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Experimental study of microwave permeability of thin Fe films
Journal of Magnetism and Magnetic Materials 258–259 (2003) 195–197
Experimental study of microwave permeability of thin Fe films Igor T. Iakubov*, Andrei N. Lagarkov, Sergei A. Maklakov, Alexei V. Osipov, Konstantin N. Rozanov, Ilya A. Ryzhikov, Nikolai A. Simonov, Sergei N. Starostenko Institute for Theoretical and Applied Electromagnetics RAS, Izhorskaya ul. 13/19, 125413 Moscow, Russia
Abstract The paper deals with the microwave permeability of thin Fe films deposited on a mylar substrate under different technological conditions. The permeability is measured in the 100 MHz–10 GHz range using coaxial techniques and below 100 MHz using the single coil and single wire techniques. Magnetically uniform films were deposited by ion beam sputtering and revealed the most suitable magnetic properties. Magnetron deposited films have inhomogeneous structure. The measured magnetic spectra have a form that is suitable to FMR. They can be modified by certain technological means such as nitrogen admixture and film thickness variation. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Ferromagnetic film; Gyromagnetic permeability; Microwave application
1. Introduction Materials exhibiting high microwave permeability are of practical importance for a number of applications. Among all magnetic materials, thin films with an inplane magnetic anisotropy are the most promising in obtaining high values of microwave permeability. The values can be much larger than those obtained from Snoek’s law that is valid for bulk magnetics [1]: m ¼ 1 þ g4pMs =f ;
ð1Þ
where g is the gyromagnetic ratio, Ms is the saturation magnetization, f is the operating frequency. This is the evidence that thin magnetic films are advantageous as high permeable materials for microwave applications. This was the motivation for the experimental study of microwave permeability of thin films reported below. The films under investigation were deposited with iron in Ar atmosphere with N2 admixture. The high value of the saturation magnetization of Fe is the key point for obtaining high microwave permeability. In addition, the microscopic structure of iron films that determines high *Corresponding author. Tel.: +095-485-83-22; fax: +095484-26-33. E-mail address: [email protected] (I.T. Iakubov).
frequency magnetic properties is greatly dependent on peculiarities of the manufacturing process. This can provide wide opportunities for tailoring the permeability dispersion curves. As the films under study are highly conducting, the effect of eddy currents can make the microwave performance of films noticeably worse. To avoid this, the thickness of the magnetic film should not exceed the skin depth, which is about a micron or less at microwaves. Nitrogen dissolved in iron leads to increase of specific resistance [2]. It lowers further eddy current losses and becomes one of the reasons of FMR shift to higher frequencies.
2. Experimental procedure The film samples of 0.1–2 mm thickness were deposited on 10 mm mylar substrates by either the ion beam sputtering (IBS) or the DC magnetron sputtering (MS) techniques. The permeability was measured in the frequency range of 100 MHz–10 GHz by the coaxial technique [3,4]. The technique is suitable for films deposited on a flexible substrate and involves winding the film into a hollow cylindrical sample that fits the
0304-8853/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 2 ) 0 1 0 4 5 - 4
I.T. Iakubov et al. / Journal of Magnetism and Magnetic Materials 258–259 (2003) 195–197
section of a standard coaxial measuring cell. After this, transmission line methods are exploited to measure the permeability of the resulting coaxial-shaped sample. The technique has proved to be useful in measuring magnetic spectra of highly conducting materials in a wide range of microwave frequencies [5]. At frequencies below 100 MHz, the permeability was measured by the single coil and single wire techniques. The different structure of magnetic field inside a coil and around a single conductor allows both components of the in-plane permeability to be measured to estimate the film anisotropy.
