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TiN underlayer and overlayer for TbFeCo perpendicular magnetic recording media
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 303 (2006) e133–e136 www.elsevier.com/locate/jmmm
TiN underlayer and overlayer for TbFeCo perpendicular magnetic recording media M. Tofizur Rahman, Xiaoxi Liu, Akimitsu Morisako Faculty of Engineering, Shinshu University, 4-17-1 Wakasato, Nagano 380-8553, Japan Available online 20 February 2006
Abstract TiN thin film is prepared by DC reactive sputtering in Ar+N2 atmosphere and its suitability as underlayer and overlayer for TbFeCo perpendicular recording media as well as its effect on the magnetic properties of the latter have been studied. Only 5 nm TiN overlayer and 20 nm under layer can successfully protect the TbFeCo film from oxidation. Initially the coercivity is increased sharply from about 2 to 6 kOe for an increase of underlayer thickness to 60 nm then the increasing rate of coercivity becomes very slow. The saturation magnetization remains almost constant with the underlayer thickness. The remanent squareness ratio remains constant at 1.0 with the underlayer thickness up to 60 nm then decreases. r 2006 Elsevier B.V. All rights reserved. PACS: 75.50.Ss Keywords: Perpendicular magnetic recording; Coercivity; TbFeCo film; TiN underlayer
1. Introduction TbFeCo films that are usually applied as the media for thermomagnetic recording have been considered as a promising candidate for ultra-high-density perpendicular magnetic storage because of its high perpendicular anisotropy, remanent squareness ratio of unity and amorphous continuous structure. The feasibility of 450 kfci perpendicular magnetic recording and its thermal stability were recently demonstrated [1,2]. TbFeCo film is extremely susceptible to oxidation [3,4]. More over, this film consists of large exchange coupling, which, however, influences the resolution of the magnetic recording. Strong exchange interaction is not desirable for keeping the nano-sized magnetic domains stable because of the minimization of the total magnetic energy of domains in the TbFeCo media. Actually, the magnetic boundary of a small sized domain shifts or vanishes through the displacement of the magnetic domain wall during the high-density magnetic recording. Magnetic pinning sites, Corresponding author. Tel.: +81 026 269 5483; fax: +81 026 269 5495.
E-mail address: potfi[email protected] (M.T. Rahman). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.01.092
which impede the motion of a magnetic domain wall, are hence required to improve the resolution of the magnetic recording. Several reports have demonstrated that the formation of the magnetic pinning sites induced by intermediate layers plays a significant role by changing the magnetization reversal process in the magnetic media and have enhanced the resolution of magnetic recording [1,5,6]. Search for a suitable overlayer and underlayer for TbFeCo media, that can enhance magnetic properties along with protection from oxidation is an extensive area of research. Titanium nitride (TiN) is one of the most important technological materials nowadays and has been studied and used as coating material in the small motor parts due to its outstanding properties such as, chemical, thermal and metallurgical stability, high hardness as well as good corrosion resistance [7–9]. Morphologically TiN consists of nano-scale rugged structure [10], which may act as pinning sites for its over layer. The aim of the present work is to prepare TiN thin film, studying its suitability as overlayer and underlayer for TbFeCo film as well as its effect on the magnetic properties of the latter.
ARTICLE IN PRESS M.T. Rahman et al. / Journal of Magnetism and Magnetic Materials 303 (2006) e133–e136
2. Experimental procedure Tb22Fe70Co8 films were deposited by a DC-magnetron sputtering system in Ar atmosphere without substrate heating on glass substrate. Base pressure was below 2 106 Torr and sputtering pressure was 5 mTorr. A composite target consisting of Fe plate overlaid by Tb and Co chips was used to deposit TbFeCo layer whereas both of TiN overlayer and underlayer was deposited by reactive sputtering in Ar+N2 atmosphere with pure Ti target without any bias voltage or substrate heating. During the preparation of TiN films, the discharge gas pressure (Ptotal), which is the sum of each partial pressure (PAr, PN), was kept constant at 5 mTorr. The partial pressure of nitrogen was varied from 0.4–1.2 mTorr. The crystal structure of TiN films was observed by an X-ray diffractometer (XRD). Magnetic properties were measured by using vibrating sample magnetometer (VSM) at a maximum field of 15 kOe. Depth profiles of the elements in the film were determined by X-ray photoemission spectroscopy (XPS) and the film composition was determined by electron probe micro-analyzer (EPMA).
