Journal of the Korean Physical Society, Vol. 51, No. 6, December 2007, pp. 20262030
Ferromagnetic Resonance Behaviors of Integrated CoPdAlO Magnetic Film on Coplanar Waveguide Ki Hyeon Kim Department of Physics, Yeungnam University, Gyeongsan 712-749
Masahiro Yamaguchi Department of Electrical and Communication Engineering, Tohoku University, Sendai 980-8579, Japan
Jongryoul Kim Department of Metallurgy and Materials Science, Hanyang University, Ansan 425-791
(Received 30 August 2007) CoPdAlO magnetic lms, whose ferromagnetic resonance (FMR) frequency was 4.2 GHz, were deposited on a coplanar waveguide (CPW) by using a RF magnetron sputtering method and were then patterned with a size of 2000 m 50 m 2 m (l w t). In the case of the patterned magnetic lms on CPW, the calculated and the measured FMR frequencies were shifted to around 7 GHz, and the measured characteristic impedance of the CPW was also changed by around 7 GHz. In addition, the propagation wavelength shortening e ect was enhanced by about 17 % up to 7 GHz compared with that of the CPW without magnetic lm. These results show that patterned magnetic lms can be used as band-pass lters and broad-band EMI noise lters in the RF and the microwave regions. PACS numbers: 75.90.+w, 85.70.k, 07.55.-w Keywords: Magnetic lm devices, Permeability, Noise suppression, Ferromagnetic resonance I. INTRODUCTION
Magnetic thin lms have been widely used for electromagnetic high-frequency devices, such as radio frequency (RF) noise suppressors, RF thin lm inductors, phase shifters, and tunable lters [1{3]. As the switching frequency in electronic devices and components has increased, these magnetic thin lms have been required to operate at higher frequencies ranges up to a few GHz with controllable switching loss. In order to control the switching and the loss of magnetic lms in GHz frequency range, many research groups have focused on tailoring the magnetic properties. For instance, micropatterning and exchange coupling using antiferromagnetic layers have been investigated to get high magnetic shape anisotropy [4{7]. When magnetic materials are employed in electronic devices and transmission lines, the change in the characteristic impedance should be minimized to avoid signal distortion in the broadband frequency region from the MHz up to the GHz range. To evaluate the switching frequency and the microwave absorption of the in E-mail:
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tegrated magnetic thin lms, transmission lines, such as micro-strip lines and coplanar waveguides (CPWs), have been widely used as broadband GHz frequency switching circuits. If the structure and the geometry of a microwave power absorption system is to be made by using a transmission line, for applications to electromagnetic devices it is critical to characterize the characteristic impedance (Zc ) of the integrated magnetic lms on the signal transmission line. In particular, a CPW shows broad pass-band frequency characteristics up to a few tenth of GHz without any signi cant change in the characteristic impedance. Therefore, we systematically investigated the microwave absorption behaviors for patterned CoPdAlO magnetic lms on a CPW. II. EXPERIMENT
The CPW had a designed characteristic impedance of 50 with a 50-m-wide, 2200-m-long and 3-m-thick signal line on a 7059 corning glass (permittivity, "r = 5.84) substrate, which was calculated using the Muller and Hilberg equations [8]. As Figure 1 (a) shown, the vertical structure is composed of a 2-m-thick magnetic
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Fig. 1. (a) Schematic of the patterned magnetic lm on CPW, (b) real photo images of the electroplated CPW and (c) the patterned magnetic lm on a CPW. Fig. 3. (a) Calculated and measured permeabilities as functions of the frequency for the unpatterned CoPdAlO lm, and (b) the calculated permeability of the patterned CoPdAlO magnetic lm (2000 m 50 m 2 m: l w t).
