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Microwave permeability of stripe patterned FeCoN thin film
Author’s Accepted Manuscript Microwave permeability of stripe patterned FeCoN thin film Yuping Wu, Yong Yang, Fusheng Ma, Baoyu Zong, Zhihong Yang, Jun Ding www.elsevier.com/locate/jmmm
PII: DOI: Reference:
S0304-8853(16)32323-X http://dx.doi.org/10.1016/j.jmmm.2016.11.064 MAGMA62139
To appear in: Journal of Magnetism and Magnetic Materials Received date: 23 September 2016 Revised date: 9 November 2016 Accepted date: 14 November 2016 Cite this article as: Yuping Wu, Yong Yang, Fusheng Ma, Baoyu Zong, Zhihong Yang and Jun Ding, Microwave permeability of stripe patterned FeCoN thin f i l m , Journal of Magnetism and Magnetic Materials, http://dx.doi.org/10.1016/j.jmmm.2016.11.064 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Microwave permeability of stripe patterned FeCoN thin film Yuping Wua, Yong Yanga,*, Fusheng Maa, Baoyu Zonga, Zhihong Yanga, Jun Dingb a
Temasek Laboratories, National University of Singapore, 5A Engineering Drive 1, Singapore 117411. b
Department of Materials Science and Engineering, National University of Singapore, Singapore 117574. *Corresponding author. [email protected]
Abstract Magnetic stripe patterns are of great importance for microwave applications owing to their highly tunable microwave permeability by adjusting the geometrical dimensions. In this work, stripe patterned FeCoN films with 160 nm thickness are fabricated by using standard UV photolithography. Their microwave permeability are investigated systematically via both experiment and micromagnetic simulation. The good agreement between experimental and simulation results suggests that stripe width is crucial for the microwave magnetic properties of the stripe pattern. It is demonstrated by simulation that with increasing stripe width from 1 to 80 μm the initial permeability shows a continuous growth from about 8 to 322, whiles the resonance frequency drops dramatically from 18.7 GHz to 3.1 GHz at 4 μm gap size. Smaller gap size would result in slightly increased initial permeability due to larger magnetic volume ratio, accompanied by decreased resonance frequency because of stronger magnetostatic interaction. Moreover, the experimental investigation on stripe length effect indicates that the stripe length should be kept as long as possible to achieve uniform bulk resonance mode and high permeability value. Insufficient stripe length would result in low frequency edge mode and decayed bulk mode. This study could provide valuable guidelines on the selection of proper geometry dimensions of FeCoN stripe patterns for high frequency applications. Keywords: Microwave permeability; FeCoN; Stripe pattern; Micromagnetic simulation
1. Introduction 1
Patterned magnetic thin film has attracted much attention due to their wide applications in high speed data storage [1], inductor [2] and microwave devices (e.g. filter, phase shifter, antenna, absorber, etc.) [3-7]. Compared with unpatterned film, the patterned films could provide additional useful features due to rich variation of static and dynamic magnetic behaviors which is unachievable in continuous films[8]. Therefore, understanding the knowledge of different patterns are vital for both fundamental physics study and technical applications. To date, various patterns (e.g. stripe, ring, anti-dot, etc.) have been reported, exhibiting fascinating magnetic properties [9, 10]. From a magnetic point of view, complex permeability (μr=μ'-jμ") is a key parameter for high frequency applications. It determines not only the performance and efficiency but also the working frequency of the patterns. Generally, the permeability is required to be as high as possible to achieve high efficiency. Moreover, tunable resonance frequency is also an advantage because it could offer great flexibility for technical applications, which is usually achieved by inducing shape anisotropy or applying external magnetic field.[11, 12] Taken these requirements into account, stripe pattern is an excellent candidate for microwave application due to the following reasons: Firstly, the strong shape anisotropy of stripe could prefer uniform spin configuration at micro- or even larger size without applying external magnetic field, which is crucial for uniform ferromagnetic resonance and sharp absorption at the resonance frequency; secondly, the ferromagnetic resonance frequency can be tuned simply by adjusting the aspect ratio of the stripe; furthermore, the rectangular shape of individual stripe could allow densely packing to enhance magnetic volume ratio for further increasing permeability. As such, kinds of stripe patterns were investigated [13-15]. However, it is worth noting that so far there is a lack of systematic study on the dynamic properties of the stripe pattern. Furthermore, most of these works focus on permalloy (Fe20Ni80) because it is a perfect soft ferromagnetic material with negligible crystalline anisotropy for fundamental study [16-19]. For practical application, iron-cobalt (FeCo) alloy is acknowledged as the best soft ferromagnetic material because of high Curie temperature, low magnetocrystalline anisotropy and the highest saturation magnetization (Ms) with respect to other ferromagnetic materials [20, 21]. According to Acher’ law, large Ms could allow high permeability and resonance frequency [22]. However, high electrical conductivity of metal material suffers from serious eddy current under electromagnetic radiation, which is also called “skin effect” [23]. In recent 2
years, nitrogen doped FeCo alloy (FeCoN) emerges as a promising material for high frequency application because of both high magnetization and reduced conductivity [24, 25]. Therefore, FeCoN stripe pattern is of great interest and there is an urgent need to investigate its high frequency magnetic properties. Based on the above facts, we are devoted to conduct a systematic study on the high frequency permeability of FeCoN stripe patterns. The patterns are fabricated through a standard standard UV photolithography and lift-off method. Furthermore, an external magnetic field is applied during film deposition to induce an in-plane uniaxial anisotropy along the stripe direction. This is because the production of permeability and resonance frequency can be enhanced by several times due to the presence of the in-plane uniaxial anisotropy [24, 26]. The microwave permeability of the fabricated FeCoN stripe patterns with different stripe widths are measured. Meanwhile, micromagnetic simulation is performed for verification of the experimental results. The effects of gap size and stripe length are also discussed systematically.
