- Email: [email protected]
Visualization of magnetic domains by near-field scanning microwave microscope
ARTICLE IN PRESS Ultramicroscopy 109 (2009) 889–893
Contents lists available at ScienceDirect
Ultramicroscopy journal homepage: www.elsevier.com/locate/ultramic
Visualization of magnetic domains by near-field scanning microwave microscope Kiejin Lee a,, Harutyun Melikyan a, Arsen Babajanyan a, Tigran Sargsyan a, Jongchel Kim a, Seungwan Kim a, Barry Friedman b a b
Department of Physics and Interdisciplinary Program of Integrated Biotechnology, Sogang University, Seoul 121-742, Republic of Korea Department of Physics, Sam Houston State University, Huntsville, TX 77341, USA
a r t i c l e in fo
PACS: 68.37.d 68.37.Uv 07.79.Pk 06.30.Ka 07.10.Cm
abstract A near-field scanning microwave microscope (NSMM) system was used for the investigation of magnetic properties of a hard disk (HD) under an external magnetic field. To demonstrate local microwave characterization of magnetic domains by NSMM, we scanned the HD surface by measuring the microwave reflection coefficient S11 of the NSMM at an operating frequency near 4.4 GHz. The NSMM offers a reliable means for quantitative measurement of magnetic domains with high spatial resolution and sensitivity. & 2009 Elsevier B.V. All rights reserved.
Keywords: Magnetic domains Hard disk Permeability Near-field Microwaves
1. Introduction Recently, methods for monitoring magnetic domains have been investigated by semiconductor Hall bar sensors, magnetic force microscopy (MFM), and the magneto-optical microscope (MOM) [1–5]. However, as magnetic storage technology is extended to the micro-scale distance and operating frequencies of integrated circuit reach the microwave regime, there is a strong need to quantitatively measure local magnetic properties at microwave frequencies and to develop tools to characterize magnetic imaging for micro-scale magnetic devices. This requires local measurement of magnetic properties such as the magnetic permeability. Electromagnetic measurements using a near-field microwave scanning microscope (NSMM) has been emerging in the past few years to study electromagnetic properties with micro-scale resolution at microwave wavelengths [6–10]. To achieve such a goal, the probe tip of the NSMM has to be improved from the conventional metal tip and developed as a quantitative metrology tool with micro-scale resolution. For a conventional metal probe tip consisting of an apex and a taper, however, it is difficult to quantitatively calculate the relationship between the probe
geometry and the sample properties. Thus, a new design of the probe tip with a distance regulation system is imperative for quantitative near-field microwave imaging. In general, high spatial resolution in near-field microscope images is determined primarily by aperture size and high sensitivity can be achieved by the quality factor of the resonator [11,12]. Thus, we used a highquality dielectric resonator coupled to a tapered probe tip utilizing the distance control sensor provided by a tuning fork. In this work, we report a NSMM system with improved sensitivity and high spatial resolution for the magnetic domain investigation of a 1.2 Gb magnetic hard disk (HD) platter under an external magnetic field at an operating frequency f ¼ 4.4 GHz. To demonstrate local microwave characterization of magnetic domains by NSMM we scanned the HD surface to visualize magnetic domains. The dependence of the magnetic domains on the external magnetic field could be imaged by measuring the microwave reflection coefficient S11 and interpreted by the transmission line theory [13]. This high-resolution measurement of the magnetic domains has a great potential for investigating the magnetic profile with high sensitivity.
2. Experimental Corresponding author. Tel.: +82 2 705 8429; fax: +82 2 715 8429.
E-mail address: [email protected] (K. Lee). 0304-3991/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.ultramic.2009.03.013
Fig. 1(a) shows the experimental setup of the NSMM [10]. In order to boost the sensitivity of the probe, we employed a
ARTICLE IN PRESS 890
K. Lee et al. / Ultramicroscopy 109 (2009) 889–893
Carbon (5 nm) CoCrPtTa (20 nm)
Feedback system Computer
Ru (2 nm) CoMo (30 nm)
Lock-in amp. 3D stage
NiP (40 nm)
Microwave resonator Tuning fork
Al substrate
PZT Probe-tip Sample x
Zms
S11 z
Z0
Network analyzer
Zm
Zs
S11
y tm Fig. 1. (a) A schematic of the NSMM system and (b) the multilayer structure, and (c) two-layer model of hard disk.
