- Email: [email protected]
Pt multilayered film
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 287 (2005) 501–506 www.elsevier.com/locate/jmmm
Heat-assisted magnetic probe recording on a CoNi/Pt multilayered film T. Onoue, M.H. Siekman, L. Abelmann, J.C. Lodder Systems & Materials for Information Storage (SMI), MESA+ Research Institute, University of Twente, P.O. Box 217, 7500AE Enschede, The Netherlands
Abstract Heat-assisted magnetic probe recording on a CoNi/Pt multilayered film is demonstrated by delivering a current through a magnetic force microscopy tip into the recording medium, in combination with an external magnetic field. Without local heating by the probe, no bits could be written because the external field exceeds a level that demagnetizes the medium in its entirety due to local variations in the coercivity of the medium. In contrast, magnetic bits were successfully written by the heat-assisted magnetic probe recording into a saturated medium even if there was no external field, because of the demagnetization field from the surrounding of the heated area. A magnetic bit as small as 80 nm in diameter was obtained by this method. r 2004 Elsevier B.V. All rights reserved. PACS: 68.37Rt; 75.50Ss; 75.70Kw; 81.16Rf; 85.70Li; 85.80Lp Keywords: Heat assist; Probe recording; Multilayer; Magnetic force microscopy
1. Introduction In the last few decades, a remarkable increase of the recording density in magnetic storage has been accomplished by shrinking the read–write elements as well as grain sizes in magnetic recording media. Corresponding author. Tel.: +31 53 489 2747; fax: +31 53 489 3343. E-mail addresses: [email protected], [email protected] (T. Onoue).
Even higher recording density can be achieved by further shrinking the writing element to the ultimate limit of using atomically sharp probes, such as used in scanning probe microscopy (SPM). Indeed, an SPM-based storage system is an attractive way to realize a storage system exhibiting high recording density by using magnetic[1–3], ferroelectric- [4], phase change- [5] materials and polymers [6] as the probe recording media. Magnetic probe recording is an especially promising candidate among them, because we can make
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2004.10.083
ARTICLE IN PRESS T. Onoue et al. / Journal of Magnetism and Magnetic Materials 287 (2005) 501–506
use of well-developed magnetic materials explored in hard disk or magneto-optical recording systems. The virtually infinite rewriteability of magnetic recording media is a major advantage. Nowadays, the target recording density of hard disk drives is moving towards several Tb/in2. In order to accomplish such high recording densities, a sufficiently high magnetic field should be exerted from a writing head to reverse the magnetization in the medium which, because it has to be thermally stable at these high densities, possesses a high coercivity (Hc). However, the strength of the magnetic field is limited by the saturation magnetization (Ms) of the writing head. Therefore, an attractive scheme is to reduce the coercivity of the medium during writing by locally heating the medium, a method often called heat-assisted magnetic recording (HAMR) [7]. In this method, it is, however, crucial to apply the heat locally, so that adjacent tracks are not erased during the writing process. Since track-widths in modern hard disk recording systems are already well below the size of a far-field focused laser spot, other means to apply local heat are of great interest. In this paper, we demonstrate heat-assisted magnetic probe recording on a CoNi/Pt multilayered film with perpendicular magnetic anisotropy. Joule heating was realized by delivering a current through an MFM tip in contact with the medium, synchronized with external magnetic field pulses. This type of heat-assisted writing is also known as Curie point writing in magneto-optical recording, which uses a laser as a heating source [8]. However, in heat-assisted magnetic probe recording, the heat is applied on a very local scale by exploiting the sharpness of the probe. The importance of heat-assisted magnetic probe recording is clarified in comparison with a magnetic probe recording without heating.
