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Variation of magnetic properties along the film thickness in perpendicular thin film media investigated by optical method
Journal of Magnetism and Magnetic Materials 235 (2001) 120–125
Variation of magnetic properties along the film thickness in perpendicular thin film media investigated by optical method Shin Saito*,1, Yoshihito Hatta, Migaku Takahashi Department of Electronics Engineering, Graduate School of Engineering, Tohoku University, Aoba-yama 05, Sendai 980-8579, Japan
Abstract Local variation of magnetic properties along the film thickness for CoNiCrTa perpendicular thin film media is discussed based on the magneto-optical analysis. It is found that the thickness of the bottom layer, which has low coercivity, is roughly estimated to be about 10 nm. Furthermore, the hysteresis loop of the bottom layer can be calculated by using magneto-optical multilayer model. r 2001 Elsevier Science B.V. All rights reserved. Keywords: Perpendicular thin film media; Polar Kerr rotation; Wavelength dependence; Light incident side; Magneto-optical multilayer model
1. Introduction Static magnetic properties, such as coercive force Hc and squareness S; of perpendicular recording media have been evaluated from hysteresis loops measured by using a vibrating sample magnetometer (VSM) and a magneto-optical polar Kerr equipment (PKE). Comparing magnetic properties obtained by VSM with the one obtained by PKE, for some samples, significant difference in the value of Hc (X1 kOe) and S (X0.15) was observed. This phenomenon is well known [1,2], but has not been comprehensively discussed from a viewpoint of local magnetic properties along the film thickness. In this paper, the variation of *Corresponding author. Department of Electronics Engineering, Graduate School of Engineering, Tohoku University, Aoba-yama 05, Sendai 980-8579, Japan. Tel.: +81-22-2177134; fax: +81-22-263-9402. E-mail address: [email protected] (S. Saito). 1 Research Fellow of the Japan Society for the Promotion of Science.
magnetic properties along the film thickness for perpendicular thin film media is discussed based on the results from the magneto-optical analysis.
2. Experimental procedure Co(67 at%)Ni(13 at%)Cr(16 at%)Ta(4 at%) perpendicular thin films were prepared by DC sputtering using the so-called ultra-clean sputtering system [3]. The same magnetic composition was used in longitudinal media in our previous study [4]. Tempered glass with the thickness of 0.64 mm and composed silica glass with the thickness of 0.1 mm were used as substrates. The films were fabricated at the substrate temperature of 2501C under Ar gas pressure of 4 mTorr. The thickness of the magnetic film, dmag ; was varied from 5 to 75 nm. The magnetic hysteresis loop was observed by VSM (swept field; 100 Oe/s) and PKE (stepped field; 7 s/each field). Polar Kerr hysteresis loop was examined from both the film surface side
0304-8853/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 0 3 7 0 - 5
S. Saito et al. / Journal of Magnetism and Magnetic Materials 235 (2001) 120–125
and from the substrate side, with wavelength in the region of 400–1000 nm. To obtain a hysteresis loop from polar Kerr rotation measured from the substrate side, background Faraday rotation coming from the glass substrate was subtracted. Complex refractive index of the film, nþik; was evaluated by the rotating-analyzer ellipsometry. The difference between complex refractive index of right- and left-polarized light for the magnetic film, DnþiDk; was consistently determined from n; k; experimentally obtained saturation polar Kerr sat: rotation and ellipticity, ysat: K and ZK ; by applying the matrix method [5].
3. Results and discussion 3.1. Measurements of hysteresis loop by PKE In this section, problems to measure hysteresis loops by using PKE with various wavelengths are explained. Fig. 1 shows spectra of saturation polar sat: Kerr rotation, ysat: K ; and ellipticity, ZK ; for a CoNiCrTa film (50 nm) measured by an incident light from the film surface side. No marked magneto-optical transition can be observed from the spectra. Measurement of a hysteresis loop by polar Kerr ellipticity at around 920 nm is found to be difficult, since the value of Zsat: K becomes zero. Therefore, in this study, we used the hysteresis loops obtained by polar Kerr rotation to evaluate magnetic properties for all the media. Fig. 2(a) shows wavelength dependence of hysteresis loops obtained by magneto-optical rotation
Fig. 1. Spectra of saturation polar Kerr rotation, ysat: K ; and ellipticity, Zsat: K ; measured from the film surface side for CoNiCrTa film (50 nm) directly deposited on glass substrate.