Fig. 1 Shows the XRD diagrams of TiN film at different partial pressure of nitrogen PN and Table 1 shows corresponding crystallographic properties. The prepared TiNx films were found to be polycrystalline. Nitrogen partial pressure PN of 0.4 mTorr is insufficient for forming TiNx compound. The lattice parameters of the films were determined form the position of the (1 1 1) peak of the FCC phase in their XRD spectra. The lattice parameter of the
200 PN=0.4mTorr Ti (110)
Intensity (cps)
200 PN=0.8mTorr
200 TiN (111)
PN=1.2mTorr
0 30
35
Partial pressure of N2 PN (mTor)
2y (deg)
Lattice constant a (A˚)
Peak
0.4 0.8 1.2
37.65 36.65 36.51
4.116 4.241 4.251
Ti (1 1 0) TiN (1 1 1) TiN (1 1 1)
Over layer OL
Magnetic layer
1.0
Under layer
40 2 (degree)
45
50
Fig. 1. XRD diagrams of TiN films prepared at different partial pressure of nitrogen, PN.
Glass
Tb
0.8
Fe 0.6
Co
0.4
Ti N2 O Si
0.2
3. Result and discussion
TiN (111)
Table 1 Lattice constant and 2y values of TiN (1 1 1) and Ti (1 1 0) peak for various partial pressure of nitrogen PN
Intensity ratio I/Imax
e134
0
0
10
20 30 Etching time, t (min)
40
Fig. 2. XPS depth profile of elements in the TiN(5 nm)/TbFeCo(40 nm)/ TiN(20 nm)/glass film.
film increased as the nitrogen partial pressure increased. The lattice parameter of the sample prepared at PN of 0.8 mTorr is 4.241 A˚, which agrees with the reported value for bulk TiN, [11,12] is assumed to have stoichiometric composition (x ¼ 1). TiN prepared in this condition is used as underlayer and overlayer for TbFeCo film. Fig. 2. Shows the XPS depth profiles of the elements in the TiN5 nm/TbFeCo40 nm/TiN20 nm glass film. It has been found that 5 nm TiN overlayer and 20 nm underlayer is sufficient to protect the film from the oxidation both from the atmosphere and substrate. Some oxygen is present in the overlayer and underlayer but not in the magnetic layer. To have an assumption about corrosion resistance, the pin hole growth through aging has been observed, keeping the film in air at room temperature for 1 month. Fig. 3 shows the AFM plane view surface image of (a) TbFeCo40 nm/glass film and (b)TiN5 nm/TbFeCo40 nm/TiN20 film after aging. It is seen that the bare TbFeCo film contains plenty of pinholes whereas the film over coated with TiN is completely free of such pin holes or surface pits. This is also confirmed by the time decay (not shown here) of the film as no change has been observed in magnetic properties with time when TiN underlayer and overlayer are used.
ARTICLE IN PRESS M.T. Rahman et al. / Journal of Magnetism and Magnetic Materials 303 (2006) e133–e136
Fig. 3. AFM plane view surface image of (a) TbFeCo(40 nm)/glass film and (b) TiN(5 nm)/TbFeCo(40 nm)/TiN(20 nm)/glass film taken after 1 month aging in air at room temperature.