coercivity (100 kA/m) as well as a squareness ratio of 1. The M–H loop, measured in perpendicular direction to the film surface by a VSM measurement, is shown in Fig. 1. The Curie temperature of this type of CoNi/Pt multilayers is around 250 1C [9]. A commercial scanning probe microscope system (Digital Instruments, Nanoscope IIIa, Dimension 3100) was adapted in order to perform the writing experiment. The MFM probe was prepared by depositing a NiFe layer (25 nm) on one side of a commercial Si AFM tip, followed by depositing a Pt layer (30 nm) all around the tip and on the cantilever as a conductive layer. The probe was used for writing as well as observing the magnetic bits. The writing experiment was performed in contact AFM mode without scanning. The measured resistance, while the probe and the CoNi/Pt film were in contact, was around 0.2 kO. The strength of the delivered current (shown in Fig. 3) was calculated by assuming the constant resistance during the writing process. Fig. 2 shows the schematic illustration of the probe writing setup. In this experiment, two pulse voltages are used; one to induce a magnetic field pulse from the coil by pulse generator (1) and the other to deliver a current pulse through the MFM probe to the sample by pulse generator (2) causing Joule heat at
400
200 M [kA/m]
502
0
-200
2. Experimental procedure A film composed of 20 bi-layers of CoNi (5.5 A˚)/ Pt (8.7 A˚) with a Pt seedlayer (230 A˚) was deposited on a Si (1 0 0) wafer by sputtering. The magnetic properties of the films were controlled by tuning the Ar pressure to achieve a moderate
-400 -400
-200
0 H[kA/m]
200
400
Fig. 1. M–H loop of the CoNi/Pt multilayered film. The virgin curve is also shown.
ARTICLE IN PRESS T. Onoue et al. / Journal of Magnetism and Magnetic Materials 287 (2005) 501–506
the contact point with the sample. The CoNi/Pt sample is initially in the saturated magnetization state and positioned above the coil. The magnetic field pulse in opposition to the magnetization direction of the sample was applied, synchronized with the current through the tip. The duration of the magnetic field pulse and the current pulse through the tip were 400 and 100 ms, respectively. The current pulse was applied after the start of the field pulse with a delay time of 200 ms. The AFM and MFM images were taken just after the writing
Fig. 2. Schematic illustration of the heat-assisted magnetic probe recording setup.
503
experiment by using the same probe as used for the writing experiment.
3. Results Writing can only be achieved when using ‘‘heat assisted’’ recording. If we only apply a combined magnetic field from the MFM probe and the coil below the sample (so the pulse generator (2) which delivers a current into the tip was not used), no single bit could be written. If we increase the amplitude of the magnetic field pulse gradually, while keeping the magnetic probe in contact with the medium, we observe stripe domains above critical pulse amplitude, instead of single magnetic bits. Careful observation revealed that the magnetization rotation started from a different part from where the probe was approached. It seems that the local magnetic field exerted from the probe is insufficient to activate a local magnetization rotation below the probe. Fig. 3 shows the MFM image of written magnetic bits by the heat-assisted magnetic probe recording. The AFM image of the corresponding part is shown in the same figure. By combining various bias magnetic fields and the delivered
Fig. 3. MFM image of written bits (right). AFM image of the same area as MFM image (left). The 4 4 bit matrix was created by varying the external magnetic field and the delivered current through the MFM probe during writing. Each value is shown in the figure.