1800
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(b)
900
Permeability
S11 (dB)
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1500
(a)
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-40 600
S11 (dB)
dielectric resonator and tuned the resonance cavity to match the impedance of 50 O. The probe tip was oriented perpendicular to the sample surface and the other end of the tip was directly connected to a coupling loop in the dielectric resonator. We used a tapered stainless-steel probe tip with about 1 mm aperture size. The apex angle of tip was 201 and the height was about 1 cm. The probe tip-sample distance was fixed at about 10 nm by using a quartz tuning fork feedback control system. To drive the tuning fork, an AC voltage was applied to one contact on the tuning fork at its resonance frequency using the oscillator of a lock-in amplifier. The resulting current from the other contact was measured by using the current input of the same lock-in amplifier. The output from the lock-in amplifier was fed into the feedback system to control the tip-sample distance (z direction) using a motorized stage combined with a piezo electric tube (PZT) driver with a 5 nm step. The movement along the x and y directions provided by a 2-dimensional motorized stage gives 100 nm minimal resolution for both directions. We measured the reflection coefficient S11 of the resonator of the NSMM at an operating frequency of about 4.4 GHz for the TM010 mode. A magnetic field was applied parallel to the 1.2 Gb magnetic hard disk platter surface. Measurements were done at room temperature. The unloaded Q factor of the microwave resonator was about 24,000. We measured the reflection coefficient S11 using a network analyzer (Agilent 8722 ES). The 1.2 Gb desk platter was manufactured by standard multilayer technology and consisted of the following layers; Carbon/ CoCrPtTa/Ru/CoMo/NiP on the bulk aluminum substrate [14] as shown in Fig. 1(b). In recent years, quaternary CoCrPtTa alloy thin films have been extensively studied for ultrahigh-density magnetic recording media. The soft magnetic recording layer, CoCrPtT is a longitudinal magnetic recording medium with a particle distributed structure (granular structure) [15]. The thickness of the soft magnetic layer was 20 nm, and using this thickness, magnetic layer near this value, magnetic anisotropy on the surface can be stabilized. The coercive force of the CoCrPtTa soft magnetic layer is preferably not less than 1 Oe and not higher than 100 Oe [16]. We used the magnetic field source with Hp100 Oe magnitude applied parallel to the sample surface.
-42
300 0
-34
20
40 60 80 Magnetic Field (Oe)
100
-36
200
400
600
800
1000
1200
Permeability Fig. 2. Microwave reflection coefficient S11 vs. the relative permeability mm of the CoCrPtTa soft magnetic layer. The inset shows (a) the measured microwave reflection coefficient of the HD and (b) the estimated relative permeability of the CoCrPtTa layer as a function of applied external magnetic field. The solid line is a guide for the eye.
3. Theory The main microwave reflection takes place from the Al substrate due to the very small thicknesses of the other layers. Thus in the theoretical calculations for the hard disk we used a two-layer model: the soft magnetic layer CoCrPtTa with varying intrinsic impedance due to the external magnetic field and the Al substrate modeled as a half-infinite medium (the thickness of bulk Al substrate, ts ¼ 1 mm, is much bigger than its skin depth at microwave frequencies, ds ¼ 2.5 mm). Fig. 1(c) shows the two-layer
ARTICLE IN PRESS K. Lee et al. / Ultramicroscopy 109 (2009) 889–893
891
layer system. The total impedance of this system can be expressed as
Here Z0 is the characteristic impedance of the probe (Z0 ¼ 50 O), Zms is the complex intrinsic impedance of the hard disk
where mm, sm, and tm are the relative permeability, the conductivity, and the thickness of the magnetic layer, respectively.