Fig. 2. Magnetization curve for the easy and the hard axes of the CoPdAlO magnetic lm.
lm /2-m-thick polyimide/3-m-thick Cu CPW/1-mmthick glass substrate stack, which was fabricated by using a micro-fabrication process. The Cu/Ti seed layers were deposited by rf sputtering to thicknesses of 100 nm and 10 nm, respectively; and then, Cu transmission lines were deposited by using an electroplating method with an electrolyte of CuSO4, H2 SO4 and DI water. The seed layers were nally etched by ion milling. Granular CoPdAlO magnetic lms were deposited by RF magnetron sputtering on Ti/polyimide layers. In order to improve the adhesion between the magnetic lm and the insulator layer, , the polyimide layer, we dei.e.
posited 10-nm-thick Ti on the polyimide layers. After photoresist patterning, the magnetic lm (2000 m 50 m 2 m: l w t) was etched by ion milling. In order to remove the polyimide on the contact pad, we used reactive ion etching (RIE) with O2 gas was used for the etching. Figures 1 (b) and (c) show the photographs of the electroplated CPW and of the patterned magnetic lm on the CPW, respectively. High-frequency measurements were performed from 0.1 GHz to 10 GHz by using a HP 8720D network analyzer with two GSG (ground-signal-ground) type microprobes. III. RESULTS AND DISCUSSION
Figure 2 shows the magnetization curve of the easy and the hard directions for a CoPdAlO magnetic lm. The saturation magnetization (4Ms), the magnetic anisotropy eld (Hk ) and the resistivity () are measured to be about 10 kG, 230 Oe and 500 cm, respectively. The relative permeability for a change of frequency for the CoPdAlO lm is shown in Figure 3 (a). The ferromagnetic resonance frequency (fF MR ) of the unpatterned CoPdAlO magnetic lm occurs around 4.2 GHz,
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which is in good agreement with the values calculated using the Landau-Lifshitz-Gilbert (LLG) equations [9]. The fF MR and the e ective permeability (eff ) of the thin magnetic lms are generally governed by the demagfF MR =
q 2 (Hk + (Ny
Nx )4Ms )(Hk + (Nx Nz )4Ms );
where fF MR , , Hk and 4Ms denote, respectively, the ferromagnetic resonance frequency, the gyromagnetic ratio, the magnetic anisotropy eld, and the saturation magnetization. The demagnetizing factors, Nx ; Ny and Nz , are determined by the dimensions of the magnetic lm [10]. Figure 3 (b) shows the calculated permeability of the patterned CoPdAlO magnetic lm (2000 m 50 m 2 m: l w t), including the demagnetizing e ect. As the gure shown, the fF MR is shifted up to 7.1 GHz in comparison with that of the unpatterned magnetic lm. In order to analyze the e ects of signal attenuation and propagation by integration of magnetic materials on a transmission line, in this study are used the scattering matrix (S21 and S11; S -parameter), which is de ned in relation to the incident and the re ected voltage. The Sparameter charterization of the transmission line is modeled as shown in Figure 1 (a). A piece of the interconnect with a charcteristic impedance Zc and propagation constant is placed in the S -parameter test system (Z0 = 50
). The incident powers a1 and a2 and re ected powers b1 and b2 are measured, then, the resulting scattering matrix is determined [12]. The S-parameter responses measured from a lossy unmatched transmission line are represented by [12{14] 2 2
l 2 Zc Z0 [s] = D1 (Zc 2ZZ0c Z) sinh (Zc2 Z02) sinh l ; (2) 0 s where Ds = 2(Zc Z0 ) cosh l(Zc2 + Z02) sinh l The S -parameter matrix is converted to ABCD parameters that incorporate the interconnect propagation constant (!) and the impedance Z (!) more explicitly. The equivalent ABCD matrix is cosh l Zc sinh l [ABCD] = sinh l cosh l ; (3) Z c
netizing eld due to the change in the magnetic lm's shape. The formula for the resonance condition, with the consideration of the demagnetization condition, is given by [9{11]
The relationship between the S -parameter and the ABCD matrix is [13] A = (1 + S11 S22 S )=(2S21 ) (4) B = (1 + S11 + S22 + S )Z0 =(2S21 ) (5) C = (1 S11 S22 + S )=(2S21 Z0 ) (6) D = (1 S11 + S22 S )=(2S21 ); (7)
(1)
where D = S11S22 S21S12, Eqs. (2)-(4) are combined to yield: 1 S112 + S212 K 1; (8) e l = 2S21 where 2 2 + 1)2 2 )2 1=2 ( S11 S21 (2 S11 K= ; (9) (2S21)2 (1 + S11)2 S212 ; (10) (1 S11)2 S212 Once and Z are determined from the Eqs. (5) and (7), then the standard transmission line relationships are p
= (R + j!L)(G + j!C ) = + j ; (11) Zc2 = Z02
Z=
s
(R + j!L) (G + j!C ) ;
(12)