ARTICLE IN PRESS 504
T. Onoue et al. / Journal of Magnetism and Magnetic Materials 287 (2005) 501–506
currents at the contact point, a 4 4 matrix of bits was created. Although the two bits at the top left are connected because of drift during writing, the 4 4 bits are successfully formed by all the writing conditions. Hence, it is obvious that the heating is indispensable to nucleate the magnetization rotation in a localized area by the probe. This experiment shows that bits can be written even without an external magnetic field. The demagnetizing magnetic field from the surroundings of the heated area, which exhibits a low value of Ms caused by the raised temperature, is sufficient to induce the magnetization rotation [3,8]. In the AFM image, no surface damage is observed, even though a current of 100 mA (a current density of 14MA/cm2 assuming the contact area to be 7 1012 cm2) was delivered. This result supports the durability of this writing scenario. Fig. 4 shows the MFM image of the smallest magnetic bit obtained in this CoNi/Pt film. The black contrast at the center is the magnetic bit. Several white spots corresponding to the topographic image are also observed, since the probe was scanned as close as 20 nm from the surface in the lift mode in order to obtain strong magnetic contrast. The bit size, which is defined as the FWHM of the cross-sectional profile, was 80 nm in diameter with relatively weak magnetic contrast compared with those obtained in Fig. 3. Therefore,
we believe that in fact the size of the bit is smaller than the resolution of the MFM measurement.
4. Discussion In order to explain the writing mechanism of both methods, the magnetization reversal process and the effective coercivity of the CoNi/Pt multilayered film to nucleate a local magnetization reversal by a magnetic probe (hereafter it is designated as the local coercivity, Hc_local) are discussed below. The M–H loop of the CoNi/Pt multilayered film (Fig. 1) shows a clear nucleation field (Hn) and a steep magnetization reversal. Additionally, this film has a relatively low Hc/Hk value of 0.1. This implies that the magnetization reversal in a uniform field (such as in the VSM) starts from a position exhibiting the smallest coercivity in the film, followed by domain wall propagation that takes place rapidly in order to minimize the magnetic energy by forming stripe domains. The local coercivity (Hc_local), which is important to understand the writing behavior by the probe, cannot be observed in the VSM measurement which measures a value much closer to the minimum coercivity in the area (Hc_min). Fig. 5 shows schematic illustrations of the writing conditions in magnetic probe recording and heatassisted magnetic probe recording. The solid line shows the distribution of the coercivity in the CoNi/Pt film. The dotted lines show the fields which contribute the magnetization rotation. Magnetic probe recording can be performed when the following condition is satisfied H probe 4H c_local -H c_min ¼ DH c ;
Fig. 4. MFM image of the smallest bit (80 nm in diameter). No external field was applied.
(1)
where Hprobe is the magnetic field exerted from the magnetic probe, Hc_local is the local coercivity, and Hc_min is the minimum coercivity obtained by, for instance, the VSM measurement. For the ‘‘magnetic’’, non-heat-assisted, probe recording experiment, Hprobe seems to be insufficient to flip the magnetization in the medium. Indeed, the field estimated from a CoCr-coated tip is reported around 30 kA/m by McVitie et al. [10]. The design of media for magnetic probe recording
ARTICLE IN PRESS T. Onoue et al. / Journal of Magnetism and Magnetic Materials 287 (2005) 501–506
(a)
Hz Hc_local Hprobe Hc_min Hcoil
O
(b)
x
Hc_local
Hprobe + Hdem Hc_local(Twrite) Hcoil O
of Ku, and therefore in the local coercivity distribution which is larger than the field from the probe. Due to this wide distribution, no magnetic bit could be obtained by the ‘‘magnetic’’ probe recording. On the other hand, heat-assisted magnetic probe recording can be successfully performed, since by local heating, the local coercivity can be temporarily lowered. The locally high coercivity region existing in the CoNi/Pt multilayer, where a high thermal stability is expected, is utilized by reducing the local coercivity during writing. In heat-assisted recording, the following conditions are required to form single bits: H probe þ H coil þ H dem 4H c_local ðT write Þ;
Hz
x
Fig. 5. Schematic illustration of: (a) magnetic probe recording; and (b) heat-assisted magnetic probe recording. The x- and yaxes show the horizontal position and the field in a vertical direction, respectively. The solid line shows the coercivity in the CoNi/Pt film, and the dotted line shows the fields which contribute the magnetization rotation.