Z s þ jZ m km t m 1 ffi 2pf m0 mm t m Z m þ jZ s km t m ss ds 1 2pf m0 mm sm t 2m , þ þj
Z ms ¼ Z m
ss ds
-33
-33
-34
-34
S11 (dB)
S11 (dB)
model schematic diagram of the multilayer HD. The reflection coefficient S11 dependence on magnetic field-induced impedance changes can be derived using the standard transmission line theory [13] and is given by, assuming impedance matching between the probe tip and the microwave source, Z ms Z 0 . (1) S11 ¼ 20log Z ms þ Z 0
-35
-36
2
4 6 Distance (µm)
8
10
-33
-33
-34
-34
S11 (dB)
S11 (dB)
-35
-36
0
-35
-36
0
2
4 6 Distance (µm)
8
10
0
2
4 6 Distance (µm)
8
10
0
2
4 6 Distance (µm)
8
10
-35
-36
0
2
4 6 Distance (µm)
8
10
-33
-33
-34
-34
S11 (dB)
S11 (dB)
ss ds
-35
-36
-35
-36
0
2
4 6 Distance (µm)
8
10
Fig. 3. The line scans of a hard disk under external magnetic field of: (a) 0 Oe, (b) +40 Oe, (c) +50 Oe, (d) +100 Oe, (e) 0 Oe, and (f) 100 Oe.
(2)
ARTICLE IN PRESS 892
K. Lee et al. / Ultramicroscopy 109 (2009) 889–893
m0 is the permeability of free space (4p 107 H/m) and f is the
4. Results and discussion
operation frequency (4.4 GHz). The complex impedance Zm and of soft magnetic layer CoCrPtTa determined as wave vector k mffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi p pare ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Z m ¼ ð1 þ jÞ= sm =ðpf m0 mm Þ and km ¼ ð1 þ jÞ= 1=ðpf m0 mm sm Þ. Zs is the impedance of the substrate where the electromagnetic characteristics are caused by the aluminum layer, thus Zs ¼ (1+j)/ dsss, and ss is the conductivity (3.8 107 S/m) and ds is the skin depth of Al (2.5 mm at 4.4 GHz). In principle, the microwave reflection coefficient S11 change is caused by the changes of the magnetic relative permeability mm and the electric conductivity sm of the CoCrPtTa magnetic layer. In the present work, only the permeability effect due to applied external magnetic field was important, the conductivity of CoCrPtTa layer was not changed by the applied magnetic field.
Fig. 2 shows the microwave reflection coefficient S11 dependence on the relative magnetic permeability of the CoCrPtTa soft magnetic layer. The above theory predicts that the microwave reflection coefficient S11 linearly decreases as the magnetic field increases due to the increased relative permeability mm. The inset shows (a) the measured microwave reflection coefficient and (b) the estimated relative permeability changes due to variation of the external magnetic field. The initial (without external magnetic field) value of the relative magnetic permeability of CoCrPtTa layer was taken to be 325. An important metric for future applications is sensitivity of measurements, which requires characterization of measurement
Fig. 4. (a) 2D and (b) 3D NSMM images of the hard disk surface with surface area of 14 14 mm.
Fig. 5. The simulated images for (a), (b) the H field and (c), (d) the E field of the electromagnetic near-field interaction between probe tip and the CoCrPtTa soft magnetic layer under external magnetic field intensities (a), (c) 0 Oe and (b), (d) 100 Oe.