200
µ
196
100
0
3. Experimental results and discussion
where ms is the static permeability, fr and f0 are the relaxation and resonance frequencies. In the frames of the same model an inequality is obtained for a film possessing in-plane magnetic anisotropy [6]: Z N p kA ¼ m00 ðf Þf df ð3Þ ðg4pMs Þ2 p1: 2 0 If a film is measured with microwave field parallel to the hard axis the coefficient kA ¼ 1: Value of kA helps to understand film quality. In Fig. 1, magnetic spectra of a typical film deposited by MS without nitrogen are given. Measured permeabilities belong to the composite film ‘‘mylar+magnetic’’, mcomp : Metal permeabilities m are taken out simply because of the absence of skin effect [7]. The permeability curves are of the pure relaxation type that points to large losses. It is consistent with the low specific resistance measured by the contact method, ¼ 18 mOhm cm. In Fig. 2 magnetic spectra of a film containing a noticeable percent of nitrogen are given. The spectra are typical for the FMR resonance, ¼ 40 mOhm cm. Variations of nitrogen content shift FMR to a desirable frequency. However, high content of nitrogen can lower Ms [8]. The film thickness d ¼ 0:3 mm is smaller than the skin depth dE0:7 mm. Hence, the thickness may be somewhat increased. In this case, the absorption line is broadened due to increase of the eddy current damping proportionally d 2 [9]. The film constant kA E0:16 is considerably lower than the best value for the isotropic in-plane sample, kA ¼
1.0
10.0
f, GHz Fig. 1. Complex permeability of a typical film deposited by MS without nitrogen, d ¼ 0:25 mm. Filled and empty dots denote m0 and m00 ; solid and dashed lines—fitting curves.
80
40 µ
Measured magnetic spectra are found to have complex shapes for the most of films under investigation. However, in the first approach magnetic permeability can be described in the frames of the Landau–Lifschitz FMR model: ms mðf Þ ¼ 1 þ ; ð2Þ 1 iðf =fr Þ ðf =f0 Þ2
0.1
0
-40 0.1
1.0 f, GHz
10.0
Fig. 2. Complex permeability of the film deposited by MS containing a noticeable percent of nitrogen, d ¼ 0:3 mm.
0:5: One can suppose the magnetic anisotropy to be inhomogeneous importantly and perpendicular magnetic anisotropy to be possible. In Fig. 3 the AFM image demonstrates the structure of the film surface. The sample morphology is non-uniform considerably. To suppress the magnetic inhomogeneity the variation of the permeability spectra under biasing magnetic field collinear with the cylinder axis was examined. With the field increase the absorption line is shifted to higher frequencies and narrowed as it should be. The coefficient kA approaches unity under 510 Oe even for the very anisotropic samples.
I.T. Iakubov et al. / Journal of Magnetism and Magnetic Materials 258–259 (2003) 195–197
197
magnetic spectra are given, d ¼ 0:26 mm. Coefficient kA E1 point to the high uniformity of the in-plane anisotropy. From the low frequency measurement made with the single coil and single wire techniques a quantitative evaluation for in-plane uniformity is provided as the ratio of magnetic susceptibilities measured in two perpendicular directions. This value for IBS films amounts to 20.
4. Conclusion
Fig. 3. AFM image of film surface. Film deposited by MS, d ¼ 1:8 mm. Vertical scale points to the thickness variation.
750
Acknowledgements The authors are grateful to Mrs. M. Sedova for her extensive help in creating the AFM images. The work is supported by the Russ. Found. Basic Res., Grants 02-02-16707 and 01-02-17962.
500
µ
The data obtained reveal a variety of types of permeability dispersion curves exhibited by Fe films. Both location and width of the magnetic absorption lines can be controlled as shown by certain technological means. The maximal microwave permeabilities were attained by IBS. Additional technological means are required to improve MS films.
250
References
0
-250 0.1
1.0
10.0
f, GHz Fig. 4. Complex permeability of the film deposited by IBS with nitrogen, d ¼ 0:26 mm.
The best films were deposited by IBS. The film formation proceeds much more slowly. And at every moment the state is close to equilibrium. In Fig. 4, the
[1] J.L. Snoek, Physica A 15 (1948) 207. [2] Y.M. Kim, IEEE Trans. Magn. 37 (2001) 2288. [3] K.N. Rozanov, N.A. Simonov, A.V. Osipov, J. Commun. Technol. Electron. 47 (2002) 229. [4] A.P. Vinogradov, S.N. Starostenko, J. Commun. Technol. Electron. 43 (1998) 455. [5] O. Acher, J. Magn. Magn. Mater. 136 (1994) 269. [6] O. Acher, A.L. Adenot, Phys. Rev. B 62 (2000) 11324. [7] S.M. Rytov, Sov. Phys. JETP 2 (1956) 466. [8] S.C. Byeon, F. Liu, G.J. Mankey, IEEE Trans. Magn. 37 (2001) 1770. [9] A.G. Gurevich, G.A. Melkov, Magnetic Oscillation and Waves, Nauka, Moscow, 1994 (in Russian).