is therefore restricted by the limited amount of magnetic field exerted from the probe, which is a similar problem as in conventional magnetic recording. In Co(Ni)/Pt multilayered films with similar compositions, we have reported 2–10 times larger coercivity in a patterned film than that in a continuous one [11,12] and we expect a similar ratio between Hc_local and the Hc_min in this work. The origin of the large difference in those coercivities is still unclear. We believe that the inhomogeneities of the CoNi layer, which is designed to be 5.5 A˚ in thickness (almost 2 atomic layers) in each layer, might result in a distribution
505
(2)
where Hcoil is the magnetic field from the coil, Hdem is the magnetic field from the surroundings of the heated area, and Hc_local(Twrite) is the local coercivity of the heated area. In Fig. 3, Hdem plays a major role among those three magnetic fields in rotating the magnetization in the medium. In principle, the dot size is determined by the size of the heated area and the critical bit size, which balances between domain wall energy and magnetostatic energy of a single bit. Bubble domain theory [13], which discusses the stability of cylindrical domains in a perfectly homogeneous material, was used to estimate the minimum domain size in the CoNi/Pt film. A CoNi/Pt has domain wall pinning sites as well, so the actual size of the bit can differ from the critical size calculated by the bubble theory. However, it might give a relevant idea in the bit size, since it has a strong magnetic interaction in the lateral direction and reverses by domain wall propagation. The critical domain size is estimated around 100 nm in diameter by this theory [13], which is in agreement with the experimental results. Hence, it seems that the heat is applied locally to the medium in a region at most equal, but probably smaller, than the bit size.
5. Conclusion The heat-assisted mechanism is indispensable to create magnetic bits on the CoNi/Pt multilayered
ARTICLE IN PRESS 506
T. Onoue et al. / Journal of Magnetism and Magnetic Materials 287 (2005) 501–506
film. The size of the bit realized by this method is smaller than 80 nm, which is much smaller than bit sizes obtained by laser heating in magneto-optical recording, in which the size of the heated area is determined by the diffraction limit of the laser. A more precise understanding of the heat conduction and the distribution of local coercivity in the CoNi/Pt film is needed to realize even smaller bits.
References [1] T. Ohkubo, J. Kishigami, K. Yanagisawa, R. Kaneko, IEEE Trans. Magn. 29 (6) (1993) 4086. [2] L. Zhang, J. Bain, J. Zhu, IEEE Trans. Magn. 38 (5) (2002) 1895. [3] H.F. Hamann, Y.C. Martin, H.K. Wickramasinghe, Appl. Phys. Lett. 84 (5) (2004) 810.
[4] Y. Cho, K. Fujimoto, Y. Hiranaga, Y. Wagatsuma, A. Onoe, K. Tarabe, K. Kitamura, Appl. Phys. Lett. 81 (23) (2002) 4401. [5] S. Gidon, et al., Digest of IMST, 2003. [6] M. Lutwyche, et al., Appl. Phys. Lett. 77 (20) (2000) 3299. [7] J.J.M. Ruigrok, R. Coehoorn, S.R. Cumpson, H.W. Kesteren, J. Appl. Phys. 87 (9) (2000) 5398. [8] H. Shieh, M. Kryder, J. Appl. Phys. 61 (3) (1987) 1108. [9] Q. Meng, D.M. Donnet, P.J.A. Schendel, J.C. Lodder, Th. Popma, J. Magn. Soc. Jpn. 20 (S1) (1996) 53. [10] S. McVitie, R.P. Ferrier, J. Scott, G.S. White, A. Gallagher, J. Appl. Phys. 89 (7) (2001) 3656. [11] M.A.M. Haast, J.R. Schuurhuis, L. Abelmann, J.C. Lodder, Th. Popma, IEEE Trans. Magn. 34 (4) (1998) 1006. [12] N. Kikuchi, J.C. Lodder, R. Murillo, L. Abelmann, Digest of PMRC, 2004. [13] A.A. Thiele, The Bell Sys. Tech. J. (1969) 3287.
