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CoPt thin film bilayers
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 272–276 (2004) 273–274
Spin wave excitations in exchange spring Co/CoPt thin film bilayers D.C. Crewa,*, R.L. Stampsa, H.Y. Liub, Z.K Wangb, M.H. Kuokb, S.C. Ngb, K. Barmakc,d, J. Kimc, L.H. Lewise a
School of Physics M013, The University of Western Australia, 35 Stirling Highway, Crawley WA 6009, Australia b Department of Physics, National University of Singapore, Singapore 117542, Singapore c Data Storage Systems Center, Carnegie Mellon University, Pittsburgh, PA 15213, USA d Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213, USA e Materials Science Department, Brookhaven National Laboratory, Upton, NY 11973, USA
Abstract We report Brillouin light scattering data taken from an exchange spring bilayer, Co/CoPt. The results are analysed using a semi-classical model in the long wavelength limit, and we demonstrate how a measure of an anomalous volume anisotropy, which we tentatively ascribe to a magnetoelastic effect, can be obtained from frequency versus field measurements in this system. r 2004 Elsevier B.V. All rights reserved. PACS: 75.30.Ds; 75.30.Et; 75.70.Cn Keywords: Spin wave; Exchange spring; Anisotropy
The exchange spring system has been found to display characteristic structure in the magnetic hysteresis, which allows important information on interface exchange and the effects of magnetic anisotropies to be obtained [1]. Spin wave frequencies can also be used [2] to provide measures of interlayer exchange and anisotropy energies. In this work, we report and analyse data taken from an exchange spring bilayer, Co/CoPt, using Brillouin light scattering (BLS). The results are analysed by determining the characteristic frequencies from a semi-classical model based on that of Nortemann et al. [3]. The sample was an RF sputtered bilayer consisting of 25 nm of near equiatomic L10 CoPt with a /1 1 1S fibre texture and 16.7 nm of Co with an HCP /0 0 0 1S texture. Details of the sample preparation can be found elsewhere [4]. The sample was pre-magnetized to
*Corresponding author. Tel.: +61-8-6488-2751; fax: +61-86488-1014. E-mail address: [email protected] (D.C. Crew).
saturation in a field of 1.7 T prior to the BLS measurements. Room temperature, p–s polarization Brillouin spectra were recorded in the 180 -backscattering geometry using 514.5 nm laser radiation and a (3+3)-pass tandem Fabry–Perot interferometer. The magnetic field was applied perpendicular to the sample surface normal, the magnon wave vector and the incident light. The applied magnetic field was varied from 1.1 to 1.1 T, where the positive sign means that the magnetic field and the premagnetization were in the same direction. The measured BLS data, shown by circles in Fig. 1, indicates a deep minimum in frequency at a field of 150 mT. From hysteresis measurements, the minimum occurs at the same reversing field as a spiral starts to form in the soft Co layer. A double minimum in frequency, predicted on the basis of a micromagnetic model of the in-plane CoPt anisotropy aligned with the field [2], is not seen. From the model used below, the double minimum is a special case for the aligned situation. For alignment angles above approximately 5 , only a single minimum in frequency is seen.
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2004.04.061
ARTICLE IN PRESS D.C. Crew et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) 273–274
274 60
Frequency (GHz)
50
40
30
20
10
0 -1.5
-1
-0.5
0 H (T)
0.5
1
1.5
Fig. 1. BLS data for decreasing field from 1.1 to 1.1 T for a Co/CoPt (16.7 nm/25 nm) exchange spring bilayer. The circles represent the experimental data and the line a fit to the data as described in the text.
The semi-classical model used to fit the data [3] treats the magnetic system in the long wavelength limit as point dipoles arranged on a simple cubic lattice interacting via Heisenberg exchange. Dipolar effects are treated classically by direct summation. The spin wave frequencies are influenced in addition by anisotropy and Zeeman terms. The spins are relaxed to their equilibrium positions as a classical vector by a simulated fully damped precession. Spin wave frequencies are found from the solution of the eigenvalue problem of the linearized equations of motion. The results presented show only the fundamental resonance frequencies, i.e. the in-plane spin wave vector is zero. The solid line in Fig. 1. shows the model results for the coupled bilayer, including an average over the distribution of in-plane anisotropy directions of the CoPt arising due to the /1 1 1S fibre texture. The parameters used for this curve are derived from the bulk values assuming a simple cubic structure with a lattice parameter of ( equivalent to the distance between basal planes 2.055 A, in HCP Co. Small changes in the exchange parameter (J) and spin dipole moment (S) would be required to treat the true HCP Co and L10 CoPt structures, but a simple cubic structure simplifies the calculation of the dipole field. The parameters used were for Co (J=28.5 meV, K=0.43 MJ/m3, S=1.31 mB) and CoPt (J=57.8 meV, K=4 MJ/m3, S=0.748 mB) where K is the uniaxial magnetocrystalline anisotropy of the material.