2. Experimental and computation methods The soft magnetic FeCoN (Fe47Co45N8) thin films with thickness of ~160 nm were prepared by DC magnetron sputtering using FeCo target with the gas mixture of nitrogen and argon as the ambient gas on the Mica substrate. To get the sputtered thin film of high microwave qualities, the mica substrates were prepared by removing a top layer. A thin SiO2 layer was deposited on the mica before the deposition of FeCoN layer. The sputtering conditions, including the background pressure, sputter pressure, supplied power, and deposition rate, were 4.0 × 10−7 Torr, 3.0 mTorr, 250 W, and 25 nm/min, respectively. The concentration of N2 was controlled by the flow rate at 8%. During deposition, a 200 Oe magnetic field was applied to induce in-plane anisotropy [24]. Standard UV photolithography and lift-off were used to define large arrays of rectangular stripes. The overall patterned area of each array is 15 × 4 mm2 and the growth-induced easy axis of in-plane anisotropy was aligned along the length of the rectangles. The lithography process is shown in Fig. 1(a). As shown in the figure, it starts with the spin coating of photoresist (AZ1518). UV light is used 3
to expose the photoresist according to the designed pattern mask followed by development to remove exposed photoresist. Then magnetic thin films were deposited into the patterned photoresist by magnetron sputtering. Finally, the unexposed photoresist and overlaying magnetic thin films are liftoffed by acetone. The micromagnetic simulations were performed using MICROMAGUS package [27]. In this package, the total energy consists of four energy contributions: external field energy, magnetocrystalline anisotropy energy, exchange stiffness energy and demagnetizing field energy. The equilibrium state is found by minimizing the total free energy of the system. To simulate dynamic remagnetization processes, it used the Bogacki-Shampine version of the Runge-Kutta-23 method. The integration step size control to achieve the required dynamical accuracy (10−6) [28]. The magnetic parameters of FeCoN used in the micromagnetic simulation are listed as follows: saturation magnetization Ms=1600 emu/cc, exchange stiffness A=1×10-6 erg/cm, magnetocrystalline anisotropy constant K1= 4.4×105 erg/cm3, in-plane uniaxial anisotropy constant Ku= 4×104 erg/cm3 (describing the induced in-plane anisotropy field of about 50 Oe along stripe direction). The cell size is 20×20 nm2 in the plane of the film, which is treated as a single layer along normal direction. The Gilbert damping coefficient α was used as 0.015. Periodic boundary condition (PBC) was applied to imitate infinitely long and periodically arranged rectangular stripes. The simulation was performed at 0K, meaning the thermal effect is excluded. To simulate the permeability of the stripe patterns, a “sinc” function excitation field h0 was applied in the film plane while perpendicular to the stripe direction. [
( (
)] )
(1)
where amplitude hmax=10 Oe, f0 =30 GHz, and t0 = 10/2f0 ns. The excitation field given by the above has a constant power spectrum up to about 30 GHz to assure that system is excited by a constant strength below the frequency f0. The complex susceptibility is calculated by using the FFT technique [29]. Finally, the complex permeability could be then obtained by using
.
The film thickness was tested by a surface step profilometer. The patterned thin films were characterized by scanning electron microscope (SEM). Static magnetic properties were measured 4
using a vibrating sample magnetometer (VSM). The permeability frequency spectra from 0.5 to 8.0 GHz was characterized by a network analyzer (Agilent 5230A) using a shorted microstrip transmission-line perturbation fixture.
3. Results and discussions
Fig. 1. (a) Schematic illustration of lift-off process for the fabrication of FeCoN stripe patterns. (b-d) SEM images of stripe patterned FeCo thin film with stripe width of 30, 20 and 10 µm, respectively. Fig. 1(a) illustrates a standard lift-off photolithography method for the fabrication of FeCoN stripe patterns. The fabrication mainly includes spin coating of the photoresist (PR), mask loading and UV exposure, development, FeCoN film deposition by sputtering and subsequent lift-off process. It should be mentioned that during film deposition a 200 Oe magnetic field was applied along the strip direction to induce an in-plane uniaxial anisotropy, implying that the shape anisotropy coincides with the induced uniaxial anisotropy. This is to ensure a single domain structure at such micrometer stripe width. Using this method, FeCoN stripe pattern with different stripe width (i.e. 5, 10, 20 and 30 µm) are fabricated. The length of all the stripes are 4000 µm, meaning the stripe is “infinitely” long with 5
respect to the stripe width. Fig. 1(b-d) provide the scanning electron microscope (SEM) image of the as fabricated 30, 20 and 10 µm width FeCoN stripe patterns. As shown in the figures, the rectangular shaped stripes are densely arranged with a gap size of around 4 µm. The densely packing of the stripes would allow high magnetic volume ratio thus achieving high magnetic permeability. The effect of gap size will be discussed later.
