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Cobalt antidot arrays on membranes: Fabrication and investigation with transmission X-ray microscopy
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 316 (2007) 99–102 www.elsevier.com/locate/jmmm
Cobalt antidot arrays on membranes: Fabrication and investigation with transmission X-ray microscopy L.J. Heydermana,, S. Czekaja, F. Noltinga, D.-H. Kimb, P. Fischerb a
b
Paul Scherrer Institut, Villigen PSI, CH-5232, Switzerland LBNL/CXRO, 1 Cyclotron Road, Berkeley, CA 94720, USA Available online 25 February 2007
Abstract We have developed a method to fabricate ferromagnetic antidot arrays on silicon nitride membrane substrates for electron or soft Xray microscopy with antidot periods ranging from 2 mm down to 200 nm. Observations of cobalt antidot arrays with magnetic soft X-ray microscopy show that for large periods, flux closure states occur between the antidots in the as-grown state and on application of a magnetic field, domain chains are created which show a spin configuration at the chain ends comprising four 901 walls. Pinning of the domain chain ends plays an important role in the magnetization reversal, determining the length of the chains and resulting in preservation of the domain chain configuration on reducing of the applied magnetic field to zero. r 2007 Elsevier B.V. All rights reserved. PACS: 75.60.Jk; 75.60.Ch; 75.60.Ej; 75.75.+a Keywords: Ferromagnetic antidot array; Magnetic soft transmission X-ray microscopy; Magnetization reversal; Silicon nitride membrane
1. Introduction Patterned magnetic thin films are currently the focus of scientific interest due to the new magnetic phenomena which occur at reduced lateral dimensions and also because of their potential for industrial applications such as highdensity information storage and sensor devices [1,2]. Antidot arrays, which comprise a continuous film containing a regular array of non-magnetic inclusions or holes, are particularly of interest because of the resulting novel domain configurations, additional magnetic anisotropies and modification of the magnetization reversal, which in turn affects the switching fields and magnetoresistance behavior [3–7]. The basic domain configuration for square antidot arrays with the antidot size greater than or equal to the antidot separation is a periodic checked domain contrast commensurate with the antidot lattice, which we have observed with photoemission electron microscopy (PEEM) [8,9]. Similar configurations were seen with magnetic force microscopy (MFM) [3,5,7] and transmisCorresponding author. Tel.: +41 56 310 2613; fax: +41 56 310 2646.
E-mail address: [email protected] (L.J. Heyderman). 0304-8853/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2007.02.010
sion electron microscopy [10], and correlate well with micromagnetic simulations [11]. We have recently performed PEEM observations of remanent magnetic states in such antidot arrays on application of an in-plane magnetic field [12]. These measurements revealed that magnetization reversal occurs by nucleation and propagation of chains of magnetic domains that have discrete lengths corresponding to multiples of the antidot period. We report here on observations of the magnetic spin structures in antidot arrays with transmission X-ray microscopy (TXM). The high spatial resolution of TXM and the possibility to image in applied magnetic fields allow us to observe the fine details of the magnetic spin configurations and the reversal behavior [13]. These studies require substrates that allow the transmission of X-rays and we describe a method to fabricate antidot arrays on silicon nitride membranes, suitable for both X-ray and transmission electron microscopy. 2. Experimental We employed electron beam lithography to fabricate the antidot arrays. It is not possible to use lift-off as a pattern
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L.J. Heyderman et al. / Journal of Magnetism and Magnetic Materials 316 (2007) 99–102
Fig. 1. Schematic diagram showing fabrication of antidot arrays on silicon nitride membranes: (a) first hole arrays were fabricated in the membrane by electron-beam lithography, and (b) a 40 nm-thick cobalt layer was deposited onto the membrane substrate. (c) SEM image of p ¼ 200 nm antidot array.