250
(a)
(b) ±250
(c) ±250
(d) ±250
(e) -250 -15
-10
-5 0 5 Applied Field (kOe)
10
The effect of TiN underlayer thickness on the M–H loop can be understood from Figs. 4(c)–(e). Coercivity increases sharply with the increase in underlayer thickness up to 60 nm maintaining rectangular loop. For an underlayer thickness of more than 60 nm the loop starts to loose its rectangular shape resulting remanent squareness ratio (S) of less than 1.0. The slant of the loop reflects a switching field distribution; probably caused by inhomogeneous pinning sites in the sample and the poorer squareness suggest that the magnetization reversal is nucleation dominated [13]. Fig. 5 shows the variation of saturation magnetization (Ms) as a function of underlayer thickness where all the films were overcoated with 5 nm TiN and magnetic layer thickness was 40 nm. Initially the Ms value decreases just after the employment of underlayer then it remains constant with the underlayer thickness. It is thought that the initial decrease in Ms value is due to the oxidation protection of Tb atom. Since the present film is ferrimagnetic, the decrease in rare-earth (RE) sub-lattice magnetization due to the oxidation causes the shifting of film composition away from the compensation composition in the current transition metal (TM) rich film. As a result, total saturation magnetization of the film increases. Protection of the film from oxidation can contribute to decrease in Ms value. The constant Ms value of the TbFeCo film deposited on TiN underlayer with different thickness also reveals that TiN can give excellent protection of the TbFeCo film from the oxidation from substrate. From Fig. 6 it has been found that perpendicular coercivity Hc sharply increases with increase in underlayer thickness of about 60 nm then it increases monotonously with little bit slower rate than that of before. The increase in Hc is supposed to be due to the induced magnetic pinning sites, which impede the motion of magnetic domain wall in the TbFeCo film, at the interface between the TbFeCo layer and the underlayer surface [14,15]. For
15
Fig. 4. M–H loops of (a) TbFeCo(40 nm)/glass film. (b)–(e) TiN(5 nm)/ TbFeCo(40 nm)/TiN(x nm)/glass film; (b) 0, (c) 10, (d) 60, (e) 80. The solid and broken lines represent perpendicular and in-plane direction, respectively.
The M–H loop (Fig. 4(a)) of TbFeCo film without underlayer and overlayer shows soft magnetic characteristics in in-plane direction. Although it is very small but this soft phase still presents even when the film is overcoated (Fig. 4(b)) and completely disappears when both of overcoat and under layer are used (Fig. 4(c)). The presence of the soft magnetic phase may be due to the oxidation of the Tb in the films. Oxidation of the Tb causes Fe- and Co- rich phase in the interfacial layer, which is magnetically soft. Films with overcoat but without underlayer still have soft magnetic phase indicating the substrate–film interaction is another oxidation source.
300 250 Magnetization , Ms (emu/cm3)
Magnetization (emu/cm3)
±250
e135
200 150 100 50 0 0
20
40 60 TiN underlayer thickness (nm)
80
100
Fig. 5. Dependence of saturation magnetization on the thickness of TiN underlayer for TiN(5 nm)/TbFeCo(40 nm)/TiN (x nm)/glass film.
ARTICLE IN PRESS M.T. Rahman et al. / Journal of Magnetism and Magnetic Materials 303 (2006) e133–e136
e136
with the increase in underlayer thickness. To decrease the noise the reverse domain formation must be reduced. Honda et al. [16] showed that reverse domain formation could be suppressed by increasing the squareness ratio. Large S is also required to prevent thermal fluctuation of perpendicular recording media [17]. TiN underlayer thickness of up to 60 nm is suitable for TbFeCo film to maintain the remanent squareness ratio at one as well as for stable TbFeCo media.
8
Coercivity, Hc (kOe)
6
4
2
0
4. Conclusion
0
20
40 60 TiN underlayer thickness (nm)
80
100
Fig. 6. Dependence of perpendicular coercivity on the thickness of TiN underlayer for TiN(5 nm)/TbFeCo(40 nm)/TiN (x nm)/glass film.
Remanent squarness ratio, S
1.5
TiN film was prepared and its suitability as underlayer and overlayer for TbFeCo perpendicular recording media were studied. The use of TiN underlayer and overlayer protect the TbFeCo film from oxidation. The coercivity in the TbFeCo film deposited on 60 nm TiN underlayer is about three fold higher than that prepared onto glass substrate. The read–write characteristics of the TbFeCo film deposited on TiN underlayer is the further scope of study. References
1.25
1
0.75
0.5
0
20
40 60 TiN underlayer thickness (nm)
80
100
Fig. 7. Dependence of remanent squarness ratio on the thickness of TiN underlayer for TiN(5 nm)/TbFeCo(40 nm)/TiN (x nm)/glass film.