Fig. 2. Room temperature hysteresis loops of (a) unpatterned FeCoN film, (b) 4000 × 30, (c) 4000 × 20, (d) 4000 × 10, (e) 4000 × 5 µm2 stripe patterns measured along easy axis (E.A.) and hard axis (H.A.). The E.A. coincides with the stripe direction. The measured hysteresis loops of the patterns with different stripe width are shown in Fig. 2. The loops of unpatterned film is also provided for comparison. For each sample, the loops are measured along the in-plane uniaxial easy axis (or stripe direction) and hard axis. It can be seen that the unpatterned film exhibits typical easy axis (E.A.) and hard axis (H.A.) type hysteresis loops, implying a well defined in-plane uniaxial anisotropy (equivalent anisotropy field is around 50 Oe). Such weak in-plane uniaxial anisotropy is crucial for achieving high permeability in magnetic thin films.[26, 30] After patterning, the loops changes dramatically. It can be seen in Fig. 2(b) that the 30 µm stripe width pattern exhibit multi-switching process when the field is along easy axis. Meanwhile, the 6
saturation field in hard axis increase obviously (>200 Oe). Further reduction in stripe width leads to the increase of coercivity in both directions (Fig. 2(b-e)), accompanied by much higher saturation fields. This is reasonable that the effective anisotropy of the stripe pattern mainly originates from the shape anisotropy. When the stripe is narrowed down, the shape anisotropy is enhanced accordingly, resulting in increased coercivity and saturation field. Since the permeability is strongly dependent on the anisotropy [30], the change of anisotropy would result in dramatic variation of microwave permeability of the FeCoN stripe patterns.
Fig. 3. Measured complex permeability of FeCoN film and stripe patterns with different stripe width. Fig. 3 plots the frequency dependent complex permeability of the FeCoN continuous film and stripe patterns measured in the frequency range of 1-8 GHz. It can be seen in Fig. 3(a) that the continuous film shows an initial permeability (here defined as real permeability value at 1 GHz) as high as 323. 7
Meanwhile, a broad resonance peak is observed at around 2.4 GHz (Fig. 3(b)), agreeing well with our previous results and other reported FeCo films with in-plane uniaxial anisotropy [24, 31]. The high permeability is attributed to the high Ms of the material and the well defined in-plane uniaxial anisotropy induced during film deposition. In comparison, the patterned film shows reduced permeability and increased resonance frequency, which is further pronounced at narrower stripe width. For 30, 20, 10, 5 µm stripe width patterns, the initial permeability is about 137, 111, 69, 18, while the resonance frequency is around 3.6, 4.0, 5.0, >8 GHz, respectively. The trends are agreed well with that reported on FeTaN stripe patterns [15]. In theory, the permeability is inversely proportional while the resonance frequency is proportional to the anisotropy [32]. Narrower stripe possess stronger shape anisotropy thus leading to lower permeability and higher resonance frequency. It is also noted in Fig. 3(b) that the patterns with 5 µm stripe width does not shows any resonance peak. This is because the shape anisotropy of the 5 µm stripe is so strong that the resonance peak is beyond the experimental frequency range, which will be confirmed by micromagnetic simulation in later section.
8
Fig. 4. (a, b) Simulated complex permeability of FeCoN continuous film and stripe patterns with different stripe width; (c) comparison of resonance frequency and initial permeability between micromagnetic simulation and experimental results; (d) simulated resonance frequency and initial permeability of FeCoN stripe patterns with different stripe width and gap size. The dashed line shows the resonance frequency calculated by Kittel’s formula. In order to verify the experimental results, microwave permeability of the FeCoN film and patterns are simulated by using micromagnetic simulation. The results are provided in Fig. 4(a-b). For the unpatterned film, the simulated initial permeability and the resonance frequency is about 343 and 2.8 GHz, respectively, which are very close to the experimental values of 323 and 2.4 GHz (Fig. 3(b)). For the stripe patterns, the simulated initial permeability is about 169, 120, 61, 30 and the resonance frequency is around 4.1, 4.8, 6.6, 9.3 GHz for the 30, 20, 10, 5 µm stripe width patterns, respectively. It confirms that the resonance frequency of 5 µm pattern, which is 9.3 GHz, is beyond the experimental frequency range therefore undetected in Fig. 3(b). A direct comparison between the experimental and simulated results is drawn in Fig. 4(c). Apparently, the simulate permeability and resonance frequency are in a very good consistence with the experimental results. It proves the validity of both experimental measurement and micromagnetic simulation. To get an overview on the microwave permeability of the FeCoN stripe patterns, the simulation is further performed in a much broader range of stripe width from 1 to 80 µm at different gap sizes (1, 2 and 4 µm). Fig. 4(d) summarizes the simulated initial permeability and resonance frequency values. Take the 2 µm gap size for example, the initial permeability increases continuously from 8 to 322 when the stripe width is increased from 1 to 80 µm. By contrast, the resonance frequency drops significantly from 18.8 GHz to about 6 GHz when the stripe width is below 10 µm. Further increase of the stripe width only leads to slight drop to about 3.1 GHz at 80 µm. Very similar resonance frequencies are obtained when the gap sizes is changed to 1 or 4 µm. However, slight difference is observed when the stripe width is below 40 µm, where larger gap size shows slightly higher resonance frequency, implying that the magnetostatic interaction would lead to the decrease of resonance frequency. In order to further confirm this, the resonance frequency is also calculated by using Kittel’s formula [33]: 9
{[ where
(
is the angular resonance frequency,
] [
)
e
(
)
]}
(2)
is the external field (zero in this study), Ni (i=x, y, z)
is the demagnetizing factor describing the shape anisotropy of the rectangular stripe (see supplementary material). This formula could describe the resonance frequency of a single rectangular stripe where the magnetostatic interaction is excluded. This is equivalent to a stripe pattern with infinite gap size. The dashed curve in Fig. 4(d) shows the resonance frequency calculated using Kittel’s formula. It is evident that the trends is almost the same as the simulation results, but the values are about 2-4 GHz higher. It confirms that the magnetostatic interaction between the stripes would lead to decrease of the resonance frequency. This is because the demagnetizing field of the stripes is reduced when the interaction is taken into account [14]. The gap between the dashed curve and simulated resonance frequency gives exactly the enhancement of resonance frequency after completely removing the magnetic interaction in stripe arrays. With respect to the initial permeability, larger gap size would lead to the drop of the permeability value due to lower magnetic volume fraction.
