ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 316 (2007) e136–e138 www.elsevier.com/locate/jmmm
Magnetic domain structure in ultrathin Au/Co/Au films grown on vicinal sapphire substrates A. Stupakiewicza,c,, M. Tekielaka, A. Maziewskia, V. Zablotskiia,d, L.T. Baczewskib, A. Wawrob a
Laboratory of Magnetism, Institute of Experimental Physics, University of Bialystok, 41 Lipowa, 15-424 Bialystok, Poland b Institute of Physics, Polish Academy of Sciences, Al. Lotnikow 32/46, 02-668 Warsaw, Poland c Institute d’Electronique Fondamentale UMR CNRS 8622, Universite Paris-Sud, 91405 Orsay, France d Institute of Physics ASCR, Na Slovance 2, 18221 Prague 8, Czech Republic Available online 27 February 2007
Abstract Study of magnetic domain structures and magnetization reversal of Au/Co/Au films grown on vicinal sapphire substrates with different miscut angles is reported. Changes of domain structure geometry were registered in a wide Co-thickness range. By tailoring the Co thickness range out-of-plane magnetization states and thickness-driven domain wall reorientation were found in the sample grown on a 5 -miscut substrate. It is shown that the domain wall orientation can be changed by magnetic anisotropy tuned by matching miscut angle and Co thickness. r 2006 Elsevier B.V. All rights reserved. PACS: 75.70.Kw; 75.70.i; 75.60.Ej Keywords: Magnetic anisotropy; Vicinal surface; Domain structure
1. Introduction Multilayered ultrathin magnetic films have been intensively investigated because of interesting physical properties and possible applications as perpendicular magnetic storage media. The magnetization reversal process, magnetic domain structure and magnetic anisotropy have been the focus points of recent studies [1–4]. Magnetic films grown on vicinal surfaces (i.e. nonmagnetic substrates with oriented monatomic steps) exhibit a strong correlation between the substrate surface structure and their magnetic properties. In Ref. [1] step-induced anisotropic domain wall propagation in Pt/Co/Pt multilayers was observed. In addition to the magnetocrystalline anisotropy a uniaxial magnetic in-plane anisotropy is induced when the film is grown on a stepped surface [5]. The magnetic domain structure and magnetic anisotropy strongly depend on the ultrathin Co film thickness. As a result a spin-reorientation Corresponding author. Tel.: +4885 745 72 28; fax: +4885 745 72 23.
E-mail address:
[email protected] (A. Stupakiewicz). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2007.02.061
phase transition (RPT) from the perpendicular to the in-plane alignment takes place with increasing Co thickness [4,5]. In this work we have studied the magnetization reversal process and magnetic domain structure in ultrathin Au/Co/ Au magnetic system grown on vicinal sapphire substrates with different miscut angles. A miscut-induced shift of the Co RPT thickness was investigated. 2. Sample preparation and experimental techniques The nanostructures were grown by molecular beam epitaxy on sapphire single-crystal (11-20) substrates with 1:2 and 5 miscut angles in the following composition: (i) first buffer layer of 20 nm Mo (1 1 0) deposited at T ¼ 1000 C, (ii) second buffer layer of 10 nm Au (1 1 1) deposited at room temperature and annealed at T ¼ 200 C for 30 min, (iii) a Co (0 0 0 1) wedge of 0odo2:35 nm thickness range; (iv) 8 nm thick Au cover layer. The structure and purity of the samples were monitored in situ by RHEED and Auger spectroscopy, respectively. Samples
ARTICLE IN PRESS A. Stupakiewicz et al. / Journal of Magnetism and Magnetic Materials 316 (2007) e136–e138
e137
prepared on such vicinal substrates with the 1:2 and 5 miscut angles were studied and the azimuthal directions were determined by X-ray diffraction in respect to the substrate edges. The study of magnetization reversal was performed at room temperature using the magneto-optical polar Kerr effect (P-MOKE) based magnetometer. A computer controlled experimental setup (with laser light of l ¼ 640 nm) enabled to determine a Kerr rotation for different amplitude of magnetic field H z applied in the direction perpendicular to the film plane. The laser beam was focused to a spot of 0.5 mm in diameter and automatically precisely focused on a Co wedge. The observations of magnetic domain structures were performed by optical polarizing microscope detecting a polar Kerr effect. The microscope was equipped with Xe lamp and a high sensitivity digital CCD camera. The domain structure images were improved by standard image processing techniques (including subtracting of the reference image). For sample demagnetization state a computer controlled setup was used. This system enabled to apply a pulse of magnetic field perpendicular to sample plane with decreasing amplitude of sequence pulses (1 s was pulse duration time). 3. Results and discussion The P-MOKE hysteresis loops for different Co thicknesses and miscut angles are shown in Fig. 1. The behavior of loops measured for 1:2 miscut sample corresponds to reorientation from the out-of-plane to the in-plane magnetization and a decreasing of coercive field to zero with increase of Co thickness. Similar effect was observed in Au/Co/Au films on flat substrates [4]. However, for 5 miscut angle, out-of-plane magnetization was observed in a wide Co thickness range. The shape of the loops is an evidence for the perpendicular magnetic anisotropy with the canted axis in the 5 miscut sample for d ¼ 2:2 nm (Fig. 1c). The coercive field measured for 5 miscut sample was not changed in a wide Co thickness range for d41:1 nm. The Co thickness dependence of normalized remnant magnetization (M R ¼ M=M S ) for different miscut angles is shown in Fig. 2. A shift of the RPT caused by the miscut is clearly visible. The change of M R for do0:5 nm could be related with sample demagnetization and/or superparamagnetic states of Co films [4]. The changes of domain structure geometry were studied in a wide Co-thickness range from 0.5 to 2.35 nm. For selected Co-thickness the images of domain structure are shown in Fig. 3. The changes were observed under the following conditions. Domain structure images were registered at remnant state after magnetic field H z pulses. Starting from the saturated state H z o0 (preferring ‘‘black’’ domain) we applied a sequence of reverse field pulses H z 40 (preferring ‘‘white’’ domain) of different amplitude. For the 1:2 miscut sample the preferential orientation is perpendicular to the miscut direction (but parallel to the
Fig. 1. P-MOKE loops measured in the samples with 1:2 and 5 miscut angles for different Co thickness d: (a) 1 nm, (b) 1.5 nm, (c) 2.2 nm. Magnetic field H z applied in the direction perpendicular to the film plane.