Journal of Magnetism and Magnetic Materials 287 (2005) 501–506 www.elsevier.com/locate/jmmm
Heat-assisted magnetic probe recording on a CoNi/Pt multilayered film T. Onoue, M.H. Siekman, L. Abelmann, J.C. Lodder Systems & Materials for Information Storage (SMI), MESA+ Research Institute, University of Twente, P.O. Box 217, 7500AE Enschede, The Netherlands
Abstract Heat-assisted magnetic probe recording on a CoNi/Pt multilayered film is demonstrated by delivering a current through a magnetic force microscopy tip into the recording medium, in combination with an external magnetic field. Without local heating by the probe, no bits could be written because the external field exceeds a level that demagnetizes the medium in its entirety due to local variations in the coercivity of the medium. In contrast, magnetic bits were successfully written by the heat-assisted magnetic probe recording into a saturated medium even if there was no external field, because of the demagnetization field from the surrounding of the heated area. A magnetic bit as small as 80 nm in diameter was obtained by this method. r 2004 Elsevier B.V. All rights reserved. PACS: 68.37Rt; 75.50Ss; 75.70Kw; 81.16Rf; 85.70Li; 85.80Lp Keywords: Heat assist; Probe recording; Multilayer; Magnetic force microscopy
1. Introduction In the last few decades, a remarkable increase of the recording density in magnetic storage has been accomplished by shrinking the read–write elements as well as grain sizes in magnetic recording media. Corresponding author. Tel.: +31 53 489 2747; fax: +31 53 489 3343. E-mail addresses: [email protected], [email protected] (T. Onoue).
Even higher recording density can be achieved by further shrinking the writing element to the ultimate limit of using atomically sharp probes, such as used in scanning probe microscopy (SPM). Indeed, an SPM-based storage system is an attractive way to realize a storage system exhibiting high recording density by using magnetic[1–3], ferroelectric- [4], phase change- [5] materials and polymers [6] as the probe recording media. Magnetic probe recording is an especially promising candidate among them, because we can make
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2004.10.083
ARTICLE IN PRESS T. Onoue et al. / Journal of Magnetism and Magnetic Materials 287 (2005) 501–506
use of well-developed magnetic materials explored in hard disk or magneto-optical recording systems. The virtually infinite rewriteability of magnetic recording media is a major advantage. Nowadays, the target recording density of hard disk drives is moving towards several Tb/in2. In order to accomplish such high recording densities, a sufficiently high magnetic field should be exerted from a writing head to reverse the magnetization in the medium which, because it has to be thermally stable at these high densities, possesses a high coercivity (Hc). However, the strength of the magnetic field is limited by the saturation magnetization (Ms) of the writing head. Therefore, an attractive scheme is to reduce the coercivity of the medium during writing by locally heating the medium, a method often called heat-assisted magnetic recording (HAMR) [7]. In this method, it is, however, crucial to apply the heat locally, so that adjacent tracks are not erased during the writing process. Since track-widths in modern hard disk recording systems are already well below the size of a far-field focused laser spot, other means to apply local heat are of great interest. In this paper, we demonstrate heat-assisted magnetic probe recording on a CoNi/Pt multilayered film with perpendicular magnetic anisotropy. Joule heating was realized by delivering a current through an MFM tip in contact with the medium, synchronized with external magnetic field pulses. This type of heat-assisted writing is also known as Curie point writing in magneto-optical recording, which uses a laser as a heating source [8]. However, in heat-assisted magnetic probe recording, the heat is applied on a very local scale by exploiting the sharpness of the probe. The importance of heat-assisted magnetic probe recording is clarified in comparison with a magnetic probe recording without heating.