transfer method because the silicon nitride membranes are too fragile to withstand the use of an ultrasound bath needed here [14]. It was therefore necessary to develop a fabrication method which avoids the use of lift-off. In a first step the antidot pattern, consisting of arrays of holes with different periods, was written with a Leica LION LV1 electron-beam writer in a polymethylmethacrylate resist (PMMA) spin coated on a silicon nitride membrane substrate coated with a 20 nm-thick chromium film. The antidot pattern in the PMMA resist was transferred using reactive ion etching (RIE) into the chromium thin film, which was subsequently used as a mask to etch holes into the silicon nitride membrane (100 nm-thick) by RIE (see Fig. 1a). Finally, a 40 nm-thick cobalt film was deposited on the membrane with holes (see Fig. 1b) by DC-magnetron sputtering (base pressure ¼ 1.5–5.5 107 mbar) and capped with a 1 nm-thick aluminum layer to prevent oxidation. We concentrated here on antidot arrays with antidot size equal to the antidot separation (both square and round holes), where the stray field energy associated with the antidots is sufficient to give a checked domain contrast and chains of magnetic domains have been observed [8,9,12]. Each of the antidot arrays cover a square area with side length of 10–20 mm, and the antidot periods, p, range from 2 mm down to 200 nm. A scanning electron microscope (SEM) image of the smallest period array is shown in Fig. 1c. TXM observations were carried out in applied fields, HA, using magnetic transmission soft X-ray microscopy [13] at the beamline 6.1.2 (XM-1, Advanced Light Source, Berkeley, CA). Employing X-ray magnetic circular dichroism (XMCD), the magnetic domains were imaged by tuning the X-ray energy to the Co L3-edge. Dividing the images by an image of the same area in a saturating magnetic field leads to an image with increased magnetic contrast. The magnetic contrast scales with the projection of the local magnetization onto the photon propagation direction. In the figures the magnetization sensitivity direction (MSD), which is parallel to HA, is indicated: Ferromagnetic domains with magnetic spins parallel or antiparallel to the MSD appear black or white in the TXM image, respectively, while domains with magnetic spins perpendicular to the MSD will have a gray contrast.
Fig. 2. Antidot arrays with 2 mm period: (a) as-grown flux closure states in array with square holes: S-state at position 1, Landau state at position 2, flower state at position 3, and (b) a domain chain forms in an antidot array with round holes on application of a magnetic field. The end of the domain chain at position 1 comprises four 901 walls.
3. TXM observations TXM images of antidot arrays with 2 mm period are given in Fig. 2. In the as-grown state, flux closure patterns, reminiscent of those in square elements [15], are present between the antidots (Fig. 2a) and include S-states associated with diagonal domains (position 1), Landau
ARTICLE IN PRESS L.J. Heyderman et al. / Journal of Magnetism and Magnetic Materials 316 (2007) 99–102
states (position 2) and the flower states associated with four whiskers (or spikes) (position 3). The first and last states have been predicted for antidot arrays by micromagnetic simulations [11]. After application of an in-plane field, chains of domains appear and at the chain ends, a spin configuration with four 901 walls is seen (position 1 in Fig. 2b). This configuration minimizes the energy when two orthogonal chains coincide and is analogous to the 1801 wall configuration observed in thinner films9 and simulated for smaller periods12. As in Ref. [12], we observed that on application of a magnetic field parallel to the antidot rows, magnetization reversal occurs by nucleation and propagation of the domain chains. In addition the magnetic configurations in applied field and at remanence, i.e. on reducing the field to zero, were found to be identical, indicating a strong pinning of the domain chain ends. Finally, we studied the magnetization reversal in detail, changing the applied magnetic field in small steps. Two stages in the magnetization reversal in a 40-nm-thick antidot array with p ¼ 800 nm are given in Fig. 3. Here, the array was first saturated with a negative field of 1000 Oe, and then the field was reduced to zero and increased in the opposite sense to 260 Oe to give the white domain chains in Fig. 3a. The field was then increased in 4 Oe steps and the domain configuration remained the same until the third step at a field of 272 Oe (Fig. 3b): In row 1 the domain chain has increased in length by one antidot period and in row 2, the domain chain has grown by more than five antidot periods. In conclusion, we have developed a method to fabricate antidot arrays with periods down to 200 nm on silicon nitride membrane substrates for electron or X-ray microscopy. First observations with TXM show that for large periods, characteristic flux closure states occur between the antidots in the as-grown state and spin configurations comprising four 901 walls occur at the ends of domain chains which form on application of a magnetic field. Such configurations result in a strong pinning of the chain ends and play an important role in determining the resulting domain chain configuration. In addition, the high-energy barrier associated with depinning the chain ends will result in magnetization reversal proceeding via nucleation and propagation of new domain chains rather than further growth of existing domain chains with strongly pinned ends [12].