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Fig. 2. Changes in hysteresis loops of magneto-optical rotation against wavelength measured from the substrate side for CoNiCrTa films (50 nm). The films are fabricated on (a) tempered glass with the thickness of 0.64 mm and (b) composed silica glass with the thickness of 0.1 mm.
measured from the substrate side for a CoNiCrTa film (50 nm) deposited on tempered glass. For any loops, Faraday rotation coming from the substrate overlaps with Kerr rotation of the magnetic film. In the case that Faraday rotation is extremely larger than Kerr rotation (at the wavelength of 420 nm, for example), it becomes hard to evaluate the hysteresis loop of the magnetic film even if the subtraction of the Faraday rotation is done. Therefore, to decrease Faraday rotation, composed silica glass with the thickness of 0.1 mm was also used as a substrate. Fig. 2(b) shows changes in hysteresis loops of magneto-optical rotation against wavelength measured from the substrate side for a CoNiCrTa film (50 nm) deposited on composed silica glass substrate. Compared with Fig. 2(a), Faraday rotation in Fig. 2(b) is reduced for each wavelength, and it becomes possible to evaluate the hysteresis loop of the magnetic film by subtracting the Faraday rotation. Therefore, composed silica glass with the thickness of 0.1 mm was used, in the case that wavelength dependence of the hysteresis loop was taken into account. Fig. 3 shows changes in polar Kerr hysteresis loops for CoNiCrTa films (50 nm) on Ti underlayer with various thicknesses, dunder ; measured by the incident light of 990 nm from the substrate side. With increasing dunder ; saturation polar Kerr rotation gradually decreases. This is caused by exponential damping of amplitude of the incident light during transmission through Ti underlayer. In our PKE with the incident light from the substrate side at 990 nm, hysteresis loops for a
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Fig. 3. Changes in polar Kerr hysteresis loops for CoNiCrTa films (50 nm) on Ti underlayer with various thicknesses measured by incident light with the wavelength of 990 nm from the substrate side.
Fig. 4. Coercivities for CoNiCrTa thin films with various film thicknesses deposited on (a) glass substrate and (b) Ti underlayer measured by PKE with the wavelength of 990 nm (solid line: film surface side, broken line: substrate side). Coercivities measured by VSM are also shown by dashed chain line.
CoNiCrTa film on Ti underlayer with dunder p15 nm can be reliably evaluated. In the following experiment, the thickness of Ti underlayer was fixed at 7.5 nm, where the coercive force of the magnetic film takes maximum against dunder : 3.2. Analysis of perpendicular magnetic properties along the film thickness In Figs. 4(a) and (b), perpendicular coercive force, Hc ; for CoNiCrTa films prepared without and with Ti underlayer is plotted against the magnetic film thickness, dmag: measured by PKE at
990 nm. Hcfilm corresponds to the Hc measured from the film surface side and Hcsub: corresponds to the Hc measured from the substrate side. Hc values measured by VSM (HcVSM ) are also shown for comparison. For the films with dmag: p25 nm shown in Fig. 4(a), the slight difference in Hc between Hcfilm and Hcsub: can be found. With increasing dmag: more than 25 nm, it is clearly seen that Hcfilm is larger than Hcsub: ; and the difference between Hcfilm and Hcsub: is 1.5 kOe at dmag: =75 nm. HcVSM shows mean value between Hcfilm and Hcsub: : These facts mean that there exist magnetic layers with high Hc near the film surface side and with low Hc near the substrate side. The behavior of the dependence of Hc on dmag: shows the same tendency for the films with Ti underlayer shown in Fig. 4(b). Polar Kerr hysteresis loops reflect the information of magnetization of a magnetic