ARTICLE IN PRESS K. Lee et al. / Ultramicroscopy 109 (2009) 889–893
noise. Over the non-saturated range (up to 50 Oe external magnetic field), the empirical dependence of the changes of reflection coefficient DS11 on magnetic field variation DH can be approximated as linear with slope of DS11/DH ¼ 104 Oe1. For our system at maximum sensitivity the root-mean-square (rms) statistical noise in the linear scale was about 5.4 107 [17]. Thus, the signal-to-noise ratio (SNR) was SNR ¼ 20log (104/ 5.4 107) ¼ 45 dB. To demonstrate the ability of NSMM and to characterize hard disk magnetic properties, we scan the HD surface area at the different external magnetic field intensities: (a) 0 Oe, (b) +40 Oe, (c) +50 Oe, (d) +100 Oe, (e) 0 Oe, and (f) 100 Oe. Fig. 3 shows a microwave reflection coefficient S11 line scan of a HD sample. The scan spatial resolution was 1 mm. The microwave line scan showed a clear difference between the magnetic domain and the substrate levels. The microwave reflection coefficient is directly related to intrinsic impedance, which is directly related to the magnetic permeability mm. Thus, the magnetic domains of magnetic layer in hard disk can be inferred by the intensity of the reflection coefficient at the resonance frequency. The line scans showed the saturation above the threshold magnetic field intensity of about 50 Oe. Under 100 Oe external fields, the line scans showed the same intensity under the parallel external magnetic field to the surface. As the external magnetic field intensity changes in the range of 40–50 Oe, the reflection coefficient S11 is significantly decreased as a function of applied external magnetic field intensity with further saturation 50–100 Oe. We directly imaged the microwave reflection coefficient S11 of the HD at 4.4 GHz. Fig. 4 shows (a) the two-dimensional (2D) and (b) the three-dimensional (3D) NSMM images of a hard disk surface with surface area of 14 14 mm under 100 Oe external magnetic field applied parallel to the surface. Note that the reflection coefficient changes originate from the changes of the magnetic permeability of the CoCrPtTa magnetic layer due to its change under external magnetic field. These results indicate that NSMM can provide a microwave image with high spatial resolution. To visualize the electromagnetic field distribution, we imaged the near-field interaction between the probe tip and CoCrPtTa soft magnetic layer using a computer simulation program Ansoft/ HFSS, as shown in Fig. 5. As the external magnetic field intensity increased, the relative permeability of CoCrPtTa soft magnetic layer increased and the H component of the electromagnetic field between the probe tip and HD decreased in the magnetic layer and increased in air as shown in Fig. 5(a) and (b). At the same time the E component of the electromagnetic field was not changed as shown in Fig. 5(c) and (d).
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5. Conclusions We demonstrate the measurement of electromagnetic properties of magnetic layers in a 1.2 Gb hard disk platter using a NSMM. As the magnetization changed, the intensity of the reflection coefficient S11 varied. The electromagnetic properties of a hard disk were estimated by measuring the microwave reflection coefficient S11, in particular, the hard disk magnetic domains hysteresis behavior was obtained using NSMM. We developed a model of how the reflection coefficient S11 depended on the magnetic permeability of the magnetic layer by the standard transmission line theory. These results clearly show the sensitivity and usefulness of this scanning microprobe microscope for investigating the electromagnetic properties of magnetic samples.
Acknowledgements This work was supported by Special Research Grant Sogang University, and Seoul Research and Business Development Program (10816). References [1] K. Lai, M. Ji, N. Leindecker, M. Kelly, Z. Shen, Rev. Sci. Instrum. 78 (2007) 063702. [2] G. Bottoni, D. Candolfo, J. Magn. Magn. Mater. 316 (2007) e149. [3] J. Saenz, N. Garcia, P. Grutter, E. Mayer, H. Heinxelmann, R. Wiesendanger, L. Rosenthaler, H.R. Hidber, H. Guntherodt, J. Appl. Phys. 62 (1987) 4293. [4] M. Kryder, R. Gustafson, J. Magn. Magn. Mater. 287 (2005) 449. [5] A. Sandhu, H. Masuda, A. Oral, S. Bending, Jpn. J. Appl. Phys. 40 (2001) 4321. [6] A. Imtiaz, S. Anlage, J. Barry, J. Melngailis, Appl. Phys. Lett. 90 (2007) 143106. [7] M. Abu-Teir, M. Golosovsky, D. Davidov, A. Frenkel, H. Goldberger, Rev. Sci. Instrum. 