Experimental study of microwave permeability of thin Fe films Igor T. Iakubov*, Andrei N. Lagarkov, Sergei A. Maklakov, Alexei V. Osipov, Konstantin N. Rozanov, Ilya A. Ryzhikov, Nikolai A. Simonov, Sergei N. Starostenko Institute for Theoretical and Applied Electromagnetics RAS, Izhorskaya ul. 13/19, 125413 Moscow, Russia
Abstract The paper deals with the microwave permeability of thin Fe films deposited on a mylar substrate under different technological conditions. The permeability is measured in the 100 MHz–10 GHz range using coaxial techniques and below 100 MHz using the single coil and single wire techniques. Magnetically uniform films were deposited by ion beam sputtering and revealed the most suitable magnetic properties. Magnetron deposited films have inhomogeneous structure. The measured magnetic spectra have a form that is suitable to FMR. They can be modified by certain technological means such as nitrogen admixture and film thickness variation. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Ferromagnetic film; Gyromagnetic permeability; Microwave application
1. Introduction Materials exhibiting high microwave permeability are of practical importance for a number of applications. Among all magnetic materials, thin films with an inplane magnetic anisotropy are the most promising in obtaining high values of microwave permeability. The values can be much larger than those obtained from Snoek’s law that is valid for bulk magnetics [1]: m ¼ 1 þ g4pMs =f ;
ð1Þ
where g is the gyromagnetic ratio, Ms is the saturation magnetization, f is the operating frequency. This is the evidence that thin magnetic films are advantageous as high permeable materials for microwave applications. This was the motivation for the experimental study of microwave permeability of thin films reported below. The films under investigation were deposited with iron in Ar atmosphere with N2 admixture. The high value of the saturation magnetization of Fe is the key point for obtaining high microwave permeability. In addition, the microscopic structure of iron films that determines high *Corresponding author. Tel.: +095-485-83-22; fax: +095484-26-33. E-mail address: [email protected] (I.T. Iakubov).
frequency magnetic properties is greatly dependent on peculiarities of the manufacturing process. This can provide wide opportunities for tailoring the permeability dispersion curves. As the films under study are highly conducting, the effect of eddy currents can make the microwave performance of films noticeably worse. To avoid this, the thickness of the magnetic film should not exceed the skin depth, which is about a micron or less at microwaves. Nitrogen dissolved in iron leads to increase of specific resistance [2]. It lowers further eddy current losses and becomes one of the reasons of FMR shift to higher frequencies.
2. Experimental procedure The film samples of 0.1–2 mm thickness were deposited on 10 mm mylar substrates by either the ion beam sputtering (IBS) or the DC magnetron sputtering (MS) techniques. The permeability was measured in the frequency range of 100 MHz–10 GHz by the coaxial technique [3,4]. The technique is suitable for films deposited on a flexible substrate and involves winding the film into a hollow cylindrical sample that fits the
0304-8853/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 2 ) 0 1 0 4 5 - 4
I.T. Iakubov et al. / Journal of Magnetism and Magnetic Materials 258–259 (2003) 195–197
section of a standard coaxial measuring cell. After this, transmission line methods are exploited to measure the permeability of the resulting coaxial-shaped sample. The technique has proved to be useful in measuring magnetic spectra of highly conducting materials in a wide range of microwave frequencies [5]. At frequencies below 100 MHz, the permeability was measured by the single coil and single wire techniques. The different structure of magnetic field inside a coil and around a single conductor allows both components of the in-plane permeability to be measured to estimate the film anisotropy.