10
Fig. 5. (a,b) SEM images of the FeCoN stripe patterns with stripes length of 250 and 20 µm. (c,d) Measured complex permeability of stripe patterns with different stripe lengths (4000, 1000, 250, 20 µm). The stripe width and gap size is 20 and 4 µm, respectively. So far, the stripe width and gap size effect have been investigated. It should be emphasized that all the above results are acquired on “infinitely long” stripe arrays where the stripe is continuous along stripe direction. As one of important geometrical parameters, the strip length could also take great effect on the dynamic permeability of the stripe patterns. It has been confirmed in micromagnetic simulation that short stripes width would cause low frequency resonance mode resulting from the splay of the spin configuration near the end of the stripe [13]. To verify this on the stripe patterned FeCoN film, stripe patterns consisting of short stripes are fabricated. The stripe length is reduced from 4000 to 1000, 250 and 20 µm with a constant width and gap size of 20 and 4 µm, respectively. Fig. 5(a-b) shows the SEM images of stripe patterns with stripes length of 250 and 20 µm. It can be seen in Fig. 5(b) that the patterns turn to be two-dimensional regular arrangement of short stripe island. The measured complex permeability of these patterns are provided in Fig. 5(c-d). It indicates that the 1000 µm stripe length pattern shows nearly the same permeability value compared with the “continuous” 4000 µm patterns. However, it is noticed that the amplitude of resonance peak drops slightly. When the stripe length is reduced to 250 µm, the initial permeability still remains almost the same, while the resonance peak decays obviously. Moreover, a noticeable resonance peak occurs at around 2.3 GHz. This is in a good agreement with the results found in permalloy stripes, where the low frequency peak is attributed to the splay of the spin configuration near the end of the stripe or so called “edge mode” in comparison to the “bulk mode” of the prior peak [13]. The “edge mode” get enhanced when the stripe length is 20 µm (squire dot arrays), associated with the split of prior peak and the drop of permeability. This could be attributed to the lack of shape anisotropy, which results in complex magnetic domain structures and corresponding resonance modes [34]. The above results provide experimental evidence that long stripe length would benefit high permeability and uniform resonance mode.
4. Conclusions 11
In summary, microwave permeability of stripe patterned FeCoN thin films have been investigated systematically. It is found in both experiment and simulation that the initial permeability and resonance frequency can be tuned in a wide range by changing width of the stripes. Simulation on gap size effect reveals that larger gap size would lead to the drop of permeability and increased resonance frequency because of lower magnetic volume fraction and weaker magnetostatic interaction, respectively. Furthermore, it has been demonstrated that the stripe length should be kept as long as possible to avoid low frequency resonance mode thus achieving uniform bulk resonance mode as well as high permeability. These results would provide valuable guidance on high frequency application of FeCoN stripes patterns, such as microwave absorption.
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[10] H. Fang, B. Caballero, E.M. Akinoglu, E.T. Papaioannou, A. García-Martín, J.C. Cuevas, M. Giersig, P. Fumagalli, Appl. Phys. Lett. 106 (2015) 153104. [11] W. Li, J. Wei, W. Wang, D. Hu, Y. Li, J. Guan, Materials & Design 110 (2016) 27. [12] W. Wang, J. Guo, C. Long, W. Li, J. Guan, J. Alloys Compd. 637 (2015) 106. [13] O. Gérardin, H. Le Gall, M.J. Donahue, N. Vukadinovic, J. Appl. Phys. 89 (2001) 7012. [14] O. Gérardin, J.B. Youssef, H. Le Gall, N. Vukadinovic, P.M. Jacquart, M.J. Donahue, J. Appl. Phys. 88 (2000) 5899. [15] X. Chen, Y.G. Ma, C.K. Ong, J. Appl. Phys. 104 (2008) 013921. [16] Y. Peng, B.M.F. Rahman, T. Wang, C. Nowrin, M. Ali, G. Wang, J. Appl. Phys. 117 (2015) 17B709. [17] G.R. Aranda, G.N. Kakazei, J. González, K.Y. Guslienko, J. Appl. Phys. 116 (2014) 093908. [18] J. Vernieres, J.F. Bobo, D. Prost, F. Issac, F. Boust, J. Appl. Phys. 109 (2011) 07A323. [19] N. Smith, T. Jagielinski, J. Freeman, P. Koeppe, J. Appl. Phys. 73 (1993) 6013. [20] Y. Yang, C. Xu, Y. Xia, T. Wang, F. Li, J. Alloys Compd. 493 (2010) 549. [21] Q. Nguyen, C.N. Chinnasamy, S.D. Yoon, S. Sivasubramanian, T. Sakai, A. Baraskar, S. Mukerjee, C. Vittoria, V.G. Harris, J. Appl. Phys. 103 (2008) 07D532. [22] O. Acher, S. Dubourg, Phys. Rev. B 77 (2008) 104440. [23] Y. Wu, M. Han, Z. Tang, L. Deng, J. Appl. Phys. 115 (2014) 163902. [24] Y.P. Wu, Y. Yang, Z.H. Yang, N.N. Phouc, F. Ma, B.Y. Zong, J. Ding, J. Appl. Phys. 118 (2015) 013902. [25] S.X. Wang, N.X. Sun, M. Yamaguchi, S. Yabukami, Nature 407 (2000) 150. [26] Y. Xu, G. Luqian, W. Jianqiang, Q. Liang, W. Tao, L. Fashen, J. Phys. D: Appl. Phys. 43 (2010) 215002. [27] For the introduction of MicroMagus: http://www.micromagus.de/ 13
[28] O. Dmytriiev, M. Dvornik, R.V. Mikhaylovskiy, M. Franchin, H. Fangohr, L. Giovannini, F. Montoncello, D.V. Berkov, E.K. Semenova, N.L. Gorn, A. Prabhakar, V.V. Kruglyak, Phys. Rev. B 86 (2012) 104405. [29] R. Liu, J. Wang, Q. Liu, H. Wang, C. Jiang, J Appl Phys 103 (2008) 013910. [30] D.S. Xue, F.S. Li, X.L. Fan, F.S. Wen, Chin. Phys. Lett. 25 (2008) 4120. [31] L. Qiao, R. Han, T. Wang, L. Tang, F. Li, J Magn Magn Mater 375 (2015) 100. [32] J. Smit.1971 Magnetic properties of materials: McGraw-Hill [33] C. Kittel, Phys. Rev. 73 (1948) 155. [34] A. Kaya, J.A. Bain, J. Appl. Phys. 99 (2006) 08B708.