101
Fig. 3. Two stages of magnetization reversal via nucleation and propagation of domain chains observed with TXM in a 40 nm-thick antidot array with p ¼ 800 nm. (a) The array is first saturated with a negative field of 1000 Oe, and then the field is reduced to zero and increased in the opposite sense to 260 Oe. (b) The field is increased in 4 Oe steps and at 272 Oe a change in the domain chain length is observed. The circular frames indicate the same location in each image.
References Acknowledgments The authors would like to thank Michael Horisberger for the sputter deposition, and Eugen Deckardt, Anja Weber and Christian David, for their support with electron-beam lithography (Paul Scherrer Institut), and Luis Lopez-Diaz for sharing valuable insight into micromagnetic configurations. D.–H. Kim and P. Fischer acknowledge support from DOE (DE-AC03-76SF00098).
[1] B.D. Terris, T. Thomson, J. Phys. D 38 (2005) R199. [2] J.M. Daughton, A.V. Pohm, R.T. Fayfield, C.H. Smith, J. Phys. D 32 (1999) R169. [3] C.T. Yu, H. Jiang, L. Shen, P.J. Flanders, G.J. Mankey, J. Appl. Phys. 87 (2000) 6322. [4] A.O. Adeyeye, J.A.C. Bland, C. Daboo, Appl. Phys. Lett. 70 (1997) 3164. [5] C.C. Wang, A.O. Adeyeye, Y.H. Wu, J. Appl. Phys. 94 (2003) 6644. [6] I. Guedes, M. Grimsditch, V. Metlushko, P. Vavassori, R. Camley, B. Ilic, P. Neuzil, R. Kumar, Phys. Rev. B 67 (2003) 024428.
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[7] P. Vavassori, G. Gubbiotti, G. Zangari, C.T. Yu, H. Yin, H. Jiang, G.J. Mankey, J. Appl. Phys. 91 (2002) 7992. [8] L.J. Heyderman, F. Nolting, C. Quitmann, Appl. Phys. Lett. 83 (2003) 1797. [9] L.J. Heyderman, H.H. Solak, F. Nolting, C. Quitmann, J. Appl. Phys. 95 (2004) 6651. [10] A.Y. Toporov, R.M. Langford, A.K. Petford-Long, Appl. Phys. Lett. 77 (2000) 3063. [11] L. Torres, L. Lopez-Diaz, J. Iniguez, Appl. Phys. Lett. 73 (1998) 3766.
[12] L.J. Heyderman, F. Nolting, D. Backes, S. Czekaj, L. Lo´pez-Dı´ az, M. Kla¨ui, U. Ru¨diger, C.A.F. Vaz, J.A.C. Bland, R.J. Matelon, U. Volkmann, P. Fischer Phys. Rev. B Accepted for publication, 2006. [13] P. Fischer, D.H. Kim, W. Chao, J.A. Liddle, E.H. Anderson, D.T. Attwood, Mater. Today 9 (1-2) (2006) 26. [14] L.J. Heyderman, M. Kla¨ui, R. Scha¨ublin, U. Ru¨diger, C.A.F. Vaz, J.A.C. Bland, C. David, J. Magn. Magn. Mater. 290 (2005) 86. [15] L.J. Heyderman, S. Czekaj, F. Nolting, E. Mu¨ller, P. Fischer, Ph. Gasser, L. Lo´pez-Dı´ az, J. Appl. Phys. 99 (2006) 063904.