in-depth understanding of the mechanism of coercivity enhancement, morphological study of TiN underlayer is necessary. Fig. 7 shows the variation of remanent squarness ratio S as a function of underlayer thickness. Initially the remanent squarness ratio S rise to 1.0 just after the employment of underlayer and kept its value constant for the underlayer thickness of about 60 nm then decreases
[1] K. Ozaki, K. Matsumoto, I. Tagawa, K. Shono, J. Magn. Soc. Japan 25 (2001) 322. [2] A. Chekanov, K. Matsumoto, K. Ozaki, J. Appl. Phys. 90 (1987) 4657. [3] S. Takayama, T. Niihara, K. Kaneko, Y. Sugita, M. Ojima, J. Appl. Phys. 61 (1987) 2610. [4] M. Takahashi, T. Niihara, N. Ohta, J. Appl. Phys. 64 (1988) 262. [5] C.H. Chang, M.H. Kryder, J. Appl. Phys. 75 (1994) 6864. [6] K. Matsumoto, H. Kawano, T. Morikawa, K. Shono, Jpn.J. Appl. Phys. 41 (Part 2) (2002) L691. [7] M. Wittmer, J. Vac. Sci. Technol. A3 (1985) 1797. [8] C.A. Dimitriadis, S. Logothetidis, I. Alexandrou, Appl. Phys. Lett. 66 (1995) 502. [9] M. Nose, T. Nagae, M. Yokota, S. Saji, Z. Min, M. Nakada, Surf. Coat. Technol. 119 (1999) 296. [10] Z.J. Liu, N. Jiang, Y.G. Shen, J. Appl. Phys. 92 (2002) 3559. [11] R.R. Manory, G. Kimmel, Surf, Coat, Technol. 63 (1994) 85. [12] B. Peez, N. Frangis, S. Lgothetidis, I. Alexandrou, P.B. Barna, J. Stoemenos, Thin Solid Films 268 (1995) 5863. [13] J. Pommier, P. Meyer, G. Pensissard, J. Ferre, P. Brouno, D. Renard, Phys. Rev. Lett. 65 (1990) 2054. [14] Y.P. Zhao, M.M. Gamache, G.C. Wang, T.M. Lu, G. Palasantzas, J. Th. M. De Hosson, J. Appl. Phys. 89 (2001) 1325. [15] M.T. Rahman, X. Liu, M. Matsumoto, A. Morisako, Trans. Magn. Soc. Jpn. 5 (2005) 69. [16] N. Honda, T. Kiya, K. Ouchi, J. Magn. Soc. Japan 21-S2 (1997) 505. [17] N. Honda, K. Ouchi, IEICE Trans. Electron. E80-C (1997) 1180.
Journal of Magnetism and Magnetic Materials 303 (2006) e133–e136 www.elsevier.com/locate/jmmm
TiN underlayer and overlayer for TbFeCo perpendicular magnetic recording media M. Tofizur Rahman, Xiaoxi Liu, Akimitsu Morisako Faculty of Engineering, Shinshu University, 4-17-1 Wakasato, Nagano 380-8553, Japan Available online 20 February 2006
Abstract TiN thin film is prepared by DC reactive sputtering in Ar+N2 atmosphere and its suitability as underlayer and overlayer for TbFeCo perpendicular recording media as well as its effect on the magnetic properties of the latter have been studied. Only 5 nm TiN overlayer and 20 nm under layer can successfully protect the TbFeCo film from oxidation. Initially the coercivity is increased sharply from about 2 to 6 kOe for an increase of underlayer thickness to 60 nm then the increasing rate of coercivity becomes very slow. The saturation magnetization remains almost constant with the underlayer thickness. The remanent squareness ratio remains constant at 1.0 with the underlayer thickness up to 60 nm then decreases. r 2006 Elsevier B.V. All rights reserved. PACS: 75.50.Ss Keywords: Perpendicular magnetic recording; Coercivity; TbFeCo film; TiN underlayer
1. Introduction TbFeCo films that are usually applied as the media for thermomagnetic recording have been considered as a promising candidate for ultra-high-density perpendicular magnetic storage because of its high perpendicular anisotropy, remanent squareness ratio of unity and amorphous continuous structure. The feasibility of 450 kfci perpendicular magnetic recording and its thermal stability were recently demonstrated [1,2]. TbFeCo film is extremely susceptible to oxidation [3,4]. More over, this film consists of large exchange coupling, which, however, influences the resolution of the magnetic recording. Strong exchange interaction is not desirable for keeping the nano-sized magnetic domains stable because of the minimization of the total magnetic energy of domains in the TbFeCo media. Actually, the magnetic boundary of a small sized domain shifts or vanishes through the displacement of the magnetic domain wall during the high-density magnetic recording. Magnetic pinning sites, Corresponding author. Tel.: +81 026 269 5483; fax: +81 026 269 5495.