Then, R = Ref Zc g; L = Imf Zc g=!; (13) G = Ref =Zc g; C = Imf =Zc g=!; (14) where R; G; L; C; and are the serial resistance, conductance, inductance, capacitance, attenuation constant, and phase (=2=), respectively. In order to verify the microwave behaviors of the patterned magnetic lm on a CPW, we measure the transmission signal attenuation (S21) and re ection (S11) are measured up to 10 GHz for the patterned magnetic lm on a CPW and for a CPW without a magnetic lms as shown in Figure 4. The behaviors of the transmission signals for the magnetic lm on a CPW are similar to those of the CPW up to 1 GHz without noticeable attenuation. The transmission signal of a CoPdAlO magnetic lm on a CPW is, however, abruptly attenuated at 7.2 GHz by up to {1.3 dB and then the magnitude of attenuation decrease. The frequency of the maximized signal attenuation is in good agreement with the calculated ferromagnetic resonance frequency for the patterned magnetic lm, as shown in Figure 3 (b). This indicates that
Ferromagnetic Resonance Behaviors of Integrated CoPdAlO { Ki Hyeon Kim et
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Fig. 4. Signal attenuation (S21 ) and re ection (S11 ) of the patterned CoPdAlO lm on a CPW in comparison with those of the CPW without a magnetic lm.
Fig. 6. (a) Attenuation constant, and (b) propagation wavelength, of the patterned CoPdAlO lm on a CPW in comparison with those of the CPW without a magnetic lm.
Fig. 5. Frequency pro les of (a) the characteristic impedance and of (b) the inductance and the capacitance of the patterned CoPdAlO lm on a CPW in comparison with those of the CPW without a magnetic lm.
the signal attenuation is caused by the microwave absorption due to the ferromagnetic resonance of the magnetic lm. Figure 5 (a) shows the characteristic impedance (Zc ) of the CoPdAlO magnetic lm on a CPW with the increments of frequency up to 10 GHz in comparison with that of a CPW. The characteristic impedance generally decreases slightly with increasing frequency. For the integrated CoPdAlO magnetic lm on a CPW, the charac-
teristic impedance abruptly changes around 7 GHz due to microwave power absorption. The behaviors of the characteristic impedance can be deduced from the ratio of the inductance to the capacitance with changing frequency, which is given by Zc (L=C )1=2. Figure 5 (b) shows the changes in the inductance (L) and the capacitance (C ) with increasing frequency, which is calculated by from Eq. (10) by using the measured S-parameter. This shows that the inductance changes at a speci c frequency of around 7 GHz, but the capacitance does not change with increasing frequency in spite of the insertion of the magnetic lm. The attenuation constant ( ) can be extracted from the propagation constant (= + j ) by using the measured S -parameter (S21; S11), as shown in Figure 6 (a). The magnitudes of the signal attenuation do not change up to 3 GHz. Above 3 GHz, these signals are drastically attenuated by up to 6.8 dB/cm around 7 GHz. This indicates that the CoPdAlO granular magnetic lm is one of good candidate materials for noise lter using power absorption around 7 GHz. The propagation wavelength () with increasing frequency in comparison with that of a CPW obtained by using Eqs. (8) (10) is shown in Figure 6 (b). The wavelengths changes with insertion of the magnetic lm on a CPW. For example, the wavelength at 1 GHz is
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obtained about 12.7 cm (17 %), which is shorter than that of a CPW (15.3 cm). This indicates that propagation wavelength shortening can e ectively reduce the size of electronic circuits and devices. IV. CONCLUSIONS
The ferromagnetic resonance behaviors of patterned CoPdAlO granular magnetic lms on CPW were analyzed. The transmission signals were attenuated, and the characteristic impedance was changed at a speci c frequency of around 7 GHz, at which frequency the ferromagnetic resonance of the patterned CoPdAlO magnetic lms occurred. In addition, the propagation wavelength was decreased by about 17 % up to 7 GHz in comparison with that of a CPW. These results show that patterned magnetic lms on CPW can be used as band-stop lters and broad-band EMI noise lters in the RF and microwave regions. ACKNOWLEDGMENTS
The authors thank Dr. Shigehiro Ohnuma. This research was supported by the Nano R & D program through the Korea Science and Engineering Foundation funded by the Ministry of Science & Technology.
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