film within the penetration depth from the interface of the light incident side. If the penetration depth can be varied, we can obtain the information of variation of magnetic properties along the film thickness. In this study, we tried to change the penetration depth by changing the wavelength of the incident light. As a first step, in order to confirm the relationship between the penetration depth and the wavelength, the calculation of Kerr hysteresis loop with various film thicknesses was carried out by applying a simple magneto-optical multilayer model. In Fig. 5, magneto-optical multilayer model, magnetic hysteresis loops of each layer, and calculated coercive force as a function of wavelength are shown. In this model, total film thickness is fixed at 50 nm, and existence of two flat layers is assumed; one layer located at the top of the film shows Hc =3.0 kOe, S=0.6 and the bottom layer located below the top layer shows Hc =0 kOe, S=0. Hysteresis loops are calculated by taking account of the interference effect on three typical cases, x=10, 25 and 40 nm, respectively, where x is the thickness of the bottom layer with Hc =0 kOe. In the case of x=10 nm, Hcfilm shows a constant value of 3.0 kOe, on the other hand, Hcsub: monotonously decreases from 1.4 to 0.7 kOe with decreasing the wavelength from 1000 to 400 nm. For x=25 nm, Hcfilm monotonously
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Fig. 5. Magneto-optical multilayer model, magnetic hysteresis loops of each layer and calculated coercive force as a function of wavelength for (left) x=10 nm, (middle) x=25 nm and (right) x=40 nm, respectively, where x is the thickness of the bottom layer which has low coercive force.
increases from 2.7 to 3.0 kOe, while Hcsub: monotonously decreases from 0.3 to 0 kOe with decreasing the wavelength. In the case of x=40 nm, Hcsub: shows a constant value of 0 kOe, while Hcfilm monotonously increases from 1.6 to 2.0 kOe with decreasing the wavelength. Based on these calculations, we can estimate the penetration depth of incident light. For example, in the case of the wavelength at 400 nm and x=10 nm, Hcsub: shows 0.9 kOe. This fact suggests that the incident light from the substrate side penetrates through the bottom layer with low Hc to the top layer with high Hc : On the other hand, in the cases of the wavelength of 400 nm and xX25 nm, Hcsub: shows nearly 0 kOe. This fact suggests that the incident light coming from the substrate side does not penetrate up to the top layer. Therefore, the
penetration depth of the incident light of 400 nm is estimated to be 10–25 nm from the incident surface. In the case of the wavelength of 1000 nm, Hcsub: shows 0 kOe for x=40 nm, 0.3 kOe for x=25 nm and 1.4 kOe for x=10 nm, respectively. These facts suggest that the penetration depth of the incident light of 1000 nm is 25–40 nm from the incident surface. Therefore, we can roughly estimate the thickness of the bottom layer, which has low Hc ; by comparing the experimental results with the calculated results. Figs. 6(a) and (b) show coercivities experimentally evaluated from polar Kerr hysteresis loops measured from the film surface side, (Hcfilm ) and from the substrate side (Hcsub: ) for CoNiCrTa films (50 nm) without and with Ti underlayer as a function of wavelength.
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Fig. 7. Calculated hysteresis loop of the bottom layer (solid line) for CoNiCrTa film (50 nm) deposited on Ti underlayer. Hysteresis loop of the top layer, which is measured by PKE from the film surface side, is also shown for comparison (broken line). Fig. 6. Coercivities experimentally evaluated from polar Kerr hysteresis loops for CoNiCrTa films (50 nm) deposited on (a) glass substrate and on (b) Ti underlayer as a function of wavelength. Coercivities measured by VSM are also shown for comparison.