E-mail address: potfi[email protected] (M.T. Rahman). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.01.092
which impede the motion of a magnetic domain wall, are hence required to improve the resolution of the magnetic recording. Several reports have demonstrated that the formation of the magnetic pinning sites induced by intermediate layers plays a significant role by changing the magnetization reversal process in the magnetic media and have enhanced the resolution of magnetic recording [1,5,6]. Search for a suitable overlayer and underlayer for TbFeCo media, that can enhance magnetic properties along with protection from oxidation is an extensive area of research. Titanium nitride (TiN) is one of the most important technological materials nowadays and has been studied and used as coating material in the small motor parts due to its outstanding properties such as, chemical, thermal and metallurgical stability, high hardness as well as good corrosion resistance [7–9]. Morphologically TiN consists of nano-scale rugged structure [10], which may act as pinning sites for its over layer. The aim of the present work is to prepare TiN thin film, studying its suitability as overlayer and underlayer for TbFeCo film as well as its effect on the magnetic properties of the latter.
ARTICLE IN PRESS M.T. Rahman et al. / Journal of Magnetism and Magnetic Materials 303 (2006) e133–e136
2. Experimental procedure Tb22Fe70Co8 films were deposited by a DC-magnetron sputtering system in Ar atmosphere without substrate heating on glass substrate. Base pressure was below 2 106 Torr and sputtering pressure was 5 mTorr. A composite target consisting of Fe plate overlaid by Tb and Co chips was used to deposit TbFeCo layer whereas both of TiN overlayer and underlayer was deposited by reactive sputtering in Ar+N2 atmosphere with pure Ti target without any bias voltage or substrate heating. During the preparation of TiN films, the discharge gas pressure (Ptotal), which is the sum of each partial pressure (PAr, PN), was kept constant at 5 mTorr. The partial pressure of nitrogen was varied from 0.4–1.2 mTorr. The crystal structure of TiN films was observed by an X-ray diffractometer (XRD). Magnetic properties were measured by using vibrating sample magnetometer (VSM) at a maximum field of 15 kOe. Depth profiles of the elements in the film were determined by X-ray photoemission spectroscopy (XPS) and the film composition was determined by electron probe micro-analyzer (EPMA).
Fig. 1 Shows the XRD diagrams of TiN film at different partial pressure of nitrogen PN and Table 1 shows corresponding crystallographic properties. The prepared TiNx films were found to be polycrystalline. Nitrogen partial pressure PN of 0.4 mTorr is insufficient for forming TiNx compound. The lattice parameters of the films were determined form the position of the (1 1 1) peak of the FCC phase in their XRD spectra. The lattice parameter of the
200 PN=0.4mTorr Ti (110)
Intensity (cps)
200 PN=0.8mTorr
200 TiN (111)
PN=1.2mTorr
0 30
35
Partial pressure of N2 PN (mTor)
2y (deg)
Lattice constant a (A˚)
Peak
0.4 0.8 1.2
37.65 36.65 36.51
4.116 4.241 4.251
Ti (1 1 0) TiN (1 1 1) TiN (1 1 1)
Over layer OL
Magnetic layer
1.0
Under layer
40 2 (degree)
45
50
Fig. 1. XRD diagrams of TiN films prepared at different partial pressure of nitrogen, PN.
Glass
Tb
0.8
Fe 0.6
Co
0.4
Ti N2 O Si
0.2
3. Result and discussion
TiN (111)
Table 1 Lattice constant and 2y values of TiN (1 1 1) and Ti (1 1 0) peak for various partial pressure of nitrogen PN
Intensity ratio I/Imax
e134
0
0
10
20 30 Etching time, t (min)
40
Fig. 2. XPS depth profile of elements in the TiN(5 nm)/TbFeCo(40 nm)/ TiN(20 nm)/glass film.