Hightlights
Magnetic stripe patterns are of great importance for microwave applications owing to their highly tunable microwave permeability by adjusting the geometrical dimensions. This work presents a systematic study on microwave permeability of FeCoN stripe patterned thin film. Through experimental investigation as well as micromagnetic simulation, geometrical dimensions (i.e. stripe width, gap size, stripe length) of the stripe pattern are systematically optimized for achieving high permeability and tunable resonance frequency. Several important conclusions has been obtained. The valuable results obtained in this study could provide guidelines on the selection of proper geometrical dimensions of FeCoN stripe patterns for high frequency applications.
14
PII: DOI: Reference:
S0304-8853(16)32323-X http://dx.doi.org/10.1016/j.jmmm.2016.11.064 MAGMA62139
To appear in: Journal of Magnetism and Magnetic Materials Received date: 23 September 2016 Revised date: 9 November 2016 Accepted date: 14 November 2016 Cite this article as: Yuping Wu, Yong Yang, Fusheng Ma, Baoyu Zong, Zhihong Yang and Jun Ding, Microwave permeability of stripe patterned FeCoN thin f i l m , Journal of Magnetism and Magnetic Materials, http://dx.doi.org/10.1016/j.jmmm.2016.11.064 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Microwave permeability of stripe patterned FeCoN thin film Yuping Wua, Yong Yanga,*, Fusheng Maa, Baoyu Zonga, Zhihong Yanga, Jun Dingb a
Temasek Laboratories, National University of Singapore, 5A Engineering Drive 1, Singapore 117411. b
Department of Materials Science and Engineering, National University of Singapore, Singapore 117574. *Corresponding author. [email protected]
Abstract Magnetic stripe patterns are of great importance for microwave applications owing to their highly tunable microwave permeability by adjusting the geometrical dimensions. In this work, stripe patterned FeCoN films with 160 nm thickness are fabricated by using standard UV photolithography. Their microwave permeability are investigated systematically via both experiment and micromagnetic simulation. The good agreement between experimental and simulation results suggests that stripe width is crucial for the microwave magnetic properties of the stripe pattern. It is demonstrated by simulation that with increasing stripe width from 1 to 80 μm the initial permeability shows a continuous growth from about 8 to 322, whiles the resonance frequency drops dramatically from 18.7 GHz to 3.1 GHz at 4 μm gap size. Smaller gap size would result in slightly increased initial permeability due to larger magnetic volume ratio, accompanied by decreased resonance frequency because of stronger magnetostatic interaction. Moreover, the experimental investigation on stripe length effect indicates that the stripe length should be kept as long as possible to achieve uniform bulk resonance mode and high permeability value. Insufficient stripe length would result in low frequency edge mode and decayed bulk mode. This study could provide valuable guidelines on the selection of proper geometry dimensions of FeCoN stripe patterns for high frequency applications. Keywords: Microwave permeability; FeCoN; Stripe pattern; Micromagnetic simulation
1. Introduction 1
Patterned magnetic thin film has attracted much attention due to their wide applications in high speed data storage [1], inductor [2] and microwave devices (e.g. filter, phase shifter, antenna, absorber, etc.) [3-7]. Compared with unpatterned film, the patterned films could provide additional useful features due to rich variation of static and dynamic magnetic behaviors which is unachievable in continuous films[8]. Therefore, understanding the knowledge of different patterns are vital for both fundamental physics study and technical applications. To date, various patterns (e.g. stripe, ring, anti-dot, etc.) have been reported, exhibiting fascinating magnetic properties [9, 10]. From a magnetic point of view, complex permeability (μr=μ'-jμ") is a key parameter for high frequency applications. It determines not only the performance and efficiency but also the working frequency of the patterns. Generally, the permeability is required to be as high as possible to achieve high efficiency. Moreover, tunable resonance frequency is also an advantage because it could offer great flexibility for technical applications, which is usually achieved by inducing shape anisotropy or applying external magnetic field.[11, 12] Taken these requirements into account, stripe pattern is an excellent candidate for microwave application due to the following reasons: Firstly, the strong shape anisotropy of stripe could prefer uniform spin configuration at micro- or even larger size without applying external magnetic field, which is crucial for uniform ferromagnetic resonance and sharp absorption at the resonance frequency; secondly, the ferromagnetic resonance frequency can be tuned simply by adjusting the aspect ratio of the stripe; furthermore, the rectangular shape of individual stripe could allow densely packing to enhance magnetic volume ratio for further increasing permeability. As such, kinds of stripe patterns were investigated [13-15]. However, it is worth noting that so far there is a lack of systematic study on the dynamic properties of the stripe pattern. Furthermore, most of these works focus on permalloy (Fe20Ni80) because it is a perfect soft ferromagnetic material with negligible crystalline anisotropy for fundamental study [16-19]. For practical application, iron-cobalt (FeCo) alloy is acknowledged as the best soft ferromagnetic material because of high Curie temperature, low magnetocrystalline anisotropy and the highest saturation magnetization (Ms) with respect to other ferromagnetic materials [20, 21]. According to Acher’ law, large Ms could allow high permeability and resonance frequency [22]. However, high electrical conductivity of metal material suffers from serious eddy current under electromagnetic radiation, which is also called “skin effect” [23]. In recent 2