72 (2001) 2073. [8] A. Babajanyan, K. Lee, E. Lim, T. Manaka, M. Iwamoto, B. Friedman, Appl. Phys. Lett. 90 (2007) 182104. [9] M. Park, H. Yoo, H. Yoo, S. Na, S. Kim, K. Lee, B. Friedman, E. Lim, M. Iwamoto, Thin Solid Films 499 (2006) 318. [10] J. Kim, M.S. Kim, K. Lee, J. Lee, D. Cha, B. Friedman, Meas. Sci. Technol. 14 (2003) 7. [11] M. Tabib-Azar, Y. Wang, IEEE Trans. Microwave Theory Tech. 52 (2004) 971. [12] S. Kalinin, A. Gruverman, Scanning Probe Microscopy—Electrical and Electromechanical Phenomena at the Nanoscale, Springer, New York, 2007. [13] D. Pozar, Microwave Engineering, Addison-Wesley, New York, 1990. [14] E. Abarra, P. Glijer, H. Kisker, I. Okarnoto, T. Suzuki, J. Magn. Magn. Mater. 175 (1997) 148. [15] K. Sin, P. Glijer, J. Sivertsen, J. Judy, J. Magn. Magn. Mater. 155 (1996) 209. [16] www.wipo.int/pctdb/en/wo.jsp?IA ¼ JP2005006228&DISPLAY ¼ DESC. [17] B. Friedman, M. Gaspar, S. Kalachikov, K. Lee, R. Levisky, G. Shen, H. Yoo, J. Am. Chem. Soc. 127 (2005) 9666.
Contents lists available at ScienceDirect
Ultramicroscopy journal homepage: www.elsevier.com/locate/ultramic
Visualization of magnetic domains by near-field scanning microwave microscope Kiejin Lee a,, Harutyun Melikyan a, Arsen Babajanyan a, Tigran Sargsyan a, Jongchel Kim a, Seungwan Kim a, Barry Friedman b a b
Department of Physics and Interdisciplinary Program of Integrated Biotechnology, Sogang University, Seoul 121-742, Republic of Korea Department of Physics, Sam Houston State University, Huntsville, TX 77341, USA
a r t i c l e in fo
PACS: 68.37.d 68.37.Uv 07.79.Pk 06.30.Ka 07.10.Cm
abstract A near-field scanning microwave microscope (NSMM) system was used for the investigation of magnetic properties of a hard disk (HD) under an external magnetic field. To demonstrate local microwave characterization of magnetic domains by NSMM, we scanned the HD surface by measuring the microwave reflection coefficient S11 of the NSMM at an operating frequency near 4.4 GHz. The NSMM offers a reliable means for quantitative measurement of magnetic domains with high spatial resolution and sensitivity. & 2009 Elsevier B.V. All rights reserved.
Keywords: Magnetic domains Hard disk Permeability Near-field Microwaves
1. Introduction Recently, methods for monitoring magnetic domains have been investigated by semiconductor Hall bar sensors, magnetic force microscopy (MFM), and the magneto-optical microscope (MOM) [1–5]. However, as magnetic storage technology is extended to the micro-scale distance and operating frequencies of integrated circuit reach the microwave regime, there is a strong need to quantitatively measure local magnetic properties at microwave frequencies and to develop tools to characterize magnetic imaging for micro-scale magnetic devices. This requires local measurement of magnetic properties such as the magnetic permeability. Electromagnetic measurements using a near-field microwave scanning microscope (NSMM) has been emerging in the past few years to study electromagnetic properties with micro-scale resolution at microwave wavelengths [6–10]. To achieve such a goal, the probe tip of the NSMM has to be improved from the conventional metal tip and developed as a quantitative metrology tool with micro-scale resolution. For a conventional metal probe tip consisting of an apex and a taper, however, it is difficult to quantitatively calculate the relationship between the probe
geometry and the sample properties. Thus, a new design of the probe tip with a distance regulation system is imperative for quantitative near-field microwave imaging. In general, high spatial resolution in near-field microscope images is determined primarily by aperture size and high sensitivity can be achieved by the quality factor of the resonator [11,12]. Thus, we used a highquality dielectric resonator coupled to a tapered probe tip utilizing the distance control sensor provided by a tuning fork. In this work, we report a NSMM system with improved sensitivity and high spatial resolution for the magnetic domain investigation of a 1.2 Gb magnetic hard disk (HD) platter under an external magnetic field at an operating frequency f ¼ 4.4 GHz. To demonstrate local microwave characterization of magnetic domains by NSMM we scanned the HD surface to visualize magnetic domains. The dependence of the magnetic domains on the external magnetic field could be imaged by measuring the microwave reflection coefficient S11 and interpreted by the transmission line theory [13]. This high-resolution measurement of the magnetic domains has a great potential for investigating the magnetic profile with high sensitivity.
