- Email: [email protected]
Effect of Ag under-layer on patterning of periodic magnetic structure using femtosecond laser-induced crystallization
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 310 (2007) 1581–1583 www.elsevier.com/locate/jmmm
Effect of Ag under-layer on patterning of periodic magnetic structure using femtosecond laser-induced crystallization Jeon Kima,, Jung H. Kima, C.K. Kima, Chong Seung Yoona,, Geon Joon Leeb, Young Pak Leeb a
Division of Materials Science and Engineering, Hanyang University, Seoul 133-791, Republic of Korea b Quantum Photonic Science Research Center, Hanyang University, Seoul 133-791, Republic of Korea Available online 13 November 2006
Abstract Femtosecond laser-interference crystallization (FLIC) was used to form a spatially periodic magnetic structure by selectively crystallizing a paramagnetic amorphous Co2MnSi thin film, which was covered with an Ag thin film. Depending on the incident beam direction, presence of the Ag capping layer improved FLIC of a-Co2MnSi by accentuating the structural difference between dark and bright band regions. Although the crystalline region remained unchanged with or without insertion of the Ag film, crystallization of the dark band region was reduced by the Ag film. The dark band region was in a microcrystalline state, containing a non-equilibrium solid solution of Co, Mn and Si. During FLIC, the Ag film acted as a heat sink, efficiently absorbing the energy released by the crystallization of amorphous Co2MnSi thin film, thus sharpening lateral thermal gradient producing FLIC and help confining the laser energy to the bright band region. r 2006 Elsevier B.V. All rights reserved. PACS: 42.62.b; 75.70.Ak; 81.65.Cf Keywords: Femtosecond laser; Crystallization; Patterning; Periodic magnetic structure
Recent advent of femtosecond lasers, producing an extremely high heating rate in a localized area, has brought about many new applications in laser processing, including precision machining of submicron-sized features, laser surgery and nanotechnology [1]. Because ultra-short pulse duration of the femtosecond laser ensures a minimal thermal damage to the surrounding material, allowing spatially selective thermal treatment of a material with high resolution [2]; femtosecond laser is ideal for inducing local phase changes in materials to fabricate nanoparticles and nanostructures and to modify physical properties [3]. Previously, we applied FLIC to an amorphous Co2MnSi film to produce a periodic magnetic structure. As-deposited amorphous Co2MnSi film, which was initially paramagnetic, was transformed to a periodic magnetic structure by Corresponding authors. Tel.: +82 2 2220 0384; fax: +82 2 2220 1838.
E-mail addresses: [email protected] (J. Kim), [email protected] (C.S. Yoon). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.10.601
locally crystallizing the amorphous thin film to ferromagnetic state using a two-beam interference pattern of femtosecond laser pulses [4]. Although a periodic magnetic structure, consisting of alternating paramagnetic and ferromagnetic bands, was successfully produced, because of the relatively high thermal conductivity of the Co2MnSi film, the region irradiated with the dark fringe (destructive interference) of the laser pulses (called dark band region hereafter) was also partially crystallized. In this work, we introduced a metal overlayer on top of the amorphous Co2MnSi film, acting as a heat sink to minimize the partial crystallization of the dark band region and to enhance the selectivity of FLIC. The resulting laser-annealed film structures were analyzed as a function of both processing conditions and incident beam directions. A 100-nm-thick amorphous Co2MnSi thin film was deposited on a glass substrate using a radio-frequency magnetron sputtering at room temperature. Preparation of the amorphous Co2MnSi thin film was described in detail
ARTICLE IN PRESS 1582
J. Kim et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 1581–1583
elsewhere [5]. On the Co2MnSi film, a 50-nm-thick Ag film was deposited using a thermal evaporator. The periodic structure was obtained by two-beam interference of femtosecond laser pulses. The grating formation and relaxation profiles were monitored using a He–Ne laser. The femtosecond laser system was a regeneratively amplified Ti:sapphire laser with 800 nm output wavelength, 130 fs pulse duration, 1.0 mJ maximum pulse energy, and 1 kHz repetition rate. Detailed description of FLIC was described elsewhere [6]. Fig. 1(a) shows a scanning electron microscopy (SEM) image of the a-Co2MnSi irradiated with 3000 shots of two 290 mJ laser pulses from the Ag side. Although a grating with a periodicity of 2 mm was observed, the grating resulted from ablation of the Ag film instead of localized crystallization of the Co2MnSi. Ablation of Ag was confirmed by both energy dispersive X-ray spectroscopy and backscattered SEM. In fact, with higher number of laser pulses, the entire Ag film was spalled off due to excessive absorption of the laser energy by the Ag film. With further decrease in number of laser pulses, grating was not observed at all using SEM. In order to minimize the absorption by Ag, the laser pulses were irradiated from the glass substrate side. When the number of pulses decreased to 300 shots at 290 mJ, as can be seen from Fig. 1(b), a grating with a periodicity of 2 mm without any surface damage to the Ag film was obtained. A plan view image of transmission electron microscopy (TEM) obtained from the sample in Fig. 1(b) is shown in Fig. 2. The TEM image clearly differentiates the bright and dark band regions. The bright band region showed a grainy contrast resulting from presence of crystalline phases, whereas the dark band region a uniform contrast, suggesting the dark band region remained amorphous or consisted of extremely small grains. Meanwhile, the dark band region produced without Ag had clearly visible nanosized grains [4]; hence, it appears that structural difference between the bright and dark band regions was accentuated by the Ag film.