Coercivities measured by VSM (HcVSM ) are also shown for comparison. For the film without Ti underlayer shown in Fig. 6(a), Hcfilm shows a constant value of 2.7 kOe independent of wavelength. On the contrary, Hcsub: monotonously decreases from 1.1 to 0.7 kOe with decreasing the wavelength from 990 to 400 nm. According to the calculated results shown in Fig. 5, this experimental result corresponds to the case (a) where x=10 nm. Therefore, we can roughly estimate the thickness of the bottom layer with low Hc in the present CoNiCrTa media at about 10 nm. The behavior of the dependence of Hc on wavelength shows the same tendency for the films with Ti underlayer shown in Fig. 6(b). It is difficult to derive a precise magnetic hysteresis loop of the bottom layer by using only polar Kerr hysteresis loops. However, if the thickness of the bottom layer can be determined by any other method, the hysteresis loop of the bottom layer can also be derived. TEM analysis revealed that in the present CoNiCrTa films (50 nm), the top layer, which shows high Hc ; corresponds to the columnar structure consisting of hexagonal grains with their c-plane parallel to the film plane and the bottom layer, which has low
Hc ; corresponds to the initial growth layer consisting of fine crystal grains with hexagonal structure [6]. Furthermore, according to perpendicular torque analysis, the films without and with Ti underlayer were found to have the initial growth layer with the thickness of 14.0 and 8.8 nm, respectively [6]. Using these experimental results, the hysteresis loop of the bottom layer was derived by another magneto-optical calculation, assuming that the magnetic properties of the top and bottom layers are homogenous, respectively. Fig. 7 shows the calculated hysteresis loop of the bottom layer for CoNiCrTa film (50 nm) with Ti underlayer, derived from the hysteresis loops measured by PKE from the film surface side and the substrate side. Here, the thicknesses of the bottom and the top layers are 8.8 and 41.2 nm, respectively. In the same figure, the measured hysteresis loop of the top layer is also shown for comparison. It is found that the bottom layer shows Hc of about 0.5 kOe and S of about 0.2, which indicates remarkably lower magnetic properties than that of the top layer. In this paper, it is shown that the hysteresis loop of the bottom layer can be calculated assuming that the magnetic properties of the bottom layer are homogenous. If the magnetic properties of the magnetic layer nearest to the substrate surface can be determined by some way, we can analyze a gradual variation in magnetic properties by applying the hysteresis loops measured from the
S. Saito et al. / Journal of Magnetism and Magnetic Materials 235 (2001) 120–125
substrate side to the magneto-optical multilayer model with increased stacking layer structure.
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loop of the bottom layer can be calculated if the thickness of the bottom layer can be determined by any other method and assuming that the magnetic properties of the bottom layer are homogenous.
4. Summary Local variation of magnetic properties along the film thickness for CoNiCrTa perpendicular thin films is discussed based on the results from the magneto-optical analysis. As a result, it is found that: (1) for CoNiCrTa perpendicular media, the top side of the film has high coercive force compared with the bottom side of the film, (2) by applying a magneto-optical multilayer model, the thickness of the bottom layer is roughly estimated to be about 10 nm, (3) the penetration depths of the incident light of 400 and 1000 nm are estimated to be 10–25 and 25–40 nm from the incident surface, respectively, and (4) the hysteresis
References [1] J.W. Smits, S.B. Luitjens, R.W.J. Geuskens, IEEE Trans. Magn. Mag- 20 (1984) 60. [2] S. Nakagawa, S. Takayama, M. Naoe, J. Magn. Soc. Japan 21 (Suppl. S1) (1997) 29. [3] M. Takahashi, A. Kikuchi, S. Kawakita, IEEE Trans. Magn. 33 (1997) 2938. [4] S. Yoshimura, D.D. Djyayaprawira, H. Shoji, M. Takahashi, J. Appl. Phys. 87 (2000) 6866. [5] A.E. Bell, F.W. Spong, IEEE J. Quantum Electron. 14 (1978) 487. [6] S. Saito, D. Hasegawa, D.D. Djayaprawira, M. Takahashi, J. Magn. Soc. Japan 25 (2001) 583 (in Japanese).