2. Experimental Corresponding author. Tel.: +82 2 705 8429; fax: +82 2 715 8429.
E-mail address: [email protected] (K. Lee). 0304-3991/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.ultramic.2009.03.013
Fig. 1(a) shows the experimental setup of the NSMM [10]. In order to boost the sensitivity of the probe, we employed a
ARTICLE IN PRESS 890
K. Lee et al. / Ultramicroscopy 109 (2009) 889–893
Carbon (5 nm) CoCrPtTa (20 nm)
Feedback system Computer
Ru (2 nm) CoMo (30 nm)
Lock-in amp. 3D stage
NiP (40 nm)
Microwave resonator Tuning fork
Al substrate
PZT Probe-tip Sample x
Zms
S11 z
Z0
Network analyzer
Zm
Zs
S11
y tm Fig. 1. (a) A schematic of the NSMM system and (b) the multilayer structure, and (c) two-layer model of hard disk.
1800
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-30 1200 -38
(b)
900
Permeability
S11 (dB)
-32
1500
(a)
-36
-40 600
S11 (dB)
dielectric resonator and tuned the resonance cavity to match the impedance of 50 O. The probe tip was oriented perpendicular to the sample surface and the other end of the tip was directly connected to a coupling loop in the dielectric resonator. We used a tapered stainless-steel probe tip with about 1 mm aperture size. The apex angle of tip was 201 and the height was about 1 cm. The probe tip-sample distance was fixed at about 10 nm by using a quartz tuning fork feedback control system. To drive the tuning fork, an AC voltage was applied to one contact on the tuning fork at its resonance frequency using the oscillator of a lock-in amplifier. The resulting current from the other contact was measured by using the current input of the same lock-in amplifier. The output from the lock-in amplifier was fed into the feedback system to control the tip-sample distance (z direction) using a motorized stage combined with a piezo electric tube (PZT) driver with a 5 nm step. The movement along the x and y directions provided by a 2-dimensional motorized stage gives 100 nm minimal resolution for both directions. We measured the reflection coefficient S11 of the resonator of the NSMM at an operating frequency of about 4.4 GHz for the TM010 mode. A magnetic field was applied parallel to the 1.2 Gb magnetic hard disk platter surface. Measurements were done at room temperature. The unloaded Q factor of the microwave resonator was about 24,000. We measured the reflection coefficient S11 using a network analyzer (Agilent 8722 ES). The 1.2 Gb desk platter was manufactured by standard multilayer technology and consisted of the following layers; Carbon/ CoCrPtTa/Ru/CoMo/NiP on the bulk aluminum substrate [14] as shown in Fig. 1(b). In recent years, quaternary CoCrPtTa alloy thin films have been extensively studied for ultrahigh-density magnetic recording media. The soft magnetic recording layer, CoCrPtT is a longitudinal magnetic recording medium with a particle distributed structure (granular structure) [15]. The thickness of the soft magnetic layer was 20 nm, and using this thickness, magnetic layer near this value, magnetic anisotropy on the surface can be stabilized. The coercive force of the CoCrPtTa soft magnetic layer is preferably not less than 1 Oe and not higher than 100 Oe [16]. We used the magnetic field source with Hp100 Oe magnitude applied parallel to the sample surface.