film increased as the nitrogen partial pressure increased. The lattice parameter of the sample prepared at PN of 0.8 mTorr is 4.241 A˚, which agrees with the reported value for bulk TiN, [11,12] is assumed to have stoichiometric composition (x ¼ 1). TiN prepared in this condition is used as underlayer and overlayer for TbFeCo film. Fig. 2. Shows the XPS depth profiles of the elements in the TiN5 nm/TbFeCo40 nm/TiN20 nm glass film. It has been found that 5 nm TiN overlayer and 20 nm underlayer is sufficient to protect the film from the oxidation both from the atmosphere and substrate. Some oxygen is present in the overlayer and underlayer but not in the magnetic layer. To have an assumption about corrosion resistance, the pin hole growth through aging has been observed, keeping the film in air at room temperature for 1 month. Fig. 3 shows the AFM plane view surface image of (a) TbFeCo40 nm/glass film and (b)TiN5 nm/TbFeCo40 nm/TiN20 film after aging. It is seen that the bare TbFeCo film contains plenty of pinholes whereas the film over coated with TiN is completely free of such pin holes or surface pits. This is also confirmed by the time decay (not shown here) of the film as no change has been observed in magnetic properties with time when TiN underlayer and overlayer are used.
ARTICLE IN PRESS M.T. Rahman et al. / Journal of Magnetism and Magnetic Materials 303 (2006) e133–e136
Fig. 3. AFM plane view surface image of (a) TbFeCo(40 nm)/glass film and (b) TiN(5 nm)/TbFeCo(40 nm)/TiN(20 nm)/glass film taken after 1 month aging in air at room temperature.
250
(a)
(b) ±250
(c) ±250
(d) ±250
(e) -250 -15
-10
-5 0 5 Applied Field (kOe)
10
The effect of TiN underlayer thickness on the M–H loop can be understood from Figs. 4(c)–(e). Coercivity increases sharply with the increase in underlayer thickness up to 60 nm maintaining rectangular loop. For an underlayer thickness of more than 60 nm the loop starts to loose its rectangular shape resulting remanent squareness ratio (S) of less than 1.0. The slant of the loop reflects a switching field distribution; probably caused by inhomogeneous pinning sites in the sample and the poorer squareness suggest that the magnetization reversal is nucleation dominated [13]. Fig. 5 shows the variation of saturation magnetization (Ms) as a function of underlayer thickness where all the films were overcoated with 5 nm TiN and magnetic layer thickness was 40 nm. Initially the Ms value decreases just after the employment of underlayer then it remains constant with the underlayer thickness. It is thought that the initial decrease in Ms value is due to the oxidation protection of Tb atom. Since the present film is ferrimagnetic, the decrease in rare-earth (RE) sub-lattice magnetization due to the oxidation causes the shifting of film composition away from the compensation composition in the current transition metal (TM) rich film. As a result, total saturation magnetization of the film increases. Protection of the film from oxidation can contribute to decrease in Ms value. The constant Ms value of the TbFeCo film deposited on TiN underlayer with different thickness also reveals that TiN can give excellent protection of the TbFeCo film from the oxidation from substrate. From Fig. 6 it has been found that perpendicular coercivity Hc sharply increases with increase in underlayer thickness of about 60 nm then it increases monotonously with little bit slower rate than that of before. The increase in Hc is supposed to be due to the induced magnetic pinning sites, which impede the motion of magnetic domain wall in the TbFeCo film, at the interface between the TbFeCo layer and the underlayer surface [14,15]. For
15
Fig. 4. M–H loops of (a) TbFeCo(40 nm)/glass film. (b)–(e) TiN(5 nm)/ TbFeCo(40 nm)/TiN(x nm)/glass film; (b) 0, (c) 10, (d) 60, (e) 80. The solid and broken lines represent perpendicular and in-plane direction, respectively.
The M–H loop (Fig. 4(a)) of TbFeCo film without underlayer and overlayer shows soft magnetic characteristics in in-plane direction. Although it is very small but this soft phase still presents even when the film is overcoated (Fig. 4(b)) and completely disappears when both of overcoat and under layer are used (Fig. 4(c)). The presence of the soft magnetic phase may be due to the oxidation of the Tb in the films. Oxidation of the Tb causes Fe- and Co- rich phase in the interfacial layer, which is magnetically soft. Films with overcoat but without underlayer still have soft magnetic phase indicating the substrate–film interaction is another oxidation source.
300 250 Magnetization , Ms (emu/cm3)
Magnetization (emu/cm3)
±250
e135
200 150 100 50 0 0
20
40 60 TiN underlayer thickness (nm)
80
100
Fig. 5. Dependence of saturation magnetization on the thickness of TiN underlayer for TiN(5 nm)/TbFeCo(40 nm)/TiN (x nm)/glass film.