Fig. 1. SEM image of Ag/a-Co2MnSi film stack irradiated: (a) with 3000 shots of two 290 mJ laser pulses with laser pulses incident from the Ag film side, (b) with 300 shots of same laser power with laser pulses incident from the substrate side.
Fig. 2. TEM image of Ag/a-Co2MnSi film stack irradiated with 300 shots of two 290 mJ laser pulses with laser pulses incident from the substrate side, (I ¼ bright band region, II ¼ dark band region).
Fig. 3. Indexed electron diffraction patterns from regions I and II in Fig. 2.
For a detailed structural analysis, selected area electron diffraction (SAD) patterns were obtained from respective regions (labeled I and II) and shown in Fig. 3. The diffraction pattern from the dark band region indicated the region crystallized into a non-equilibrium solid solution of Co, Mn and Si, having an FCC structure that was previously observed in FLIC of a-Co2MnSi. In comparison, the diffraction pattern from the bright band region was matched to a mixture of the non-equilibrium FCC phase, FCC-Co, b-Mn and Co2Si, which is consistent with our previous work. The diffraction patterns convincingly support the structural difference between two regions. Extremely small grain size and non-equilibrium state of Co, Mn and Si ensure that the dark band region remained paramagnetic, while the crystallized region became ferromagnetic as confirmed by magnetic force microscopy. It is conjectured that during FLIC, the Ag film acted as a heat sink, efficiently absorbing the energy released by the
ARTICLE IN PRESS J. Kim et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 1581–1583
crystallization of a-Co2MnSi, thus sharpening lateral thermal gradient produced FLIC and improving the confinement of the laser energy to the bright band region. In conclusion, we demonstrated that the Ag overlayer on aCo2MnSi film was beneficial in producing a periodic magnetic structure and that the resulting structure was sensitive to the incident beam direction. This work was supported by the Ministry of Science and Technology through the Nanoscopia Center of Excellence at Hanyang University and also partially supported by the research fund of Hanyang University (HY-2003).
1583
References [1] V.R. Bhardwaj, E. Simova, P.B. Corkum, D.M. Rayner, C. Hnatovsky, B. Schreder, M. Kluge, J. Zimmer, J. Appl. Phys. 97 (2005) 083102. [2] P. Galinetto, D. Ballarini, D. Grando, G. Samoggia, Appl. Surf. Sci. 248 (2005) 291. [3] G. Zhang, D. Gu, F. Gan, X. Jiang, Q. Chen, Thin Solid Films 474 (2005) 169. [4] J.H. Kim, J. Kim, S.U. Lim, C.K. Kim, C.S. Yoon, G.J. Lee, Y.P. Lee, J. Appl. Phys. 99 (2006) 08G311. [5] S.J. Kim, D.H. Lim, C.S. Yoon, C.K. Kim, Solid State Commun. 132 (2004) 361. [6] G.J. Lee, J. Park, E.K. Kim, Y.P. Lee, K.M. Kim, H. Cheong, C.S. Yoon, Y.-D. Son, J. Jang, Opt. Express 13 (2005) 6445.