coercivity (100 kA/m) as well as a squareness ratio of 1. The M–H loop, measured in perpendicular direction to the film surface by a VSM measurement, is shown in Fig. 1. The Curie temperature of this type of CoNi/Pt multilayers is around 250 1C [9]. A commercial scanning probe microscope system (Digital Instruments, Nanoscope IIIa, Dimension 3100) was adapted in order to perform the writing experiment. The MFM probe was prepared by depositing a NiFe layer (25 nm) on one side of a commercial Si AFM tip, followed by depositing a Pt layer (30 nm) all around the tip and on the cantilever as a conductive layer. The probe was used for writing as well as observing the magnetic bits. The writing experiment was performed in contact AFM mode without scanning. The measured resistance, while the probe and the CoNi/Pt film were in contact, was around 0.2 kO. The strength of the delivered current (shown in Fig. 3) was calculated by assuming the constant resistance during the writing process. Fig. 2 shows the schematic illustration of the probe writing setup. In this experiment, two pulse voltages are used; one to induce a magnetic field pulse from the coil by pulse generator (1) and the other to deliver a current pulse through the MFM probe to the sample by pulse generator (2) causing Joule heat at
400
200 M [kA/m]
502
0
-200
2. Experimental procedure A film composed of 20 bi-layers of CoNi (5.5 A˚)/ Pt (8.7 A˚) with a Pt seedlayer (230 A˚) was deposited on a Si (1 0 0) wafer by sputtering. The magnetic properties of the films were controlled by tuning the Ar pressure to achieve a moderate
-400 -400
-200
0 H[kA/m]
200
400
Fig. 1. M–H loop of the CoNi/Pt multilayered film. The virgin curve is also shown.
ARTICLE IN PRESS T. Onoue et al. / Journal of Magnetism and Magnetic Materials 287 (2005) 501–506
the contact point with the sample. The CoNi/Pt sample is initially in the saturated magnetization state and positioned above the coil. The magnetic field pulse in opposition to the magnetization direction of the sample was applied, synchronized with the current through the tip. The duration of the magnetic field pulse and the current pulse through the tip were 400 and 100 ms, respectively. The current pulse was applied after the start of the field pulse with a delay time of 200 ms. The AFM and MFM images were taken just after the writing
Fig. 2. Schematic illustration of the heat-assisted magnetic probe recording setup.
503
experiment by using the same probe as used for the writing experiment.
3. Results Writing can only be achieved when using ‘‘heat assisted’’ recording. If we only apply a combined magnetic field from the MFM probe and the coil below the sample (so the pulse generator (2) which delivers a current into the tip was not used), no single bit could be written. If we increase the amplitude of the magnetic field pulse gradually, while keeping the magnetic probe in contact with the medium, we observe stripe domains above critical pulse amplitude, instead of single magnetic bits. Careful observation revealed that the magnetization rotation started from a different part from where the probe was approached. It seems that the local magnetic field exerted from the probe is insufficient to activate a local magnetization rotation below the probe. Fig. 3 shows the MFM image of written magnetic bits by the heat-assisted magnetic probe recording. The AFM image of the corresponding part is shown in the same figure. By combining various bias magnetic fields and the delivered
Fig. 3. MFM image of written bits (right). AFM image of the same area as MFM image (left). The 4 4 bit matrix was created by varying the external magnetic field and the delivered current through the MFM probe during writing. Each value is shown in the figure.