The assumed g factor was 2.25 which is appropriate for Co. To fit the minimum in frequency shown in Fig. 1, an anomolous uniaxial in-plane anisotropy, separate from the magnetocrystalline anisotropy, is required for the Co layers, with magnitude of 0.21 MJ/m3 directed along the in-plane CoPt easy axis. This anomolous anisotropy is too large to be a surface anisotropy alone, which typically has a magnitude below 1 mJ/m2 in Co [5], equivalent (in energy) in a 16.7 nm thick film to a volume anisotropy of 0.06 MJ/m3. The value of the anomolous anisotropy required is approximately twice the volume magnetoelastic anisotropy extrapolated from the measurements of Fritzsche et al. [6] for Co (0 0 0 1) films grown on W [1 1 0]. In the Co on W [1 1 0] system however the lattice misfit is approximately 3% compared with 6% for Co on CoPt [1 1 1], indicating that an increase in film strain, and hence volume magnetoelastic anisotropy, is not unreasonable. From Ref. [6] a volume magnetoelastic anisotropy of 0.21 MJ/m3 requires an inplane strain of approximately 1.8%. The frequencies determined by the model do not fit the experimental frequencies well for low fields below 0.5 T. This is because in this field regime the CoPt layer is reversing, which is not simulated well by the simple relaxation mechanism assumed in the model. The experimental frequencies lie between the curve shown in Fig. 1 for unreversed CoPt, and a similar curve determined if the CoPt is assumed to have fully reversed. Research performed in part at Brookhaven National Laboratory under the auspices of the U.S. D.O.E., Division of Materials Sciences, Office of Basic Energy Sciences under Contract No. DE-AC02-98CH10886. K.B. and J.K. acknowledge NSF-ECD-8907068, NSF DMR-9458000, NSF DMR-9411146 and the Horner Fellowship from Lehigh University for partial funding support.
References [1] D.C. Crew, J. Kim, K. Barmak, L.H. Lewis, J. Appl. Phys. 93 (2003) 7235. [2] D.C. Crew, R.L. Stamps, J. Appl. Phys. 93 (2003) 6483. [3] F.C. Nortemann, R.L. Stamps, R.E. Camley, Phys. Rev. B 47 (1993) 11910. [4] J. Kim, K. Barmak, L.H. Lewis, Acta Mater. 51 (2003) 313. [5] W.J.M. de Jonge, P.J.H. Bloemen, F.J.A. den Broeder, in: J.A.C. Bland, B. Heinrich (Eds.), Ultrathin Magnetic Structures, Vol. I, Springer, Berlin, 1994, p. 65. [6] H. Fritzsche, J. Kohlhepp, U. Gradmann, Phys. Rev. B 51 (1995) 15933.
Journal of Magnetism and Magnetic Materials 272–276 (2004) 273–274
Spin wave excitations in exchange spring Co/CoPt thin film bilayers D.C. Crewa,*, R.L. Stampsa, H.Y. Liub, Z.K Wangb, M.H. Kuokb, S.C. Ngb, K. Barmakc,d, J. Kimc, L.H. Lewise a
School of Physics M013, The University of Western Australia, 35 Stirling Highway, Crawley WA 6009, Australia b Department of Physics, National University of Singapore, Singapore 117542, Singapore c Data Storage Systems Center, Carnegie Mellon University, Pittsburgh, PA 15213, USA d Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213, USA e Materials Science Department, Brookhaven National Laboratory, Upton, NY 11973, USA
Abstract We report Brillouin light scattering data taken from an exchange spring bilayer, Co/CoPt. The results are analysed using a semi-classical model in the long wavelength limit, and we demonstrate how a measure of an anomalous volume anisotropy, which we tentatively ascribe to a magnetoelastic effect, can be obtained from frequency versus field measurements in this system. r 2004 Elsevier B.V. All rights reserved. PACS: 75.30.Ds; 75.30.Et; 75.70.Cn Keywords: Spin wave; Exchange spring; Anisotropy
The exchange spring system has been found to display characteristic structure in the magnetic hysteresis, which allows important information on interface exchange and the effects of magnetic anisotropies to be obtained [1]. Spin wave frequencies can also be used [2] to provide measures of interlayer exchange and anisotropy energies. In this work, we report and analyse data taken from an exchange spring bilayer, Co/CoPt, using Brillouin light scattering (BLS). The results are analysed by determining the characteristic frequencies from a semi-classical model based on that of Nortemann et al. [3]. The sample was an RF sputtered bilayer consisting of 25 nm of near equiatomic L10 CoPt with a /1 1 1S fibre texture and 16.7 nm of Co with an HCP /0 0 0 1S texture. Details of the sample preparation can be found elsewhere [4]. The sample was pre-magnetized to
*Corresponding author. Tel.: +61-8-6488-2751; fax: +61-86488-1014. E-mail address: [email protected] (D.C. Crew).
saturation in a field of 1.7 T prior to the BLS measurements. Room temperature, p–s polarization Brillouin spectra were recorded in the 180 -backscattering geometry using 514.5 nm laser radiation and a (3+3)-pass tandem Fabry–Perot interferometer. The magnetic field was applied perpendicular to the sample surface normal, the magnon wave vector and the incident light. The applied magnetic field was varied from 1.1 to 1.1 T, where the positive sign means that the magnetic field and the premagnetization were in the same direction. The measured BLS data, shown by circles in Fig. 1, indicates a deep minimum in frequency at a field of 150 mT. From hysteresis measurements, the minimum occurs at the same reversing field as a spiral starts to form in the soft Co layer. A double minimum in frequency, predicted on the basis of a micromagnetic model of the in-plane CoPt anisotropy aligned with the field [2], is not seen. From the model used below, the double minimum is a special case for the aligned situation. For alignment angles above approximately 5 , only a single minimum in frequency is seen.