Journal of Magnetism and Magnetic Materials 316 (2007) 99–102 www.elsevier.com/locate/jmmm
Cobalt antidot arrays on membranes: Fabrication and investigation with transmission X-ray microscopy L.J. Heydermana,, S. Czekaja, F. Noltinga, D.-H. Kimb, P. Fischerb a
b
Paul Scherrer Institut, Villigen PSI, CH-5232, Switzerland LBNL/CXRO, 1 Cyclotron Road, Berkeley, CA 94720, USA Available online 25 February 2007
Abstract We have developed a method to fabricate ferromagnetic antidot arrays on silicon nitride membrane substrates for electron or soft Xray microscopy with antidot periods ranging from 2 mm down to 200 nm. Observations of cobalt antidot arrays with magnetic soft X-ray microscopy show that for large periods, flux closure states occur between the antidots in the as-grown state and on application of a magnetic field, domain chains are created which show a spin configuration at the chain ends comprising four 901 walls. Pinning of the domain chain ends plays an important role in the magnetization reversal, determining the length of the chains and resulting in preservation of the domain chain configuration on reducing of the applied magnetic field to zero. r 2007 Elsevier B.V. All rights reserved. PACS: 75.60.Jk; 75.60.Ch; 75.60.Ej; 75.75.+a Keywords: Ferromagnetic antidot array; Magnetic soft transmission X-ray microscopy; Magnetization reversal; Silicon nitride membrane
1. Introduction Patterned magnetic thin films are currently the focus of scientific interest due to the new magnetic phenomena which occur at reduced lateral dimensions and also because of their potential for industrial applications such as highdensity information storage and sensor devices [1,2]. Antidot arrays, which comprise a continuous film containing a regular array of non-magnetic inclusions or holes, are particularly of interest because of the resulting novel domain configurations, additional magnetic anisotropies and modification of the magnetization reversal, which in turn affects the switching fields and magnetoresistance behavior [3–7]. The basic domain configuration for square antidot arrays with the antidot size greater than or equal to the antidot separation is a periodic checked domain contrast commensurate with the antidot lattice, which we have observed with photoemission electron microscopy (PEEM) [8,9]. Similar configurations were seen with magnetic force microscopy (MFM) [3,5,7] and transmisCorresponding author. Tel.: +41 56 310 2613; fax: +41 56 310 2646.
E-mail address: [email protected] (L.J. Heyderman). 0304-8853/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2007.02.010
sion electron microscopy [10], and correlate well with micromagnetic simulations [11]. We have recently performed PEEM observations of remanent magnetic states in such antidot arrays on application of an in-plane magnetic field [12]. These measurements revealed that magnetization reversal occurs by nucleation and propagation of chains of magnetic domains that have discrete lengths corresponding to multiples of the antidot period. We report here on observations of the magnetic spin structures in antidot arrays with transmission X-ray microscopy (TXM). The high spatial resolution of TXM and the possibility to image in applied magnetic fields allow us to observe the fine details of the magnetic spin configurations and the reversal behavior [13]. These studies require substrates that allow the transmission of X-rays and we describe a method to fabricate antidot arrays on silicon nitride membranes, suitable for both X-ray and transmission electron microscopy. 2. Experimental We employed electron beam lithography to fabricate the antidot arrays. It is not possible to use lift-off as a pattern
ARTICLE IN PRESS 100
L.J. Heyderman et al. / Journal of Magnetism and Magnetic Materials 316 (2007) 99–102
Fig. 1. Schematic diagram showing fabrication of antidot arrays on silicon nitride membranes: (a) first hole arrays were fabricated in the membrane by electron-beam lithography, and (b) a 40 nm-thick cobalt layer was deposited onto the membrane substrate. (c) SEM image of p ¼ 200 nm antidot array.