Variation of magnetic properties along the film thickness in perpendicular thin film media investigated by optical method Shin Saito*,1, Yoshihito Hatta, Migaku Takahashi Department of Electronics Engineering, Graduate School of Engineering, Tohoku University, Aoba-yama 05, Sendai 980-8579, Japan
Abstract Local variation of magnetic properties along the film thickness for CoNiCrTa perpendicular thin film media is discussed based on the magneto-optical analysis. It is found that the thickness of the bottom layer, which has low coercivity, is roughly estimated to be about 10 nm. Furthermore, the hysteresis loop of the bottom layer can be calculated by using magneto-optical multilayer model. r 2001 Elsevier Science B.V. All rights reserved. Keywords: Perpendicular thin film media; Polar Kerr rotation; Wavelength dependence; Light incident side; Magneto-optical multilayer model
1. Introduction Static magnetic properties, such as coercive force Hc and squareness S; of perpendicular recording media have been evaluated from hysteresis loops measured by using a vibrating sample magnetometer (VSM) and a magneto-optical polar Kerr equipment (PKE). Comparing magnetic properties obtained by VSM with the one obtained by PKE, for some samples, significant difference in the value of Hc (X1 kOe) and S (X0.15) was observed. This phenomenon is well known [1,2], but has not been comprehensively discussed from a viewpoint of local magnetic properties along the film thickness. In this paper, the variation of *Corresponding author. Department of Electronics Engineering, Graduate School of Engineering, Tohoku University, Aoba-yama 05, Sendai 980-8579, Japan. Tel.: +81-22-2177134; fax: +81-22-263-9402. E-mail address: [email protected] (S. Saito). 1 Research Fellow of the Japan Society for the Promotion of Science.
magnetic properties along the film thickness for perpendicular thin film media is discussed based on the results from the magneto-optical analysis.
2. Experimental procedure Co(67 at%)Ni(13 at%)Cr(16 at%)Ta(4 at%) perpendicular thin films were prepared by DC sputtering using the so-called ultra-clean sputtering system [3]. The same magnetic composition was used in longitudinal media in our previous study [4]. Tempered glass with the thickness of 0.64 mm and composed silica glass with the thickness of 0.1 mm were used as substrates. The films were fabricated at the substrate temperature of 2501C under Ar gas pressure of 4 mTorr. The thickness of the magnetic film, dmag ; was varied from 5 to 75 nm. The magnetic hysteresis loop was observed by VSM (swept field; 100 Oe/s) and PKE (stepped field; 7 s/each field). Polar Kerr hysteresis loop was examined from both the film surface side
0304-8853/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 0 3 7 0 - 5
S. Saito et al. / Journal of Magnetism and Magnetic Materials 235 (2001) 120–125
and from the substrate side, with wavelength in the region of 400–1000 nm. To obtain a hysteresis loop from polar Kerr rotation measured from the substrate side, background Faraday rotation coming from the glass substrate was subtracted. Complex refractive index of the film, nþik; was evaluated by the rotating-analyzer ellipsometry. The difference between complex refractive index of right- and left-polarized light for the magnetic film, DnþiDk; was consistently determined from n; k; experimentally obtained saturation polar Kerr sat: rotation and ellipticity, ysat: K and ZK ; by applying the matrix method [5].
3. Results and discussion 3.1. Measurements of hysteresis loop by PKE In this section, problems to measure hysteresis loops by using PKE with various wavelengths are explained. Fig. 1 shows spectra of saturation polar sat: Kerr rotation, ysat: K ; and ellipticity, ZK ; for a CoNiCrTa film (50 nm) measured by an incident light from the film surface side. No marked magneto-optical transition can be observed from the spectra. Measurement of a hysteresis loop by polar Kerr ellipticity at around 920 nm is found to be difficult, since the value of Zsat: K becomes zero. Therefore, in this study, we used the hysteresis loops obtained by polar Kerr rotation to evaluate magnetic properties for all the media. Fig. 2(a) shows wavelength dependence of hysteresis loops obtained by magneto-optical rotation
Fig. 1. Spectra of saturation polar Kerr rotation, ysat: K ; and ellipticity, Zsat: K ; measured from the film surface side for CoNiCrTa film (50 nm) directly deposited on glass substrate.
