several methods including induced magnetic field , induced stress  during deposition, multilayer design , and post-annealing under external magnetic field [13, 14], were investigated. Besides, patterned magnetic films with induced shape anisotropy designed by artificial structure draw great public attention due to its controllable and robust properties [15, 16]. In light of this, double-stripe patterned FeCo-based magnetic film were proposed in our former work . Broad resonance band with double resonance peaks phenomenon was observed during experiment, which ascribed to superposition of double resonance source contributed by independent magnetic stripes.
Therefore, in this paper, in order to expand the resonance band furthermore, we introduced a unique combined stripe patterned FeCoBSi thin films containing various stripe with five different widths and analyzed microwave resonance characterization due to multiple resonance peaks with the Landau-Lifshitz-Gilbert (LLG) processional motion formulism. The broad resonance band phenomenon was enhanced with full width half maximum (FWHM) of 4 GHz at thin thickness, i.e., 45 nm for our experiments. Meanwhile, the alteration of resonance frequency could be predicted by the mathematic formula related to demagnetization factors. The results could be further illustrated by the shape-induced effective anisotropy filed due to distinguished stripe width, which made it possible to control by the traditional lithography process in the actual application.
The thickness of the films was determined by the cross-sectional observation with scanning electron microscopy (SEM). Correspondent static properties of magnetic film, i.e., hysteresis loops, were measured by vibrating sample magnetometer (VSM). Microwave properties were characterized by a shorted micro-strip transmission line perturbation method connected to a vector net analyzer in the frequency range of 0.5–6 GHz.
Results and discussion
Figure 1a shows the scheme of the deposition setup with external induced magnetic field. A 500 Oe external magnetic field was applied during deposition in order to induce in-plane uniaxial anisotropy. Lift-off method was processed after deposition to expose to patterned structure of films. Figure 1b exhibits the combined stripe patterned structure of our magnetic films. The sequence of width for each stripe corresponds to 5, 10, 15, 20, 25 μm, respectively, while the gap between each stripe was fixed at 5 μm. According to our previous work, there were no obvious crystalline peaks except Si (111) from the substrates during XRD measurement . Hence, the crystalline structure of our films was amorphous or nanocrystalline.
where 4πM s is defined as saturation magnetization, α is damping factor, γ is gyromagnetic ratio (1.76 × 107Oe−1s−1 for FeCo alloy), H e is effective anisotropy filed, and N x , N y , N z is the demagnetization factor along three orthogonal directions, respectively. f r can be derived by the Kittle equation as
In light of stripes with different width included in our films, which induced distinctive shape anisotropy leading to split resonance peaks, the entire spectrum should be characterized as mathematical addition of five separate one. The demagnetization factor along x, y, and z direction can be written as 
where L is the length along z-axis, W is the width along x-axis, and T is thickness along y-axis. With formula (3), (4), (5), and the LLG formula, the resonant frequency corresponding to different width of the magnetic stripe from 5 to 25 μm can be calculated, respectively.
In conclusion, we have studied the magnetic and microwave resonance characterization of combined stripe patterned FeCoBSi with different thickness. Compared to the former double-striped patterned films, five-striped patterned FeCoBSi pattern could extend the resonance band (FWHM) furthermore to 4 GHz. The broaden band phenomenon could be controlled by tuning width of different stripe as well as thickness of magnetic films in order to meet the requirement in the actual application, which may be useful in the future EMI devices.
The authors would like to thank Dr. Yin Zhang for her valuable discussion.
This work was supported by the National Natural Science Foundation of China (No. 51201025, No. 51301031, and No. 51772042), Sichuan Science and Technology plan (No. 2017JY0348), and “111” Center (No. B13042).
Availability of data and materials
The datasets supporting the conclusions of this article are included within the article.
WZ,YL did the most experiments in this work; LZ, HZ, and YL drafted the manuscript; LZ made the research plan; MZ, PHZ, LBZ, HYC, HPL, JXL, LJD and XW provided helpful discussions and suggestions; and LZ supervised all of the study. All the authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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