ARTICLE IN PRESS 504
T. Onoue et al. / Journal of Magnetism and Magnetic Materials 287 (2005) 501–506
currents at the contact point, a 4 4 matrix of bits was created. Although the two bits at the top left are connected because of drift during writing, the 4 4 bits are successfully formed by all the writing conditions. Hence, it is obvious that the heating is indispensable to nucleate the magnetization rotation in a localized area by the probe. This experiment shows that bits can be written even without an external magnetic field. The demagnetizing magnetic field from the surroundings of the heated area, which exhibits a low value of Ms caused by the raised temperature, is sufficient to induce the magnetization rotation [3,8]. In the AFM image, no surface damage is observed, even though a current of 100 mA (a current density of 14MA/cm2 assuming the contact area to be 7 1012 cm2) was delivered. This result supports the durability of this writing scenario. Fig. 4 shows the MFM image of the smallest magnetic bit obtained in this CoNi/Pt film. The black contrast at the center is the magnetic bit. Several white spots corresponding to the topographic image are also observed, since the probe was scanned as close as 20 nm from the surface in the lift mode in order to obtain strong magnetic contrast. The bit size, which is defined as the FWHM of the cross-sectional profile, was 80 nm in diameter with relatively weak magnetic contrast compared with those obtained in Fig. 3. Therefore,
we believe that in fact the size of the bit is smaller than the resolution of the MFM measurement.
4. Discussion In order to explain the writing mechanism of both methods, the magnetization reversal process and the effective coercivity of the CoNi/Pt multilayered film to nucleate a local magnetization reversal by a magnetic probe (hereafter it is designated as the local coercivity, Hc_local) are discussed below. The M–H loop of the CoNi/Pt multilayered film (Fig. 1) shows a clear nucleation field (Hn) and a steep magnetization reversal. Additionally, this film has a relatively low Hc/Hk value of 0.1. This implies that the magnetization reversal in a uniform field (such as in the VSM) starts from a position exhibiting the smallest coercivity in the film, followed by domain wall propagation that takes place rapidly in order to minimize the magnetic energy by forming stripe domains. The local coercivity (Hc_local), which is important to understand the writing behavior by the probe, cannot be observed in the VSM measurement which measures a value much closer to the minimum coercivity in the area (Hc_min). Fig. 5 shows schematic illustrations of the writing conditions in magnetic probe recording and heatassisted magnetic probe recording. The solid line shows the distribution of the coercivity in the CoNi/Pt film. The dotted lines show the fields which contribute the magnetization rotation. Magnetic probe recording can be performed when the following condition is satisfied H probe 4H c_local -H c_min ¼ DH c ;
Fig. 4. MFM image of the smallest bit (80 nm in diameter). No external field was applied.
(1)
where Hprobe is the magnetic field exerted from the magnetic probe, Hc_local is the local coercivity, and Hc_min is the minimum coercivity obtained by, for instance, the VSM measurement. For the ‘‘magnetic’’, non-heat-assisted, probe recording experiment, Hprobe seems to be insufficient to flip the magnetization in the medium. Indeed, the field estimated from a CoCr-coated tip is reported around 30 kA/m by McVitie et al. [10]. The design of media for magnetic probe recording
ARTICLE IN PRESS T. Onoue et al. / Journal of Magnetism and Magnetic Materials 287 (2005) 501–506
(a)
Hz Hc_local Hprobe Hc_min Hcoil
O
(b)
x
Hc_local
Hprobe + Hdem Hc_local(Twrite) Hcoil O
of Ku, and therefore in the local coercivity distribution which is larger than the field from the probe. Due to this wide distribution, no magnetic bit could be obtained by the ‘‘magnetic’’ probe recording. On the other hand, heat-assisted magnetic probe recording can be successfully performed, since by local heating, the local coercivity can be temporarily lowered. The locally high coercivity region existing in the CoNi/Pt multilayer, where a high thermal stability is expected, is utilized by reducing the local coercivity during writing. In heat-assisted recording, the following conditions are required to form single bits: H probe þ H coil þ H dem 4H c_local ðT write Þ;
Hz
x
Fig. 5. Schematic illustration of: (a) magnetic probe recording; and (b) heat-assisted magnetic probe recording. The x- and yaxes show the horizontal position and the field in a vertical direction, respectively. The solid line shows the coercivity in the CoNi/Pt film, and the dotted line shows the fields which contribute the magnetization rotation.