-42
300 0
-34
20
40 60 80 Magnetic Field (Oe)
100
-36
200
400
600
800
1000
1200
Permeability Fig. 2. Microwave reflection coefficient S11 vs. the relative permeability mm of the CoCrPtTa soft magnetic layer. The inset shows (a) the measured microwave reflection coefficient of the HD and (b) the estimated relative permeability of the CoCrPtTa layer as a function of applied external magnetic field. The solid line is a guide for the eye.
3. Theory The main microwave reflection takes place from the Al substrate due to the very small thicknesses of the other layers. Thus in the theoretical calculations for the hard disk we used a two-layer model: the soft magnetic layer CoCrPtTa with varying intrinsic impedance due to the external magnetic field and the Al substrate modeled as a half-infinite medium (the thickness of bulk Al substrate, ts ¼ 1 mm, is much bigger than its skin depth at microwave frequencies, ds ¼ 2.5 mm). Fig. 1(c) shows the two-layer
ARTICLE IN PRESS K. Lee et al. / Ultramicroscopy 109 (2009) 889–893
891
layer system. The total impedance of this system can be expressed as
Here Z0 is the characteristic impedance of the probe (Z0 ¼ 50 O), Zms is the complex intrinsic impedance of the hard disk
where mm, sm, and tm are the relative permeability, the conductivity, and the thickness of the magnetic layer, respectively.
Z s þ jZ m km t m 1 ffi 2pf m0 mm t m Z m þ jZ s km t m ss ds 1 2pf m0 mm sm t 2m , þ þj
Z ms ¼ Z m
ss ds
-33
-33
-34
-34
S11 (dB)
S11 (dB)
model schematic diagram of the multilayer HD. The reflection coefficient S11 dependence on magnetic field-induced impedance changes can be derived using the standard transmission line theory [13] and is given by, assuming impedance matching between the probe tip and the microwave source, Z ms Z 0 . (1) S11 ¼ 20log Z ms þ Z 0
-35
-36
2
4 6 Distance (µm)
8
10
-33
-33
-34
-34
S11 (dB)
S11 (dB)
-35
-36
0
-35
-36
0
2
4 6 Distance (µm)
8
10
0
2
4 6 Distance (µm)
8
10
0
2
4 6 Distance (µm)
8
10
-35
-36
0
2
4 6 Distance (µm)
8
10
-33
-33
-34
-34
S11 (dB)
S11 (dB)
ss ds
-35
-36
-35
-36
0
2
4 6 Distance (µm)
8
10
Fig. 3. The line scans of a hard disk under external magnetic field of: (a) 0 Oe, (b) +40 Oe, (c) +50 Oe, (d) +100 Oe, (e) 0 Oe, and (f) 100 Oe.
(2)
ARTICLE IN PRESS 892
K. Lee et al. / Ultramicroscopy 109 (2009) 889–893
m0 is the permeability of free space (4p 107 H/m) and f is the
4. Results and discussion
operation frequency (4.4 GHz). The complex impedance Zm and of soft magnetic layer CoCrPtTa determined as wave vector k mffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi p pare ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Z m ¼ ð1 þ jÞ= sm =ðpf m0 mm Þ and km ¼ ð1 þ jÞ= 1=ðpf m0 mm sm Þ. Zs is the impedance of the substrate where the electromagnetic characteristics are caused by the aluminum layer, thus Zs ¼ (1+j)/ dsss, and ss is the conductivity (3.8 107 S/m) and ds is the skin depth of Al (2.5 mm at 4.4 GHz). In principle, the microwave reflection coefficient S11 change is caused by the changes of the magnetic relative permeability mm and the electric conductivity sm of the CoCrPtTa magnetic layer. In the present work, only the permeability effect due to applied external magnetic field was important, the conductivity of CoCrPtTa layer was not changed by the applied magnetic field.