years, nitrogen doped FeCo alloy (FeCoN) emerges as a promising material for high frequency application because of both high magnetization and reduced conductivity [24, 25]. Therefore, FeCoN stripe pattern is of great interest and there is an urgent need to investigate its high frequency magnetic properties. Based on the above facts, we are devoted to conduct a systematic study on the high frequency permeability of FeCoN stripe patterns. The patterns are fabricated through a standard standard UV photolithography and lift-off method. Furthermore, an external magnetic field is applied during film deposition to induce an in-plane uniaxial anisotropy along the stripe direction. This is because the production of permeability and resonance frequency can be enhanced by several times due to the presence of the in-plane uniaxial anisotropy [24, 26]. The microwave permeability of the fabricated FeCoN stripe patterns with different stripe widths are measured. Meanwhile, micromagnetic simulation is performed for verification of the experimental results. The effects of gap size and stripe length are also discussed systematically.
2. Experimental and computation methods The soft magnetic FeCoN (Fe47Co45N8) thin films with thickness of ~160 nm were prepared by DC magnetron sputtering using FeCo target with the gas mixture of nitrogen and argon as the ambient gas on the Mica substrate. To get the sputtered thin film of high microwave qualities, the mica substrates were prepared by removing a top layer. A thin SiO2 layer was deposited on the mica before the deposition of FeCoN layer. The sputtering conditions, including the background pressure, sputter pressure, supplied power, and deposition rate, were 4.0 × 10−7 Torr, 3.0 mTorr, 250 W, and 25 nm/min, respectively. The concentration of N2 was controlled by the flow rate at 8%. During deposition, a 200 Oe magnetic field was applied to induce in-plane anisotropy [24]. Standard UV photolithography and lift-off were used to define large arrays of rectangular stripes. The overall patterned area of each array is 15 × 4 mm2 and the growth-induced easy axis of in-plane anisotropy was aligned along the length of the rectangles. The lithography process is shown in Fig. 1(a). As shown in the figure, it starts with the spin coating of photoresist (AZ1518). UV light is used 3
to expose the photoresist according to the designed pattern mask followed by development to remove exposed photoresist. Then magnetic thin films were deposited into the patterned photoresist by magnetron sputtering. Finally, the unexposed photoresist and overlaying magnetic thin films are liftoffed by acetone. The micromagnetic simulations were performed using MICROMAGUS package [27]. In this package, the total energy consists of four energy contributions: external field energy, magnetocrystalline anisotropy energy, exchange stiffness energy and demagnetizing field energy. The equilibrium state is found by minimizing the total free energy of the system. To simulate dynamic remagnetization processes, it used the Bogacki-Shampine version of the Runge-Kutta-23 method. The integration step size control to achieve the required dynamical accuracy (10−6) [28]. The magnetic parameters of FeCoN used in the micromagnetic simulation are listed as follows: saturation magnetization Ms=1600 emu/cc, exchange stiffness A=1×10-6 erg/cm, magnetocrystalline anisotropy constant K1= 4.4×105 erg/cm3, in-plane uniaxial anisotropy constant Ku= 4×104 erg/cm3 (describing the induced in-plane anisotropy field of about 50 Oe along stripe direction). The cell size is 20×20 nm2 in the plane of the film, which is treated as a single layer along normal direction. The Gilbert damping coefficient α was used as 0.015. Periodic boundary condition (PBC) was applied to imitate infinitely long and periodically arranged rectangular stripes. The simulation was performed at 0K, meaning the thermal effect is excluded. To simulate the permeability of the stripe patterns, a “sinc” function excitation field h0 was applied in the film plane while perpendicular to the stripe direction. [
( (
)] )
(1)
where amplitude hmax=10 Oe, f0 =30 GHz, and t0 = 10/2f0 ns. The excitation field given by the above has a constant power spectrum up to about 30 GHz to assure that system is excited by a constant strength below the frequency f0. The complex susceptibility is calculated by using the FFT technique [29]. Finally, the complex permeability could be then obtained by using
.
The film thickness was tested by a surface step profilometer. The patterned thin films were characterized by scanning electron microscope (SEM). Static magnetic properties were measured 4
using a vibrating sample magnetometer (VSM). The permeability frequency spectra from 0.5 to 8.0 GHz was characterized by a network analyzer (Agilent 5230A) using a shorted microstrip transmission-line perturbation fixture.