ARTICLE IN PRESS M.T. Rahman et al. / Journal of Magnetism and Magnetic Materials 303 (2006) e133–e136
e136
with the increase in underlayer thickness. To decrease the noise the reverse domain formation must be reduced. Honda et al. [16] showed that reverse domain formation could be suppressed by increasing the squareness ratio. Large S is also required to prevent thermal fluctuation of perpendicular recording media [17]. TiN underlayer thickness of up to 60 nm is suitable for TbFeCo film to maintain the remanent squareness ratio at one as well as for stable TbFeCo media.
8
Coercivity, Hc (kOe)
6
4
2
0
4. Conclusion
0
20
40 60 TiN underlayer thickness (nm)
80
100
Fig. 6. Dependence of perpendicular coercivity on the thickness of TiN underlayer for TiN(5 nm)/TbFeCo(40 nm)/TiN (x nm)/glass film.
Remanent squarness ratio, S
1.5
TiN film was prepared and its suitability as underlayer and overlayer for TbFeCo perpendicular recording media were studied. The use of TiN underlayer and overlayer protect the TbFeCo film from oxidation. The coercivity in the TbFeCo film deposited on 60 nm TiN underlayer is about three fold higher than that prepared onto glass substrate. The read–write characteristics of the TbFeCo film deposited on TiN underlayer is the further scope of study. References
1.25
1
0.75
0.5
0
20
40 60 TiN underlayer thickness (nm)
80
100
Fig. 7. Dependence of remanent squarness ratio on the thickness of TiN underlayer for TiN(5 nm)/TbFeCo(40 nm)/TiN (x nm)/glass film.
in-depth understanding of the mechanism of coercivity enhancement, morphological study of TiN underlayer is necessary. Fig. 7 shows the variation of remanent squarness ratio S as a function of underlayer thickness. Initially the remanent squarness ratio S rise to 1.0 just after the employment of underlayer and kept its value constant for the underlayer thickness of about 60 nm then decreases
[1] K. Ozaki, K. Matsumoto, I. Tagawa, K. Shono, J. Magn. Soc. Japan 25 (2001) 322. [2] A. Chekanov, K. Matsumoto, K. Ozaki, J. Appl. Phys. 90 (1987) 4657. [3] S. Takayama, T. Niihara, K. Kaneko, Y. Sugita, M. Ojima, J. Appl. Phys. 61 (1987) 2610. [4] M. Takahashi, T. Niihara, N. Ohta, J. Appl. Phys. 64 (1988) 262. [5] C.H. Chang, M.H. Kryder, J. Appl. Phys. 75 (1994) 6864. [6] K. Matsumoto, H. Kawano, T. Morikawa, K. Shono, Jpn.J. Appl. Phys. 41 (Part 2) (2002) L691. [7] M. Wittmer, J. Vac. Sci. Technol. A3 (1985) 1797. [8] C.A. Dimitriadis, S. Logothetidis, I. Alexandrou, Appl. Phys. Lett. 66 (1995) 502. [9] M. Nose, T. Nagae, M. Yokota, S. Saji, Z. Min, M. Nakada, Surf. Coat. Technol. 119 (1999) 296. [10] Z.J. Liu, N. Jiang, Y.G. Shen, J. Appl. Phys. 92 (2002) 3559. [11] R.R. Manory, G. Kimmel, Surf, Coat, Technol. 63 (1994) 85. [12] B. Peez, N. Frangis, S. Lgothetidis, I. Alexandrou, P.B. Barna, J. Stoemenos, Thin Solid Films 268 (1995) 5863. [13] J. Pommier, P. Meyer, G. Pensissard, J. Ferre, P. Brouno, D. Renard, Phys. Rev. Lett. 65 (1990) 2054. [14] Y.P. Zhao, M.M. Gamache, G.C. Wang, T.M. Lu, G. Palasantzas, J. Th. M. De Hosson, J. Appl. Phys. 89 (2001) 1325. [15] M.T. Rahman, X. Liu, M. Matsumoto, A. Morisako, Trans. Magn. Soc. Jpn. 5 (2005) 69. [16] N. Honda, T. Kiya, K. Ouchi, J. Magn. Soc. Japan 21-S2 (1997) 505. [17] N. Honda, K. Ouchi, IEICE Trans. Electron. E80-C (1997) 1180.