step direction) in the whole range of Co-layer thickness. This preference could be related to the vicinal substrate surface structure causing two-fold symmetry of the magnetic in-plane anisotropy. A similar effect was observed in Co films deposited on Au monocrystals [6]. The preferential orientation is retained in a wide Co thickness range. The domain size [7] decreases while approaching the RPT Co-thickness (Fig. 3a) and it is difficult to distinguish the wall preference orientation for these thicknesses—see areas with d41:6 nm. For 5 miscut sample the magnetization reversal is accompanied by a creation of large domains
ARTICLE IN PRESS e138
A. Stupakiewicz et al. / Journal of Magnetism and Magnetic Materials 316 (2007) e136–e138
Fig. 2. Normalized remnant magnetization versus the Co-thickness for different substrates miscut angles.
sample is that the domain wall preferential direction is turning towards the miscut orientation as the Co film thickness increases (Fig. 3(d–f)). The wall preference along the direction perpendicular to that defined by the miscut is observed for do1:8 nm. However, above d ¼ 1:8 nm the domain preferential direction turns. We suppose that the domain preferential orientation change is related to a more complicated form of magnetic anisotropy for 5 miscut sample [8]. Correlation between the domain wall preference and magnetic in-plane anisotropy was recently reported for Co grown on Mo buffer [9]. The observed anisotropic domain wall propagation can be explained by the competition of the step-induced magnetic anisotropy and inclined uniaxial anisotropy in Co films grown on vicinal substrates [8]. The above presented results show that the domain wall orientation can be changed by matching miscut angle and Co thickness. 4. Conclusions Changes of domain structure geometry were observed in Co wedge samples grown on 1:2 and 5 miscut substrates. Thickness-driven turning of the domain wall orientation was observed for 5 miscut sample. For different wedge thickness the preference of the domain wall orientation could be explained by the competition of the step-induced magnetic anisotropy and inclined uniaxial anisotropy in Co grown on vicinal substrates. A possibility of anisotropy tuning by a proper choice of miscut angle and Co thickness has been demonstrated. Acknowledgments This work was supported by the Polish State Committee for Scientific Research (Grant no. 4 T11B 006 24) and Marie Curie Fellowships for ‘‘Transfer of Knowledge’’ (‘‘NANOMAG-LAB’’ N 2004-003177). References
Fig. 3. Images of magnetic domain structure in Co wedges for different substrate miscut angles. Image size is 200 200 mm. Co thickness for the sample 1:2 miscut—(a) 1.6 nm, (b) 1.5 nm, (c) 1 nm and for 5 miscut—(d) 2.2 nm, (e) 2 nm, (f) 1.8 nm. Black arrows indicate the preferential domain wall orientation. Co thickness increases along vertical axis.
(Fig. 3 (d–f)) in the studied Co-thickness range. This observation corresponds to magnetization reversal process shown in Fig. 1. The new effect observed for 5 -miscut
[1] P. Haibach, M. Huth, H. Adrian, Phys. Rev. Lett. 84 (2000) 1312. [2] E.Y. Vedmedenko, A. Kubetzka, K. von Bergmann, O. Pietzsch, M. Bode, J. Kirschner, H.P. Oepen, R. Wiesendanger, Phys. Rev. Lett. 92 (2004) 77207. [3] R. Ramchal, A.K. Schmid, M. Farle, H. Poppa, Phys. Rev. B 69 (2004) 214401. [4] M. Kisielewski, A. Maziewski, M. Tekielak, A. Wawro, L.T. Baczewski, Phys. Rev. Lett. 89 (2002) 087203. [5] Z.Q. Qiu, J. Pearson, S.D. Bader, Phys. Rev. Lett. 70 (1993) 1006. [6] S. Rousset, V. Repain, G. Baudot, H. Ellmer, Y. Garreau, V. Etgens, J.M. Berroir, B. Croset, M. Sotto, P. Zeppenfeld, J. Ferre, J.P. Jamet, C. Chappert, J. Lecoeur, Mat. Sci. Eng. B 96 (2002) 169. [7] M. Kisielewski, A. Maziewski, Z. Kurant, M. Tekielak, A. Wawro, L.T. Baczewski, J. Appl. Phys. 93 (2003) 7628. [8] A. Stupakiewicz, R. Gieniusz, A. Maziewski, K. Postava, A. Wawro, L.T. Baczewski, Phys. Stat. Sol. (b) 243 (1) (2006) 202. [9] M. Tekielak, Z. Kurant, A. Maziewski, L.T. Baczewski, A. Maneikis, A. Wawro, J. Magn. Magn. Mater. 290–291 (2005) 783.