3. Results and discussions
Fig. 1. (a) Schematic illustration of lift-off process for the fabrication of FeCoN stripe patterns. (b-d) SEM images of stripe patterned FeCo thin film with stripe width of 30, 20 and 10 µm, respectively. Fig. 1(a) illustrates a standard lift-off photolithography method for the fabrication of FeCoN stripe patterns. The fabrication mainly includes spin coating of the photoresist (PR), mask loading and UV exposure, development, FeCoN film deposition by sputtering and subsequent lift-off process. It should be mentioned that during film deposition a 200 Oe magnetic field was applied along the strip direction to induce an in-plane uniaxial anisotropy, implying that the shape anisotropy coincides with the induced uniaxial anisotropy. This is to ensure a single domain structure at such micrometer stripe width. Using this method, FeCoN stripe pattern with different stripe width (i.e. 5, 10, 20 and 30 µm) are fabricated. The length of all the stripes are 4000 µm, meaning the stripe is “infinitely” long with 5
respect to the stripe width. Fig. 1(b-d) provide the scanning electron microscope (SEM) image of the as fabricated 30, 20 and 10 µm width FeCoN stripe patterns. As shown in the figures, the rectangular shaped stripes are densely arranged with a gap size of around 4 µm. The densely packing of the stripes would allow high magnetic volume ratio thus achieving high magnetic permeability. The effect of gap size will be discussed later.
Fig. 2. Room temperature hysteresis loops of (a) unpatterned FeCoN film, (b) 4000 × 30, (c) 4000 × 20, (d) 4000 × 10, (e) 4000 × 5 µm2 stripe patterns measured along easy axis (E.A.) and hard axis (H.A.). The E.A. coincides with the stripe direction. The measured hysteresis loops of the patterns with different stripe width are shown in Fig. 2. The loops of unpatterned film is also provided for comparison. For each sample, the loops are measured along the in-plane uniaxial easy axis (or stripe direction) and hard axis. It can be seen that the unpatterned film exhibits typical easy axis (E.A.) and hard axis (H.A.) type hysteresis loops, implying a well defined in-plane uniaxial anisotropy (equivalent anisotropy field is around 50 Oe). Such weak in-plane uniaxial anisotropy is crucial for achieving high permeability in magnetic thin films.[26, 30] After patterning, the loops changes dramatically. It can be seen in Fig. 2(b) that the 30 µm stripe width pattern exhibit multi-switching process when the field is along easy axis. Meanwhile, the 6
saturation field in hard axis increase obviously (>200 Oe). Further reduction in stripe width leads to the increase of coercivity in both directions (Fig. 2(b-e)), accompanied by much higher saturation fields. This is reasonable that the effective anisotropy of the stripe pattern mainly originates from the shape anisotropy. When the stripe is narrowed down, the shape anisotropy is enhanced accordingly, resulting in increased coercivity and saturation field. Since the permeability is strongly dependent on the anisotropy [30], the change of anisotropy would result in dramatic variation of microwave permeability of the FeCoN stripe patterns.
Fig. 3. Measured complex permeability of FeCoN film and stripe patterns with different stripe width. Fig. 3 plots the frequency dependent complex permeability of the FeCoN continuous film and stripe patterns measured in the frequency range of 1-8 GHz. It can be seen in Fig. 3(a) that the continuous film shows an initial permeability (here defined as real permeability value at 1 GHz) as high as 323. 7
Meanwhile, a broad resonance peak is observed at around 2.4 GHz (Fig. 3(b)), agreeing well with our previous results and other reported FeCo films with in-plane uniaxial anisotropy [24, 31]. The high permeability is attributed to the high Ms of the material and the well defined in-plane uniaxial anisotropy induced during film deposition. In comparison, the patterned film shows reduced permeability and increased resonance frequency, which is further pronounced at narrower stripe width. For 30, 20, 10, 5 µm stripe width patterns, the initial permeability is about 137, 111, 69, 18, while the resonance frequency is around 3.6, 4.0, 5.0, >8 GHz, respectively. The trends are agreed well with that reported on FeTaN stripe patterns [15]. In theory, the permeability is inversely proportional while the resonance frequency is proportional to the anisotropy [32]. Narrower stripe possess stronger shape anisotropy thus leading to lower permeability and higher resonance frequency. It is also noted in Fig. 3(b) that the patterns with 5 µm stripe width does not shows any resonance peak. This is because the shape anisotropy of the 5 µm stripe is so strong that the resonance peak is beyond the experimental frequency range, which will be confirmed by micromagnetic simulation in later section.