is therefore restricted by the limited amount of magnetic field exerted from the probe, which is a similar problem as in conventional magnetic recording. In Co(Ni)/Pt multilayered films with similar compositions, we have reported 2–10 times larger coercivity in a patterned film than that in a continuous one [11,12] and we expect a similar ratio between Hc_local and the Hc_min in this work. The origin of the large difference in those coercivities is still unclear. We believe that the inhomogeneities of the CoNi layer, which is designed to be 5.5 A˚ in thickness (almost 2 atomic layers) in each layer, might result in a distribution
505
(2)
where Hcoil is the magnetic field from the coil, Hdem is the magnetic field from the surroundings of the heated area, and Hc_local(Twrite) is the local coercivity of the heated area. In Fig. 3, Hdem plays a major role among those three magnetic fields in rotating the magnetization in the medium. In principle, the dot size is determined by the size of the heated area and the critical bit size, which balances between domain wall energy and magnetostatic energy of a single bit. Bubble domain theory [13], which discusses the stability of cylindrical domains in a perfectly homogeneous material, was used to estimate the minimum domain size in the CoNi/Pt film. A CoNi/Pt has domain wall pinning sites as well, so the actual size of the bit can differ from the critical size calculated by the bubble theory. However, it might give a relevant idea in the bit size, since it has a strong magnetic interaction in the lateral direction and reverses by domain wall propagation. The critical domain size is estimated around 100 nm in diameter by this theory [13], which is in agreement with the experimental results. Hence, it seems that the heat is applied locally to the medium in a region at most equal, but probably smaller, than the bit size.
5. Conclusion The heat-assisted mechanism is indispensable to create magnetic bits on the CoNi/Pt multilayered
ARTICLE IN PRESS 506
T. Onoue et al. / Journal of Magnetism and Magnetic Materials 287 (2005) 501–506
film. The size of the bit realized by this method is smaller than 80 nm, which is much smaller than bit sizes obtained by laser heating in magneto-optical recording, in which the size of the heated area is determined by the diffraction limit of the laser. A more precise understanding of the heat conduction and the distribution of local coercivity in the CoNi/Pt film is needed to realize even smaller bits.
References [1] T. Ohkubo, J. Kishigami, K. Yanagisawa, R. Kaneko, IEEE Trans. Magn. 29 (6) (1993) 4086. [2] L. Zhang, J. Bain, J. Zhu, IEEE Trans. Magn. 38 (5) (2002) 1895. [3] H.F. Hamann, Y.C. Martin, H.K. Wickramasinghe, Appl. Phys. Lett. 84 (5) (2004) 810.
[4] Y. Cho, K. Fujimoto, Y. Hiranaga, Y. Wagatsuma, A. Onoe, K. Tarabe, K. Kitamura, Appl. Phys. Lett. 81 (23) (2002) 4401. [5] S. Gidon, et al., Digest of IMST, 2003. [6] M. Lutwyche, et al., Appl. Phys. Lett. 77 (20) (2000) 3299. [7] J.J.M. Ruigrok, R. Coehoorn, S.R. Cumpson, H.W. Kesteren, J. Appl. Phys. 87 (9) (2000) 5398. [8] H. Shieh, M. Kryder, J. Appl. Phys. 61 (3) (1987) 1108. [9] Q. Meng, D.M. Donnet, P.J.A. Schendel, J.C. Lodder, Th. Popma, J. Magn. Soc. Jpn. 20 (S1) (1996) 53. [10] S. McVitie, R.P. Ferrier, J. Scott, G.S. White, A. Gallagher, J. Appl. Phys. 89 (7) (2001) 3656. [11] M.A.M. Haast, J.R. Schuurhuis, L. Abelmann, J.C. Lodder, Th. Popma, IEEE Trans. Magn. 34 (4) (1998) 1006. [12] N. Kikuchi, J.C. Lodder, R. Murillo, L. Abelmann, Digest of PMRC, 2004. [13] A.A. Thiele, The Bell Sys. Tech. J. (1969) 3287.