transfer method because the silicon nitride membranes are too fragile to withstand the use of an ultrasound bath needed here [14]. It was therefore necessary to develop a fabrication method which avoids the use of lift-off. In a first step the antidot pattern, consisting of arrays of holes with different periods, was written with a Leica LION LV1 electron-beam writer in a polymethylmethacrylate resist (PMMA) spin coated on a silicon nitride membrane substrate coated with a 20 nm-thick chromium film. The antidot pattern in the PMMA resist was transferred using reactive ion etching (RIE) into the chromium thin film, which was subsequently used as a mask to etch holes into the silicon nitride membrane (100 nm-thick) by RIE (see Fig. 1a). Finally, a 40 nm-thick cobalt film was deposited on the membrane with holes (see Fig. 1b) by DC-magnetron sputtering (base pressure ¼ 1.5–5.5 107 mbar) and capped with a 1 nm-thick aluminum layer to prevent oxidation. We concentrated here on antidot arrays with antidot size equal to the antidot separation (both square and round holes), where the stray field energy associated with the antidots is sufficient to give a checked domain contrast and chains of magnetic domains have been observed [8,9,12]. Each of the antidot arrays cover a square area with side length of 10–20 mm, and the antidot periods, p, range from 2 mm down to 200 nm. A scanning electron microscope (SEM) image of the smallest period array is shown in Fig. 1c. TXM observations were carried out in applied fields, HA, using magnetic transmission soft X-ray microscopy [13] at the beamline 6.1.2 (XM-1, Advanced Light Source, Berkeley, CA). Employing X-ray magnetic circular dichroism (XMCD), the magnetic domains were imaged by tuning the X-ray energy to the Co L3-edge. Dividing the images by an image of the same area in a saturating magnetic field leads to an image with increased magnetic contrast. The magnetic contrast scales with the projection of the local magnetization onto the photon propagation direction. In the figures the magnetization sensitivity direction (MSD), which is parallel to HA, is indicated: Ferromagnetic domains with magnetic spins parallel or antiparallel to the MSD appear black or white in the TXM image, respectively, while domains with magnetic spins perpendicular to the MSD will have a gray contrast.
Fig. 2. Antidot arrays with 2 mm period: (a) as-grown flux closure states in array with square holes: S-state at position 1, Landau state at position 2, flower state at position 3, and (b) a domain chain forms in an antidot array with round holes on application of a magnetic field. The end of the domain chain at position 1 comprises four 901 walls.
3. TXM observations TXM images of antidot arrays with 2 mm period are given in Fig. 2. In the as-grown state, flux closure patterns, reminiscent of those in square elements [15], are present between the antidots (Fig. 2a) and include S-states associated with diagonal domains (position 1), Landau
ARTICLE IN PRESS L.J. Heyderman et al. / Journal of Magnetism and Magnetic Materials 316 (2007) 99–102
states (position 2) and the flower states associated with four whiskers (or spikes) (position 3). The first and last states have been predicted for antidot arrays by micromagnetic simulations [11]. After application of an in-plane field, chains of domains appear and at the chain ends, a spin configuration with four 901 walls is seen (position 1 in Fig. 2b). This configuration minimizes the energy when two orthogonal chains coincide and is analogous to the 1801 wall configuration observed in thinner films9 and simulated for smaller periods12. As in Ref. [12], we observed that on application of a magnetic field parallel to the antidot rows, magnetization reversal occurs by nucleation and propagation of the domain chains. In addition the magnetic configurations in applied field and at remanence, i.e. on reducing the field to zero, were found to be identical, indicating a strong pinning of the domain chain ends. Finally, we studied the magnetization reversal in detail, changing the applied magnetic field in small steps. Two stages in the magnetization reversal in a 40-nm-thick antidot array with p ¼ 800 nm are given in Fig. 3. Here, the array was first saturated with a negative field of 1000 Oe, and then the field was reduced to zero and increased in the opposite sense to 260 Oe to give the white domain chains in Fig. 3a. The field was then increased in 4 Oe steps and the domain configuration remained the same until the third step at a field of 272 Oe (Fig. 3b): In row 1 the domain chain has increased in length by one antidot period and in row 2, the domain chain has grown by more than five antidot periods. In conclusion, we have developed a method to fabricate antidot arrays with periods down to 200 nm on silicon nitride membrane substrates for electron or X-ray microscopy. First observations with TXM show that for large periods, characteristic flux closure states occur between the antidots in the as-grown state and spin configurations comprising four 901 walls occur at the ends of domain chains which form on application of a magnetic field. Such configurations result in a strong pinning of the chain ends and play an important role in determining the resulting domain chain configuration. In addition, the high-energy barrier associated with depinning the chain ends will result in magnetization reversal proceeding via nucleation and propagation of new domain chains rather than further growth of existing domain chains with strongly pinned ends [12].