Journal of Magnetism and Magnetic Materials 310 (2007) 1581–1583 www.elsevier.com/locate/jmmm
Effect of Ag under-layer on patterning of periodic magnetic structure using femtosecond laser-induced crystallization Jeon Kima,, Jung H. Kima, C.K. Kima, Chong Seung Yoona,, Geon Joon Leeb, Young Pak Leeb a
Division of Materials Science and Engineering, Hanyang University, Seoul 133-791, Republic of Korea b Quantum Photonic Science Research Center, Hanyang University, Seoul 133-791, Republic of Korea Available online 13 November 2006
Abstract Femtosecond laser-interference crystallization (FLIC) was used to form a spatially periodic magnetic structure by selectively crystallizing a paramagnetic amorphous Co2MnSi thin film, which was covered with an Ag thin film. Depending on the incident beam direction, presence of the Ag capping layer improved FLIC of a-Co2MnSi by accentuating the structural difference between dark and bright band regions. Although the crystalline region remained unchanged with or without insertion of the Ag film, crystallization of the dark band region was reduced by the Ag film. The dark band region was in a microcrystalline state, containing a non-equilibrium solid solution of Co, Mn and Si. During FLIC, the Ag film acted as a heat sink, efficiently absorbing the energy released by the crystallization of amorphous Co2MnSi thin film, thus sharpening lateral thermal gradient producing FLIC and help confining the laser energy to the bright band region. r 2006 Elsevier B.V. All rights reserved. PACS: 42.62.b; 75.70.Ak; 81.65.Cf Keywords: Femtosecond laser; Crystallization; Patterning; Periodic magnetic structure
Recent advent of femtosecond lasers, producing an extremely high heating rate in a localized area, has brought about many new applications in laser processing, including precision machining of submicron-sized features, laser surgery and nanotechnology [1]. Because ultra-short pulse duration of the femtosecond laser ensures a minimal thermal damage to the surrounding material, allowing spatially selective thermal treatment of a material with high resolution [2]; femtosecond laser is ideal for inducing local phase changes in materials to fabricate nanoparticles and nanostructures and to modify physical properties [3]. Previously, we applied FLIC to an amorphous Co2MnSi film to produce a periodic magnetic structure. As-deposited amorphous Co2MnSi film, which was initially paramagnetic, was transformed to a periodic magnetic structure by Corresponding authors. Tel.: +82 2 2220 0384; fax: +82 2 2220 1838.
E-mail addresses: [email protected] (J. Kim), [email protected] (C.S. Yoon). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.10.601
locally crystallizing the amorphous thin film to ferromagnetic state using a two-beam interference pattern of femtosecond laser pulses [4]. Although a periodic magnetic structure, consisting of alternating paramagnetic and ferromagnetic bands, was successfully produced, because of the relatively high thermal conductivity of the Co2MnSi film, the region irradiated with the dark fringe (destructive interference) of the laser pulses (called dark band region hereafter) was also partially crystallized. In this work, we introduced a metal overlayer on top of the amorphous Co2MnSi film, acting as a heat sink to minimize the partial crystallization of the dark band region and to enhance the selectivity of FLIC. The resulting laser-annealed film structures were analyzed as a function of both processing conditions and incident beam directions. A 100-nm-thick amorphous Co2MnSi thin film was deposited on a glass substrate using a radio-frequency magnetron sputtering at room temperature. Preparation of the amorphous Co2MnSi thin film was described in detail
ARTICLE IN PRESS 1582
J. Kim et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 1581–1583
elsewhere [5]. On the Co2MnSi film, a 50-nm-thick Ag film was deposited using a thermal evaporator. The periodic structure was obtained by two-beam interference of femtosecond laser pulses. The grating formation and relaxation profiles were monitored using a He–Ne laser. The femtosecond laser system was a regeneratively amplified Ti:sapphire laser with 800 nm output wavelength, 130 fs pulse duration, 1.0 mJ maximum pulse energy, and 1 kHz repetition rate. Detailed description of FLIC was described elsewhere [6]. Fig. 1(a) shows a scanning electron microscopy (SEM) image of the a-Co2MnSi irradiated with 3000 shots of two 290 mJ laser pulses from the Ag side. Although a grating with a periodicity of 2 mm was observed, the grating resulted from ablation of the Ag film instead of localized crystallization of the Co2MnSi. Ablation of Ag was confirmed by both energy dispersive X-ray spectroscopy and backscattered SEM. In fact, with higher number of laser pulses, the entire Ag film was spalled off due to excessive absorption of the laser energy by the Ag film. With further decrease in number of laser pulses, grating was not observed at all using SEM. In order to minimize the absorption by Ag, the laser pulses were irradiated from the glass substrate side. When the number of pulses decreased to 300 shots at 290 mJ, as can be seen from Fig. 1(b), a grating with a periodicity of 2 mm without any surface damage to the Ag film was obtained. A plan view image of transmission electron microscopy (TEM) obtained from the sample in Fig. 1(b) is shown in Fig. 2. The TEM image clearly differentiates the bright and dark band regions. The bright band region showed a grainy contrast resulting from presence of crystalline phases, whereas the dark band region a uniform contrast, suggesting the dark band region remained amorphous or consisted of extremely small grains. Meanwhile, the dark band region produced without Ag had clearly visible nanosized grains [4]; hence, it appears that structural difference between the bright and dark band regions was accentuated by the Ag film.