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Fig. 2. Changes in hysteresis loops of magneto-optical rotation against wavelength measured from the substrate side for CoNiCrTa films (50 nm). The films are fabricated on (a) tempered glass with the thickness of 0.64 mm and (b) composed silica glass with the thickness of 0.1 mm.
measured from the substrate side for a CoNiCrTa film (50 nm) deposited on tempered glass. For any loops, Faraday rotation coming from the substrate overlaps with Kerr rotation of the magnetic film. In the case that Faraday rotation is extremely larger than Kerr rotation (at the wavelength of 420 nm, for example), it becomes hard to evaluate the hysteresis loop of the magnetic film even if the subtraction of the Faraday rotation is done. Therefore, to decrease Faraday rotation, composed silica glass with the thickness of 0.1 mm was also used as a substrate. Fig. 2(b) shows changes in hysteresis loops of magneto-optical rotation against wavelength measured from the substrate side for a CoNiCrTa film (50 nm) deposited on composed silica glass substrate. Compared with Fig. 2(a), Faraday rotation in Fig. 2(b) is reduced for each wavelength, and it becomes possible to evaluate the hysteresis loop of the magnetic film by subtracting the Faraday rotation. Therefore, composed silica glass with the thickness of 0.1 mm was used, in the case that wavelength dependence of the hysteresis loop was taken into account. Fig. 3 shows changes in polar Kerr hysteresis loops for CoNiCrTa films (50 nm) on Ti underlayer with various thicknesses, dunder ; measured by the incident light of 990 nm from the substrate side. With increasing dunder ; saturation polar Kerr rotation gradually decreases. This is caused by exponential damping of amplitude of the incident light during transmission through Ti underlayer. In our PKE with the incident light from the substrate side at 990 nm, hysteresis loops for a
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Fig. 3. Changes in polar Kerr hysteresis loops for CoNiCrTa films (50 nm) on Ti underlayer with various thicknesses measured by incident light with the wavelength of 990 nm from the substrate side.
Fig. 4. Coercivities for CoNiCrTa thin films with various film thicknesses deposited on (a) glass substrate and (b) Ti underlayer measured by PKE with the wavelength of 990 nm (solid line: film surface side, broken line: substrate side). Coercivities measured by VSM are also shown by dashed chain line.
CoNiCrTa film on Ti underlayer with dunder p15 nm can be reliably evaluated. In the following experiment, the thickness of Ti underlayer was fixed at 7.5 nm, where the coercive force of the magnetic film takes maximum against dunder : 3.2. Analysis of perpendicular magnetic properties along the film thickness In Figs. 4(a) and (b), perpendicular coercive force, Hc ; for CoNiCrTa films prepared without and with Ti underlayer is plotted against the magnetic film thickness, dmag: measured by PKE at
990 nm. Hcfilm corresponds to the Hc measured from the film surface side and Hcsub: corresponds to the Hc measured from the substrate side. Hc values measured by VSM (HcVSM ) are also shown for comparison. For the films with dmag: p25 nm shown in Fig. 4(a), the slight difference in Hc between Hcfilm and Hcsub: can be found. With increasing dmag: more than 25 nm, it is clearly seen that Hcfilm is larger than Hcsub: ; and the difference between Hcfilm and Hcsub: is 1.5 kOe at dmag: =75 nm. HcVSM shows mean value between Hcfilm and Hcsub: : These facts mean that there exist magnetic layers with high Hc near the film surface side and with low Hc near the substrate side. The behavior of the dependence of Hc on dmag: shows the same tendency for the films with Ti underlayer shown in Fig. 4(b). Polar Kerr hysteresis loops reflect the information of magnetization of a magnetic