200
µ
196
100
0
3. Experimental results and discussion
where ms is the static permeability, fr and f0 are the relaxation and resonance frequencies. In the frames of the same model an inequality is obtained for a film possessing in-plane magnetic anisotropy [6]: Z N p kA ¼ m00 ðf Þf df ð3Þ ðg4pMs Þ2 p1: 2 0 If a film is measured with microwave field parallel to the hard axis the coefficient kA ¼ 1: Value of kA helps to understand film quality. In Fig. 1, magnetic spectra of a typical film deposited by MS without nitrogen are given. Measured permeabilities belong to the composite film ‘‘mylar+magnetic’’, mcomp : Metal permeabilities m are taken out simply because of the absence of skin effect [7]. The permeability curves are of the pure relaxation type that points to large losses. It is consistent with the low specific resistance measured by the contact method, ¼ 18 mOhm cm. In Fig. 2 magnetic spectra of a film containing a noticeable percent of nitrogen are given. The spectra are typical for the FMR resonance, ¼ 40 mOhm cm. Variations of nitrogen content shift FMR to a desirable frequency. However, high content of nitrogen can lower Ms [8]. The film thickness d ¼ 0:3 mm is smaller than the skin depth dE0:7 mm. Hence, the thickness may be somewhat increased. In this case, the absorption line is broadened due to increase of the eddy current damping proportionally d 2 [9]. The film constant kA E0:16 is considerably lower than the best value for the isotropic in-plane sample, kA ¼
1.0
10.0
f, GHz Fig. 1. Complex permeability of a typical film deposited by MS without nitrogen, d ¼ 0:25 mm. Filled and empty dots denote m0 and m00 ; solid and dashed lines—fitting curves.
80
40 µ
Measured magnetic spectra are found to have complex shapes for the most of films under investigation. However, in the first approach magnetic permeability can be described in the frames of the Landau–Lifschitz FMR model: ms mðf Þ ¼ 1 þ ; ð2Þ 1 iðf =fr Þ ðf =f0 Þ2
0.1
0
-40 0.1
1.0 f, GHz
10.0
Fig. 2. Complex permeability of the film deposited by MS containing a noticeable percent of nitrogen, d ¼ 0:3 mm.
0:5: One can suppose the magnetic anisotropy to be inhomogeneous importantly and perpendicular magnetic anisotropy to be possible. In Fig. 3 the AFM image demonstrates the structure of the film surface. The sample morphology is non-uniform considerably. To suppress the magnetic inhomogeneity the variation of the permeability spectra under biasing magnetic field collinear with the cylinder axis was examined. With the field increase the absorption line is shifted to higher frequencies and narrowed as it should be. The coefficient kA approaches unity under 510 Oe even for the very anisotropic samples.
I.T. Iakubov et al. / Journal of Magnetism and Magnetic Materials 258–259 (2003) 195–197
197
magnetic spectra are given, d ¼ 0:26 mm. Coefficient kA E1 point to the high uniformity of the in-plane anisotropy. From the low frequency measurement made with the single coil and single wire techniques a quantitative evaluation for in-plane uniformity is provided as the ratio of magnetic susceptibilities measured in two perpendicular directions. This value for IBS films amounts to 20.
4. Conclusion
Fig. 3. AFM image of film surface. Film deposited by MS, d ¼ 1:8 mm. Vertical scale points to the thickness variation.
750
Acknowledgements The authors are grateful to Mrs. M. Sedova for her extensive help in creating the AFM images. The work is supported by the Russ. Found. Basic Res., Grants 02-02-16707 and 01-02-17962.
500
µ
The data obtained reveal a variety of types of permeability dispersion curves exhibited by Fe films. Both location and width of the magnetic absorption lines can be controlled as shown by certain technological means. The maximal microwave permeabilities were attained by IBS. Additional technological means are required to improve MS films.
250
References
0
-250 0.1
1.0
10.0
f, GHz Fig. 4. Complex permeability of the film deposited by IBS with nitrogen, d ¼ 0:26 mm.
The best films were deposited by IBS. The film formation proceeds much more slowly. And at every moment the state is close to equilibrium. In Fig. 4, the
[1] J.L. Snoek, Physica A 15 (1948) 207. [2] Y.M. Kim, IEEE Trans. Magn. 37 (2001) 2288. [3] K.N. Rozanov, N.A. Simonov, A.V. Osipov, J. Commun. Technol. Electron. 47 (2002) 229. [4] A.P. Vinogradov, S.N. Starostenko, J. Commun. Technol. Electron. 43 (1998) 455. [5] O. Acher, J. Magn. Magn. Mater. 136 (1994) 269. [6] O. Acher, A.L. Adenot, Phys. Rev. B 62 (2000) 11324. [7] S.M. Rytov, Sov. Phys. JETP 2 (1956) 466. [8] S.C. Byeon, F. Liu, G.J. Mankey, IEEE Trans. Magn. 37 (2001) 1770. [9] A.G. Gurevich, G.A. Melkov, Magnetic Oscillation and Waves, Nauka, Moscow, 1994 (in Russian).