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Fig. 4. (a, b) Simulated complex permeability of FeCoN continuous film and stripe patterns with different stripe width; (c) comparison of resonance frequency and initial permeability between micromagnetic simulation and experimental results; (d) simulated resonance frequency and initial permeability of FeCoN stripe patterns with different stripe width and gap size. The dashed line shows the resonance frequency calculated by Kittel’s formula. In order to verify the experimental results, microwave permeability of the FeCoN film and patterns are simulated by using micromagnetic simulation. The results are provided in Fig. 4(a-b). For the unpatterned film, the simulated initial permeability and the resonance frequency is about 343 and 2.8 GHz, respectively, which are very close to the experimental values of 323 and 2.4 GHz (Fig. 3(b)). For the stripe patterns, the simulated initial permeability is about 169, 120, 61, 30 and the resonance frequency is around 4.1, 4.8, 6.6, 9.3 GHz for the 30, 20, 10, 5 µm stripe width patterns, respectively. It confirms that the resonance frequency of 5 µm pattern, which is 9.3 GHz, is beyond the experimental frequency range therefore undetected in Fig. 3(b). A direct comparison between the experimental and simulated results is drawn in Fig. 4(c). Apparently, the simulate permeability and resonance frequency are in a very good consistence with the experimental results. It proves the validity of both experimental measurement and micromagnetic simulation. To get an overview on the microwave permeability of the FeCoN stripe patterns, the simulation is further performed in a much broader range of stripe width from 1 to 80 µm at different gap sizes (1, 2 and 4 µm). Fig. 4(d) summarizes the simulated initial permeability and resonance frequency values. Take the 2 µm gap size for example, the initial permeability increases continuously from 8 to 322 when the stripe width is increased from 1 to 80 µm. By contrast, the resonance frequency drops significantly from 18.8 GHz to about 6 GHz when the stripe width is below 10 µm. Further increase of the stripe width only leads to slight drop to about 3.1 GHz at 80 µm. Very similar resonance frequencies are obtained when the gap sizes is changed to 1 or 4 µm. However, slight difference is observed when the stripe width is below 40 µm, where larger gap size shows slightly higher resonance frequency, implying that the magnetostatic interaction would lead to the decrease of resonance frequency. In order to further confirm this, the resonance frequency is also calculated by using Kittel’s formula [33]: 9
{[ where
(
is the angular resonance frequency,
] [
)
e
(
)
]}
(2)
is the external field (zero in this study), Ni (i=x, y, z)
is the demagnetizing factor describing the shape anisotropy of the rectangular stripe (see supplementary material). This formula could describe the resonance frequency of a single rectangular stripe where the magnetostatic interaction is excluded. This is equivalent to a stripe pattern with infinite gap size. The dashed curve in Fig. 4(d) shows the resonance frequency calculated using Kittel’s formula. It is evident that the trends is almost the same as the simulation results, but the values are about 2-4 GHz higher. It confirms that the magnetostatic interaction between the stripes would lead to decrease of the resonance frequency. This is because the demagnetizing field of the stripes is reduced when the interaction is taken into account [14]. The gap between the dashed curve and simulated resonance frequency gives exactly the enhancement of resonance frequency after completely removing the magnetic interaction in stripe arrays. With respect to the initial permeability, larger gap size would lead to the drop of the permeability value due to lower magnetic volume fraction.
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Fig. 5. (a,b) SEM images of the FeCoN stripe patterns with stripes length of 250 and 20 µm. (c,d) Measured complex permeability of stripe patterns with different stripe lengths (4000, 1000, 250, 20 µm). The stripe width and gap size is 20 and 4 µm, respectively. So far, the stripe width and gap size effect have been investigated. It should be emphasized that all the above results are acquired on “infinitely long” stripe arrays where the stripe is continuous along stripe direction. As one of important geometrical parameters, the strip length could also take great effect on the dynamic permeability of the stripe patterns. It has been confirmed in micromagnetic simulation that short stripes width would cause low frequency resonance mode resulting from the splay of the spin configuration near the end of the stripe [13]. To verify this on the stripe patterned FeCoN film, stripe patterns consisting of short stripes are fabricated. The stripe length is reduced from 4000 to 1000, 250 and 20 µm with a constant width and gap size of 20 and 4 µm, respectively. Fig. 5(a-b) shows the SEM images of stripe patterns with stripes length of 250 and 20 µm. It can be seen in Fig. 5(b) that the patterns turn to be two-dimensional regular arrangement of short stripe island. The measured complex permeability of these patterns are provided in Fig. 5(c-d). It indicates that the 1000 µm stripe length pattern shows nearly the same permeability value compared with the “continuous” 4000 µm patterns. However, it is noticed that the amplitude of resonance peak drops slightly. When the stripe length is reduced to 250 µm, the initial permeability still remains almost the same, while the resonance peak decays obviously. Moreover, a noticeable resonance peak occurs at around 2.3 GHz. This is in a good agreement with the results found in permalloy stripes, where the low frequency peak is attributed to the splay of the spin configuration near the end of the stripe or so called “edge mode” in comparison to the “bulk mode” of the prior peak [13]. The “edge mode” get enhanced when the stripe length is 20 µm (squire dot arrays), associated with the split of prior peak and the drop of permeability. This could be attributed to the lack of shape anisotropy, which results in complex magnetic domain structures and corresponding resonance modes [34]. The above results provide experimental evidence that long stripe length would benefit high permeability and uniform resonance mode.
4. Conclusions 11
In summary, microwave permeability of stripe patterned FeCoN thin films have been investigated systematically. It is found in both experiment and simulation that the initial permeability and resonance frequency can be tuned in a wide range by changing width of the stripes. Simulation on gap size effect reveals that larger gap size would lead to the drop of permeability and increased resonance frequency because of lower magnetic volume fraction and weaker magnetostatic interaction, respectively. Furthermore, it has been demonstrated that the stripe length should be kept as long as possible to avoid low frequency resonance mode thus achieving uniform bulk resonance mode as well as high permeability. These results would provide valuable guidance on high frequency application of FeCoN stripes patterns, such as microwave absorption.
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Hightlights
Magnetic stripe patterns are of great importance for microwave applications owing to their highly tunable microwave permeability by adjusting the geometrical dimensions. This work presents a systematic study on microwave permeability of FeCoN stripe patterned thin film. Through experimental investigation as well as micromagnetic simulation, geometrical dimensions (i.e. stripe width, gap size, stripe length) of the stripe pattern are systematically optimized for achieving high permeability and tunable resonance frequency. Several important conclusions has been obtained. The valuable results obtained in this study could provide guidelines on the selection of proper geometrical dimensions of FeCoN stripe patterns for high frequency applications.
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