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2004.04.061
ARTICLE IN PRESS D.C. Crew et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) 273–274
274 60
Frequency (GHz)
50
40
30
20
10
0 -1.5
-1
-0.5
0 H (T)
0.5
1
1.5
Fig. 1. BLS data for decreasing field from 1.1 to 1.1 T for a Co/CoPt (16.7 nm/25 nm) exchange spring bilayer. The circles represent the experimental data and the line a fit to the data as described in the text.
The semi-classical model used to fit the data [3] treats the magnetic system in the long wavelength limit as point dipoles arranged on a simple cubic lattice interacting via Heisenberg exchange. Dipolar effects are treated classically by direct summation. The spin wave frequencies are influenced in addition by anisotropy and Zeeman terms. The spins are relaxed to their equilibrium positions as a classical vector by a simulated fully damped precession. Spin wave frequencies are found from the solution of the eigenvalue problem of the linearized equations of motion. The results presented show only the fundamental resonance frequencies, i.e. the in-plane spin wave vector is zero. The solid line in Fig. 1. shows the model results for the coupled bilayer, including an average over the distribution of in-plane anisotropy directions of the CoPt arising due to the /1 1 1S fibre texture. The parameters used for this curve are derived from the bulk values assuming a simple cubic structure with a lattice parameter of ( equivalent to the distance between basal planes 2.055 A, in HCP Co. Small changes in the exchange parameter (J) and spin dipole moment (S) would be required to treat the true HCP Co and L10 CoPt structures, but a simple cubic structure simplifies the calculation of the dipole field. The parameters used were for Co (J=28.5 meV, K=0.43 MJ/m3, S=1.31 mB) and CoPt (J=57.8 meV, K=4 MJ/m3, S=0.748 mB) where K is the uniaxial magnetocrystalline anisotropy of the material.
The assumed g factor was 2.25 which is appropriate for Co. To fit the minimum in frequency shown in Fig. 1, an anomolous uniaxial in-plane anisotropy, separate from the magnetocrystalline anisotropy, is required for the Co layers, with magnitude of 0.21 MJ/m3 directed along the in-plane CoPt easy axis. This anomolous anisotropy is too large to be a surface anisotropy alone, which typically has a magnitude below 1 mJ/m2 in Co [5], equivalent (in energy) in a 16.7 nm thick film to a volume anisotropy of 0.06 MJ/m3. The value of the anomolous anisotropy required is approximately twice the volume magnetoelastic anisotropy extrapolated from the measurements of Fritzsche et al. [6] for Co (0 0 0 1) films grown on W [1 1 0]. In the Co on W [1 1 0] system however the lattice misfit is approximately 3% compared with 6% for Co on CoPt [1 1 1], indicating that an increase in film strain, and hence volume magnetoelastic anisotropy, is not unreasonable. From Ref. [6] a volume magnetoelastic anisotropy of 0.21 MJ/m3 requires an inplane strain of approximately 1.8%. The frequencies determined by the model do not fit the experimental frequencies well for low fields below 0.5 T. This is because in this field regime the CoPt layer is reversing, which is not simulated well by the simple relaxation mechanism assumed in the model. The experimental frequencies lie between the curve shown in Fig. 1 for unreversed CoPt, and a similar curve determined if the CoPt is assumed to have fully reversed. Research performed in part at Brookhaven National Laboratory under the auspices of the U.S. D.O.E., Division of Materials Sciences, Office of Basic Energy Sciences under Contract No. DE-AC02-98CH10886. K.B. and J.K. acknowledge NSF-ECD-8907068, NSF DMR-9458000, NSF DMR-9411146 and the Horner Fellowship from Lehigh University for partial funding support.
References [1] D.C. Crew, J. Kim, K. Barmak, L.H. Lewis, J. Appl. Phys. 93 (2003) 7235. [2] D.C. Crew, R.L. Stamps, J. Appl. Phys. 93 (2003) 6483. [3] F.C. Nortemann, R.L. Stamps, R.E. Camley, Phys. Rev. B 47 (1993) 11910. [4] J. Kim, K. Barmak, L.H. Lewis, Acta Mater. 51 (2003) 313. [5] W.J.M. de Jonge, P.J.H. Bloemen, F.J.A. den Broeder, in: J.A.C. Bland, B. Heinrich (Eds.), Ultrathin Magnetic Structures, Vol. I, Springer, Berlin, 1994, p. 65. [6] H. Fritzsche, J. Kohlhepp, U. Gradmann, Phys. Rev. B 51 (1995) 15933.