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Pt multilayered thin films by using an MFM tip
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 272–276 (2004) 2317–2318
Probe recording on CoNi/Pt multilayered thin films by using an MFM tip T. Onoue, M.H. Siekman, L. Abelmann, J.C. Lodder* Systems and Materials for Information storage (SMI), MESA+, University of Twente, P.O.Box 217, Enschede 7500AE, The Netherlands
Abstract Reversed single magnetic bits of 130 nm in diameter were successfully written in a CoNi/Pt multilayered film by means of magnetic probe recording. A weak exchange coupled layer deposited onto a stronger exchange coupled layer effectively induces a local magnetic reversal, which results in the formation of small and stable magnetic bits. r 2003 Elsevier B.V. All rights reserved. PACS: 75.50. Ss; 85.70. Li; 75.70. Kw Keywords: Probe recording; CoNi/Pt multilayer; Exchange coupling; Magnetic bits
This work demonstrates the formation of reversed single magnetic bits on a CoNi/Pt multilayered film with strong perpendicular anisotropy by means of MFM probe recording with an external magnetic field. A CoNi/Pt multilayered film composed of [CoNi ( ( 20/Pt seedlayer (230 A) ( was deposited (5.5 A)/Pt (8.7 A)] by sputtering onto a thermally oxidized Si (1 0 0) wafer. The magnetic properties of the films were controlled by tuning the Ar pressure during deposition. A commercial MFM (Nanoscope3100, Digital Instruments) was equipped with a coil that applies magnetic field pulses in order to perform the writing experiment. Fig. 1 shows a schematic illustration of the set-up. A CoNi/Pt multilayered sample in the saturated magnetization state was positioned onto the coil, which was connected to a pulse generator with a current amplifier which is able to deliver current pulses of 40 A in order to generate a strong magnetic field in a short period. The pulse width in this experiment was set at 270 ms. The magnetic field pulse was applied in opposition to the magnetized direction of the film. The combination of the pulsed magnetic field and the field from the MFM tip locally reverses the magnetization of the film. A Si AFM tip coated on one side with a Co layer was used *Corresponding author. Tel.: +31-53-489-2750; fax: +3153-489-3343. E-mail address: [email protected] (J.C. Lodder).
for both writing as well as observing the magnetic domain structure. Fig. 2 shows M–H loops of the CoNi/Pt multilayered films. It is well known that the Ar pressure during deposition strongly affects the microstructure of the film, which results in a change of its magnetic properties [1,2]. A film deposited with an Ar pressure of 1.6 10 2 mbar (see sample A) shows a squareness ratio (SQR) of unity and a steep magnetization reversal. This suggests that once a reversed domain was nucleated in the film, the domain will rapidly expand to minimize the total magnetic energy. The writing experiment performed on sample A showed that a reversed single magnetic domain was hardly obtained, or it was several microns in size with an irregular shape. Furthermore, some domains collapsed during scanning due to stray fields from the MFM probe. The instability and low controllability of the written bits in sample A is attributable to the strong exchange interaction in the film. On the other hand, a film deposited with an Ar pressure of 2.8 10 2 mbar (see sample B) showed a shared M–H loop with high coercivity. The shearing of the M2H loop is related to the weak exchange interaction between magnetic grains [3,4]. The magnetic reversal mode tends to change from domain wall motion to isolated magnetic reversal in each grains that is desirable to form local magnetic reversal. However, sample B is difficult to saturate with the external coil
0304-8853/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.12.940
ARTICLE IN PRESS T. Onoue et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) 2317–2318
2318
Fig. 3. MFM images of the written bits on sample C with applied fields of (a) 34 kA/m, and (b) 68 kA/m from the coil.
Fig. 1. Schematic illustration of writing set-up.
600 400
M [kA/m]
200 0 -200 A B C
-400 -600 -600
-400
-200
0 200 H [kA/m]
400
600
( ( 20/Pt(230 A) ( films Fig. 2. M–H loops of [CoNi(5.5 A)/Pt(8.7 A)] deposited with various Ar pressure.