Fig. 2 shows the microwave reflection coefficient S11 dependence on the relative magnetic permeability of the CoCrPtTa soft magnetic layer. The above theory predicts that the microwave reflection coefficient S11 linearly decreases as the magnetic field increases due to the increased relative permeability mm. The inset shows (a) the measured microwave reflection coefficient and (b) the estimated relative permeability changes due to variation of the external magnetic field. The initial (without external magnetic field) value of the relative magnetic permeability of CoCrPtTa layer was taken to be 325. An important metric for future applications is sensitivity of measurements, which requires characterization of measurement
Fig. 4. (a) 2D and (b) 3D NSMM images of the hard disk surface with surface area of 14 14 mm.
Fig. 5. The simulated images for (a), (b) the H field and (c), (d) the E field of the electromagnetic near-field interaction between probe tip and the CoCrPtTa soft magnetic layer under external magnetic field intensities (a), (c) 0 Oe and (b), (d) 100 Oe.
ARTICLE IN PRESS K. Lee et al. / Ultramicroscopy 109 (2009) 889–893
noise. Over the non-saturated range (up to 50 Oe external magnetic field), the empirical dependence of the changes of reflection coefficient DS11 on magnetic field variation DH can be approximated as linear with slope of DS11/DH ¼ 104 Oe1. For our system at maximum sensitivity the root-mean-square (rms) statistical noise in the linear scale was about 5.4 107 [17]. Thus, the signal-to-noise ratio (SNR) was SNR ¼ 20log (104/ 5.4 107) ¼ 45 dB. To demonstrate the ability of NSMM and to characterize hard disk magnetic properties, we scan the HD surface area at the different external magnetic field intensities: (a) 0 Oe, (b) +40 Oe, (c) +50 Oe, (d) +100 Oe, (e) 0 Oe, and (f) 100 Oe. Fig. 3 shows a microwave reflection coefficient S11 line scan of a HD sample. The scan spatial resolution was 1 mm. The microwave line scan showed a clear difference between the magnetic domain and the substrate levels. The microwave reflection coefficient is directly related to intrinsic impedance, which is directly related to the magnetic permeability mm. Thus, the magnetic domains of magnetic layer in hard disk can be inferred by the intensity of the reflection coefficient at the resonance frequency. The line scans showed the saturation above the threshold magnetic field intensity of about 50 Oe. Under 100 Oe external fields, the line scans showed the same intensity under the parallel external magnetic field to the surface. As the external magnetic field intensity changes in the range of 40–50 Oe, the reflection coefficient S11 is significantly decreased as a function of applied external magnetic field intensity with further saturation 50–100 Oe. We directly imaged the microwave reflection coefficient S11 of the HD at 4.4 GHz. Fig. 4 shows (a) the two-dimensional (2D) and (b) the three-dimensional (3D) NSMM images of a hard disk surface with surface area of 14 14 mm under 100 Oe external magnetic field applied parallel to the surface. Note that the reflection coefficient changes originate from the changes of the magnetic permeability of the CoCrPtTa magnetic layer due to its change under external magnetic field. These results indicate that NSMM can provide a microwave image with high spatial resolution. To visualize the electromagnetic field distribution, we imaged the near-field interaction between the probe tip and CoCrPtTa soft magnetic layer using a computer simulation program Ansoft/ HFSS, as shown in Fig. 5. As the external magnetic field intensity increased, the relative permeability of CoCrPtTa soft magnetic layer increased and the H component of the electromagnetic field between the probe tip and HD decreased in the magnetic layer and increased in air as shown in Fig. 5(a) and (b). At the same time the E component of the electromagnetic field was not changed as shown in Fig. 5(c) and (d).
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5. Conclusions We demonstrate the measurement of electromagnetic properties of magnetic layers in a 1.2 Gb hard disk platter using a NSMM. As the magnetization changed, the intensity of the reflection coefficient S11 varied. The electromagnetic properties of a hard disk were estimated by measuring the microwave reflection coefficient S11, in particular, the hard disk magnetic domains hysteresis behavior was obtained using NSMM. We developed a model of how the reflection coefficient S11 depended on the magnetic permeability of the magnetic layer by the standard transmission line theory. These results clearly show the sensitivity and usefulness of this scanning microprobe microscope for investigating the electromagnetic properties of magnetic samples.
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