film within the penetration depth from the interface of the light incident side. If the penetration depth can be varied, we can obtain the information of variation of magnetic properties along the film thickness. In this study, we tried to change the penetration depth by changing the wavelength of the incident light. As a first step, in order to confirm the relationship between the penetration depth and the wavelength, the calculation of Kerr hysteresis loop with various film thicknesses was carried out by applying a simple magneto-optical multilayer model. In Fig. 5, magneto-optical multilayer model, magnetic hysteresis loops of each layer, and calculated coercive force as a function of wavelength are shown. In this model, total film thickness is fixed at 50 nm, and existence of two flat layers is assumed; one layer located at the top of the film shows Hc =3.0 kOe, S=0.6 and the bottom layer located below the top layer shows Hc =0 kOe, S=0. Hysteresis loops are calculated by taking account of the interference effect on three typical cases, x=10, 25 and 40 nm, respectively, where x is the thickness of the bottom layer with Hc =0 kOe. In the case of x=10 nm, Hcfilm shows a constant value of 3.0 kOe, on the other hand, Hcsub: monotonously decreases from 1.4 to 0.7 kOe with decreasing the wavelength from 1000 to 400 nm. For x=25 nm, Hcfilm monotonously
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Fig. 5. Magneto-optical multilayer model, magnetic hysteresis loops of each layer and calculated coercive force as a function of wavelength for (left) x=10 nm, (middle) x=25 nm and (right) x=40 nm, respectively, where x is the thickness of the bottom layer which has low coercive force.
increases from 2.7 to 3.0 kOe, while Hcsub: monotonously decreases from 0.3 to 0 kOe with decreasing the wavelength. In the case of x=40 nm, Hcsub: shows a constant value of 0 kOe, while Hcfilm monotonously increases from 1.6 to 2.0 kOe with decreasing the wavelength. Based on these calculations, we can estimate the penetration depth of incident light. For example, in the case of the wavelength at 400 nm and x=10 nm, Hcsub: shows 0.9 kOe. This fact suggests that the incident light from the substrate side penetrates through the bottom layer with low Hc to the top layer with high Hc : On the other hand, in the cases of the wavelength of 400 nm and xX25 nm, Hcsub: shows nearly 0 kOe. This fact suggests that the incident light coming from the substrate side does not penetrate up to the top layer. Therefore, the
penetration depth of the incident light of 400 nm is estimated to be 10–25 nm from the incident surface. In the case of the wavelength of 1000 nm, Hcsub: shows 0 kOe for x=40 nm, 0.3 kOe for x=25 nm and 1.4 kOe for x=10 nm, respectively. These facts suggest that the penetration depth of the incident light of 1000 nm is 25–40 nm from the incident surface. Therefore, we can roughly estimate the thickness of the bottom layer, which has low Hc ; by comparing the experimental results with the calculated results. Figs. 6(a) and (b) show coercivities experimentally evaluated from polar Kerr hysteresis loops measured from the film surface side, (Hcfilm ) and from the substrate side (Hcsub: ) for CoNiCrTa films (50 nm) without and with Ti underlayer as a function of wavelength.
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Fig. 7. Calculated hysteresis loop of the bottom layer (solid line) for CoNiCrTa film (50 nm) deposited on Ti underlayer. Hysteresis loop of the top layer, which is measured by PKE from the film surface side, is also shown for comparison (broken line). Fig. 6. Coercivities experimentally evaluated from polar Kerr hysteresis loops for CoNiCrTa films (50 nm) deposited on (a) glass substrate and on (b) Ti underlayer as a function of wavelength. Coercivities measured by VSM are also shown for comparison.