used in this work due to high saturation field (Hs ). Therefore, a combination film C was prepared to take advantages of both films. This film was prepared by depositing 10 bi-layers of CoNi/Pt with an Ar pressure of 1.6 10 2 mbar followed by the deposition of 10 bilayers of CoNi/Pt with that of 2.8 10 2 mbar. The M– H loop of sample C shows a similar shape to that of sample A, which suggests that domain wall motion is still the dominant magnetization reversal mode. Thus, the double-layered structure with different exchange coupling enables us to control the value of Hc and Hn while maintaining steep magnetization reversal that results in high writing sensitivity. Fig. 3 shows MFM images of written magnetic bits in sample C. The formation of single bits with a diameter of 130 nm was successfully performed in sample C. The size of the dot increases with an increase in the applied field. It suggests that the magnetic bit cannot expand or shrink its size that minimize the summation of magnetostatic energy and domain wall energy, but the domain wall propagation is restricted such as pinning sites which affects the size of the domain according to the strength
of applied field. The dots obtained in sample C were quite stable during scanning by an MFM probe, which is consistent with the discussion above. It should be noted that the smaller dot shows a rounder shape which implies high applicability of CoNi/Pt multilayers for probe recording media. In the following we will discuss the mechanism of formation of a single bit in sample C. The weak exchange-coupled layer deposited with high Ar pressure facilitates initial local magnetization rotation when a localized magnetic field is applied by the MFM probe and the external coil. The strong exchange-coupled layer greatly affects the whole magnetization reversal process in such a way that sample C shows smooth magnetization reversal with a small magnetic reversal field distribution, even if it has double-layered structure with different magnetic properties in each layers. Hence, a magnetic bit is reversed completely to the bottom of the film because of the exchange interaction in the thickness direction in the film. The weak exchange coupled layer, which increases the values of Hc and Hn in sample C, contributes to form uniform dots due to pinning sites. In sample C, the magnetic wall propagation is restricted due to high coercivity, which gives rise to high stability of the domains as well as the controllability of the bits. The research was supported by the FET arm of the European IST programme under contract name InProM (Integrated Probe Memory), IST-2001-33065.
References [1] W.B. Zeper, H.W. van Kesteren, B.A.J. Jacobs, J.H.M. Spruit, P.F. Carcia, J. Appl. Phys. 70 (1991) 2264. [2] Q. Meng, P. de Haan, W.P. van Drent, J.C. Lodder, Th.J.A. Popma, IEEE Trans. Magn. 32 (1996) 4064. [3] I. Tagawa, A. Takeo, Y. Nakamura, J. Magn. Magn. Mater. 155 (1996) 341. [4] N. Honda, J. Ariake, K. Ouchi, IEEE Trans. Magn. 34 (1998) 1651.
Journal of Magnetism and Magnetic Materials 272–276 (2004) 2317–2318
Probe recording on CoNi/Pt multilayered thin films by using an MFM tip T. Onoue, M.H. Siekman, L. Abelmann, J.C. Lodder* Systems and Materials for Information storage (SMI), MESA+, University of Twente, P.O.Box 217, Enschede 7500AE, The Netherlands
Abstract Reversed single magnetic bits of 130 nm in diameter were successfully written in a CoNi/Pt multilayered film by means of magnetic probe recording. A weak exchange coupled layer deposited onto a stronger exchange coupled layer effectively induces a local magnetic reversal, which results in the formation of small and stable magnetic bits. r 2003 Elsevier B.V. All rights reserved. PACS: 75.50. Ss; 85.70. Li; 75.70. Kw Keywords: Probe recording; CoNi/Pt multilayer; Exchange coupling; Magnetic bits
This work demonstrates the formation of reversed single magnetic bits on a CoNi/Pt multilayered film with strong perpendicular anisotropy by means of MFM probe recording with an external magnetic field. A CoNi/Pt multilayered film composed of [CoNi ( ( 20/Pt seedlayer (230 A) ( was deposited (5.5 A)/Pt (8.7 A)] by sputtering onto a thermally oxidized Si (1 0 0) wafer. The magnetic properties of the films were controlled by tuning the Ar pressure during deposition. A commercial MFM (Nanoscope3100, Digital Instruments) was equipped with a coil that applies magnetic field pulses in order to perform the writing experiment. Fig. 1 shows a schematic illustration of the set-up. A CoNi/Pt multilayered sample in the saturated magnetization state was positioned onto the coil, which was connected to a pulse generator with a current amplifier which is able to deliver current pulses of 40 A in order to generate a strong magnetic field in a short period. The pulse width in this experiment was set at 270 ms. The magnetic field pulse was applied in opposition to the magnetized direction of the film. The combination of the pulsed magnetic field and the field from the MFM tip locally reverses the magnetization of the film. A Si AFM tip coated on one side with a Co layer was used *Corresponding author. Tel.: +31-53-489-2750; fax: +3153-489-3343. E-mail address: [email protected] (J.C. Lodder).