101
Fig. 3. Two stages of magnetization reversal via nucleation and propagation of domain chains observed with TXM in a 40 nm-thick antidot array with p ¼ 800 nm. (a) The array is first saturated with a negative field of 1000 Oe, and then the field is reduced to zero and increased in the opposite sense to 260 Oe. (b) The field is increased in 4 Oe steps and at 272 Oe a change in the domain chain length is observed. The circular frames indicate the same location in each image.
References Acknowledgments The authors would like to thank Michael Horisberger for the sputter deposition, and Eugen Deckardt, Anja Weber and Christian David, for their support with electron-beam lithography (Paul Scherrer Institut), and Luis Lopez-Diaz for sharing valuable insight into micromagnetic configurations. D.–H. Kim and P. Fischer acknowledge support from DOE (DE-AC03-76SF00098).
[1] B.D. Terris, T. Thomson, J. Phys. D 38 (2005) R199. [2] J.M. Daughton, A.V. Pohm, R.T. Fayfield, C.H. Smith, J. Phys. D 32 (1999) R169. [3] C.T. Yu, H. Jiang, L. Shen, P.J. Flanders, G.J. Mankey, J. Appl. Phys. 87 (2000) 6322. [4] A.O. Adeyeye, J.A.C. Bland, C. Daboo, Appl. Phys. Lett. 70 (1997) 3164. [5] C.C. Wang, A.O. Adeyeye, Y.H. Wu, J. Appl. Phys. 94 (2003) 6644. [6] I. Guedes, M. Grimsditch, V. Metlushko, P. Vavassori, R. Camley, B. Ilic, P. Neuzil, R. Kumar, Phys. Rev. B 67 (2003) 024428.
ARTICLE IN PRESS 102
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[7] P. Vavassori, G. Gubbiotti, G. Zangari, C.T. Yu, H. Yin, H. Jiang, G.J. Mankey, J. Appl. Phys. 91 (2002) 7992. [8] L.J. Heyderman, F. Nolting, C. Quitmann, Appl. Phys. Lett. 83 (2003) 1797. [9] L.J. Heyderman, H.H. Solak, F. Nolting, C. Quitmann, J. Appl. Phys. 95 (2004) 6651. [10] A.Y. Toporov, R.M. Langford, A.K. Petford-Long, Appl. Phys. Lett. 77 (2000) 3063. [11] L. Torres, L. Lopez-Diaz, J. Iniguez, Appl. Phys. Lett. 73 (1998) 3766.
[12] L.J. Heyderman, F. Nolting, D. Backes, S. Czekaj, L. Lo´pez-Dı´ az, M. Kla¨ui, U. Ru¨diger, C.A.F. Vaz, J.A.C. Bland, R.J. Matelon, U. Volkmann, P. Fischer Phys. Rev. B Accepted for publication, 2006. [13] P. Fischer, D.H. Kim, W. Chao, J.A. Liddle, E.H. Anderson, D.T. Attwood, Mater. Today 9 (1-2) (2006) 26. [14] L.J. Heyderman, M. Kla¨ui, R. Scha¨ublin, U. Ru¨diger, C.A.F. Vaz, J.A.C. Bland, C. David, J. Magn. Magn. Mater. 290 (2005) 86. [15] L.J. Heyderman, S. Czekaj, F. Nolting, E. Mu¨ller, P. Fischer, Ph. Gasser, L. Lo´pez-Dı´ az, J. Appl. Phys. 99 (2006) 063904.