Coercivities measured by VSM (HcVSM ) are also shown for comparison. For the film without Ti underlayer shown in Fig. 6(a), Hcfilm shows a constant value of 2.7 kOe independent of wavelength. On the contrary, Hcsub: monotonously decreases from 1.1 to 0.7 kOe with decreasing the wavelength from 990 to 400 nm. According to the calculated results shown in Fig. 5, this experimental result corresponds to the case (a) where x=10 nm. Therefore, we can roughly estimate the thickness of the bottom layer with low Hc in the present CoNiCrTa media at about 10 nm. The behavior of the dependence of Hc on wavelength shows the same tendency for the films with Ti underlayer shown in Fig. 6(b). It is difficult to derive a precise magnetic hysteresis loop of the bottom layer by using only polar Kerr hysteresis loops. However, if the thickness of the bottom layer can be determined by any other method, the hysteresis loop of the bottom layer can also be derived. TEM analysis revealed that in the present CoNiCrTa films (50 nm), the top layer, which shows high Hc ; corresponds to the columnar structure consisting of hexagonal grains with their c-plane parallel to the film plane and the bottom layer, which has low
Hc ; corresponds to the initial growth layer consisting of fine crystal grains with hexagonal structure [6]. Furthermore, according to perpendicular torque analysis, the films without and with Ti underlayer were found to have the initial growth layer with the thickness of 14.0 and 8.8 nm, respectively [6]. Using these experimental results, the hysteresis loop of the bottom layer was derived by another magneto-optical calculation, assuming that the magnetic properties of the top and bottom layers are homogenous, respectively. Fig. 7 shows the calculated hysteresis loop of the bottom layer for CoNiCrTa film (50 nm) with Ti underlayer, derived from the hysteresis loops measured by PKE from the film surface side and the substrate side. Here, the thicknesses of the bottom and the top layers are 8.8 and 41.2 nm, respectively. In the same figure, the measured hysteresis loop of the top layer is also shown for comparison. It is found that the bottom layer shows Hc of about 0.5 kOe and S of about 0.2, which indicates remarkably lower magnetic properties than that of the top layer. In this paper, it is shown that the hysteresis loop of the bottom layer can be calculated assuming that the magnetic properties of the bottom layer are homogenous. If the magnetic properties of the magnetic layer nearest to the substrate surface can be determined by some way, we can analyze a gradual variation in magnetic properties by applying the hysteresis loops measured from the
S. Saito et al. / Journal of Magnetism and Magnetic Materials 235 (2001) 120–125
substrate side to the magneto-optical multilayer model with increased stacking layer structure.
125
loop of the bottom layer can be calculated if the thickness of the bottom layer can be determined by any other method and assuming that the magnetic properties of the bottom layer are homogenous.
4. Summary Local variation of magnetic properties along the film thickness for CoNiCrTa perpendicular thin films is discussed based on the results from the magneto-optical analysis. As a result, it is found that: (1) for CoNiCrTa perpendicular media, the top side of the film has high coercive force compared with the bottom side of the film, (2) by applying a magneto-optical multilayer model, the thickness of the bottom layer is roughly estimated to be about 10 nm, (3) the penetration depths of the incident light of 400 and 1000 nm are estimated to be 10–25 and 25–40 nm from the incident surface, respectively, and (4) the hysteresis
References [1] J.W. Smits, S.B. Luitjens, R.W.J. Geuskens, IEEE Trans. Magn. Mag- 20 (1984) 60. [2] S. Nakagawa, S. Takayama, M. Naoe, J. Magn. Soc. Japan 21 (Suppl. S1) (1997) 29. [3] M. Takahashi, A. Kikuchi, S. Kawakita, IEEE Trans. Magn. 33 (1997) 2938. [4] S. Yoshimura, D.D. Djyayaprawira, H. Shoji, M. Takahashi, J. Appl. Phys. 87 (2000) 6866. [5] A.E. Bell, F.W. Spong, IEEE J. Quantum Electron. 14 (1978) 487. [6] S. Saito, D. Hasegawa, D.D. Djayaprawira, M. Takahashi, J. Magn. Soc. Japan 25 (2001) 583 (in Japanese).