Fig. 1. SEM image of Ag/a-Co2MnSi film stack irradiated: (a) with 3000 shots of two 290 mJ laser pulses with laser pulses incident from the Ag film side, (b) with 300 shots of same laser power with laser pulses incident from the substrate side.
Fig. 2. TEM image of Ag/a-Co2MnSi film stack irradiated with 300 shots of two 290 mJ laser pulses with laser pulses incident from the substrate side, (I ¼ bright band region, II ¼ dark band region).
Fig. 3. Indexed electron diffraction patterns from regions I and II in Fig. 2.
For a detailed structural analysis, selected area electron diffraction (SAD) patterns were obtained from respective regions (labeled I and II) and shown in Fig. 3. The diffraction pattern from the dark band region indicated the region crystallized into a non-equilibrium solid solution of Co, Mn and Si, having an FCC structure that was previously observed in FLIC of a-Co2MnSi. In comparison, the diffraction pattern from the bright band region was matched to a mixture of the non-equilibrium FCC phase, FCC-Co, b-Mn and Co2Si, which is consistent with our previous work. The diffraction patterns convincingly support the structural difference between two regions. Extremely small grain size and non-equilibrium state of Co, Mn and Si ensure that the dark band region remained paramagnetic, while the crystallized region became ferromagnetic as confirmed by magnetic force microscopy. It is conjectured that during FLIC, the Ag film acted as a heat sink, efficiently absorbing the energy released by the
ARTICLE IN PRESS J. Kim et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 1581–1583
crystallization of a-Co2MnSi, thus sharpening lateral thermal gradient produced FLIC and improving the confinement of the laser energy to the bright band region. In conclusion, we demonstrated that the Ag overlayer on aCo2MnSi film was beneficial in producing a periodic magnetic structure and that the resulting structure was sensitive to the incident beam direction. This work was supported by the Ministry of Science and Technology through the Nanoscopia Center of Excellence at Hanyang University and also partially supported by the research fund of Hanyang University (HY-2003).
1583
References [1] V.R. Bhardwaj, E. Simova, P.B. Corkum, D.M. Rayner, C. Hnatovsky, B. Schreder, M. Kluge, J. Zimmer, J. Appl. Phys. 97 (2005) 083102. [2] P. Galinetto, D. Ballarini, D. Grando, G. Samoggia, Appl. Surf. Sci. 248 (2005) 291. [3] G. Zhang, D. Gu, F. Gan, X. Jiang, Q. Chen, Thin Solid Films 474 (2005) 169. [4] J.H. Kim, J. Kim, S.U. Lim, C.K. Kim, C.S. Yoon, G.J. Lee, Y.P. Lee, J. Appl. Phys. 99 (2006) 08G311. [5] S.J. Kim, D.H. Lim, C.S. Yoon, C.K. Kim, Solid State Commun. 132 (2004) 361. [6] G.J. Lee, J. Park, E.K. Kim, Y.P. Lee, K.M. Kim, H. Cheong, C.S. Yoon, Y.-D. Son, J. Jang, Opt. Express 13 (2005) 6445.