for both writing as well as observing the magnetic domain structure. Fig. 2 shows M–H loops of the CoNi/Pt multilayered films. It is well known that the Ar pressure during deposition strongly affects the microstructure of the film, which results in a change of its magnetic properties [1,2]. A film deposited with an Ar pressure of 1.6 10 2 mbar (see sample A) shows a squareness ratio (SQR) of unity and a steep magnetization reversal. This suggests that once a reversed domain was nucleated in the film, the domain will rapidly expand to minimize the total magnetic energy. The writing experiment performed on sample A showed that a reversed single magnetic domain was hardly obtained, or it was several microns in size with an irregular shape. Furthermore, some domains collapsed during scanning due to stray fields from the MFM probe. The instability and low controllability of the written bits in sample A is attributable to the strong exchange interaction in the film. On the other hand, a film deposited with an Ar pressure of 2.8 10 2 mbar (see sample B) showed a shared M–H loop with high coercivity. The shearing of the M2H loop is related to the weak exchange interaction between magnetic grains [3,4]. The magnetic reversal mode tends to change from domain wall motion to isolated magnetic reversal in each grains that is desirable to form local magnetic reversal. However, sample B is difficult to saturate with the external coil
0304-8853/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.12.940
ARTICLE IN PRESS T. Onoue et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) 2317–2318
2318
Fig. 3. MFM images of the written bits on sample C with applied fields of (a) 34 kA/m, and (b) 68 kA/m from the coil.
Fig. 1. Schematic illustration of writing set-up.
600 400
M [kA/m]
200 0 -200 A B C
-400 -600 -600
-400
-200
0 200 H [kA/m]
400
600
( ( 20/Pt(230 A) ( films Fig. 2. M–H loops of [CoNi(5.5 A)/Pt(8.7 A)] deposited with various Ar pressure.
used in this work due to high saturation field (Hs ). Therefore, a combination film C was prepared to take advantages of both films. This film was prepared by depositing 10 bi-layers of CoNi/Pt with an Ar pressure of 1.6 10 2 mbar followed by the deposition of 10 bilayers of CoNi/Pt with that of 2.8 10 2 mbar. The M– H loop of sample C shows a similar shape to that of sample A, which suggests that domain wall motion is still the dominant magnetization reversal mode. Thus, the double-layered structure with different exchange coupling enables us to control the value of Hc and Hn while maintaining steep magnetization reversal that results in high writing sensitivity. Fig. 3 shows MFM images of written magnetic bits in sample C. The formation of single bits with a diameter of 130 nm was successfully performed in sample C. The size of the dot increases with an increase in the applied field. It suggests that the magnetic bit cannot expand or shrink its size that minimize the summation of magnetostatic energy and domain wall energy, but the domain wall propagation is restricted such as pinning sites which affects the size of the domain according to the strength
of applied field. The dots obtained in sample C were quite stable during scanning by an MFM probe, which is consistent with the discussion above. It should be noted that the smaller dot shows a rounder shape which implies high applicability of CoNi/Pt multilayers for probe recording media. In the following we will discuss the mechanism of formation of a single bit in sample C. The weak exchange-coupled layer deposited with high Ar pressure facilitates initial local magnetization rotation when a localized magnetic field is applied by the MFM probe and the external coil. The strong exchange-coupled layer greatly affects the whole magnetization reversal process in such a way that sample C shows smooth magnetization reversal with a small magnetic reversal field distribution, even if it has double-layered structure with different magnetic properties in each layers. Hence, a magnetic bit is reversed completely to the bottom of the film because of the exchange interaction in the thickness direction in the film. The weak exchange coupled layer, which increases the values of Hc and Hn in sample C, contributes to form uniform dots due to pinning sites. In sample C, the magnetic wall propagation is restricted due to high coercivity, which gives rise to high stability of the domains as well as the controllability of the bits. The research was supported by the FET arm of the European IST programme under contract name InProM (Integrated Probe Memory), IST-2001-33065.
References [1] W.B. Zeper, H.W. van Kesteren, B.A.J. Jacobs, J.H.M. Spruit, P.F. Carcia, J. Appl. Phys. 70 (1991) 2264. [2] Q. Meng, P. de Haan, W.P. van Drent, J.C. Lodder, Th.J.A. Popma, IEEE Trans. Magn. 32 (1996) 4064. [3] I. Tagawa, A. Takeo, Y. Nakamura, J. Magn. Magn. Mater. 155 (1996) 341. [4] N. Honda, J. Ariake, K. Ouchi, IEEE Trans. Magn. 34 (1998) 1651.