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Growth of Fe nanostructures Article · May 2004 DOI: 10.1016/j.jmmm.2003.12.545
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ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 272–276 (2004) 1588–1589
Growth of Fe nanostructures J. Ostera,*, M. Huthb, L. Wiehla, H. Adriana b
a Johannes Gutenberg-Universitat . Mainz, Institut fur . Physik, Staudinger Weg 7, Mainz 55099, Germany Johann Wolfgang Goethe-Universitat . Frankfurt, Physikalisches Institut, Robert Mayer-Str. 2-4, Frankfurt/M. 60054, Germany
Abstract Highly ordered arrays of epitaxial iron thin film nanostructures were grown by molecular beam epitaxy techniques on m-plane sapphire a-Al2O3 ð1 0 1% 0Þ substrates. Iron was deposited by electron beam evaporation under shallow incidence onto faceted sapphire substrates held at elevated temperatures of 450 C. Scanning electron microscopy suggests the formation of morphologically and electrically isolated nanowire structures on the ridges of the facets. The topology of the structures depends strongly on the iron deposition angle. r 2003 Elsevier B.V. All rights reserved. PACS: 08.d.; 68.55.Jk; 75.50.Bb; 75.75.+a; 81.16.Dn Keywords: Nanowires; Iron; Magnetic anisotropy; Faceting; Self-assembly
1. Faceting of m-plane Al2O3
2. Iron nanostructures
ð1 0 1% 0Þ (m-plane)-oriented a-Al2O3 substrates show a pronounced faceting after one-day annealing in air. They are suited for the epitaxial growth of cubic 3dtransition metals. Compared to preparation techniques exploiting the steps of vicinal single crystal substrates [1], faceted Al2O3 m-plane substrates enable the epitaxial growth of metallic nanowires of several 10 nm thickness [2]. Al2O3 m-plane substrates were annealed in an alumina tube oven for 24 h at temperatures up to 1700 C. Atomic force microscopy (AFM) shows the formation of facets with the facet size and periodicity depending on the annealing temperature. The maximum observed width and height of the facets amount to about 300 and 70 nm, respectively. The facet angles of the observed ð1% 1 0 1Þ- and ð1 1% 0 2Þ-surfaces amount to 17.6 and 32.4 , respectively, with regard to the original ð101% 0Þ surface. In plane the facets run parallel to the Al2O3 [0 1 0] axis (b-axis) and perpendicular to the Al2O3 [0 0 1] axis (c-axis). The length and width of the individual facet is limited by the formation process [3]. We estimate the average length to be about 100 mm.
The iron nanostructures were prepared as follows: Iron was deposited by electron beam evaporation under shallow incidence onto the faceted sapphire substrates held at elevated temperatures of 450 C. The iron ( for normal incidence. The deposition rate was 0.12 A/s incident angle of the iron beam with respect to the ð1 0 1% 0Þ-plane of the substrate was varied from 5 up to 15.5 . Subsequently, the surface was covered with a 2 nm thick layer of molybdenum in normal incidence in order to protect the iron from oxidation. Scanning electron microscopy (SEM) suggests the formation of morphologically and electrically isolated nanowire structures on the ridges of the facets. The length of these structures corresponds to the length of the individual facets. For a deposition angle of 5 iron is forming oval drops strung together in a pearl-neckletlike structure. The SEM image in Fig. 1 suggests the single drops to be electrically isolated from each other. Their width amounts from 25 to 70 nm. This size is mainly influenced by the width of the individual facets. We estimate the thickness of the Fe structures to 19 nm. For 8.5 deposition angle iron forms elongated structures. According to Fig. 2 these structures form due to coalescence of droplets during the growth. The width of the structures amounts to 100 nm. The length
*Corresponding author. Tel.: +4961313923633; fax: +4961313925156. E-mail address:
[email protected] (J. Oster).
0304-8853/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.12.545
ARTICLE IN PRESS J. Oster et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) 1588–1589
Fig. 1. SEM image of Fe nanostructures. Deposition angle 5 .
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Fig. 3. SEM image of Fe nanostructures. Deposition angle 15.5 .
and molybdenum films the growth of iron is more complex. On non-faceted and faceted sapphire m-plane niobium and molybdenum grow epitaxially in one single (2 1 1) orientation. On the other hand, iron shows at least four different domain orientations with approximately (2 1 1) growth axes. In plane they have a common [1 1 1]-axis parallel to Al2O3 c: The (2 1 1)-directions are tilted along the factes by about 3 to the right and left, respectively. The other two domains are related pairwise by a twin law parallel (1 1 1). 3. Further experiments
Fig. 2. SEM image of Fe nanostructures. Deposition angle
of the structures varies from 100 to 500 nm. The thickness amounts to approximately 25 nm. For 15.5 deposition angle iron forms continuous nanowires as shown in Fig. 3. These wires still show the typical lacings. Their width amounts to 100 nm. The thickness is estimated to 40 nm. The formation process of iron nanostructures is thought to be driven by two effects as it was deduced for the formation of niobium nanowires [2]. Firstly, due to a self-shadowing of the facets the iron deposition is mainly limited to the ridges of the facets. Secondly, the different surface energies of transition metals and the sapphire surfaces cause a de-wetting of the metal from the facet surfaces at elevated temperatures. This leads to an accumulation of the metal on the facet ridges with an overgrowth over the edge of the facets. The crystallographic orientations of thin iron films deposited on faceted and non-faceted Al2O3 ð1 0 1% 0Þ were investigated in a four circle X-ray diffractometer. Compared to prior experiments with thin niobium [2]
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Preliminary magnetic measurements of the iron nanostructures were performed in a vibrating sample magnetometer at T ¼ 300 K. The films show an uniaxial magnetic anisotropy within the film plane. Hereby the easy axis of magnetization is parallel and the hard axis is perpendicular to the facet ridges. Furthermore the shape of the hystersis loops and the saturation fields depend strongly on the topology of the structures. More detailed measurements are in progress. Acknowledgements This work is supported by the Multifunctional Materials and Miniaturized Devices Center at the University of Mainz.
References [1] H.J. Elmers, J. Hauschild, U. Gradmann, J. Magn. Magn. Mater. 221 (2000) 219. [2] M. Huth, K.A. Ritley, J. Oster, H. Dosch, H. Adrian, Adv. Funct. Mater. 12 (2002) 333. [3] J.R. Heffelfinger, C.B. Carter, Surf. Sci. 389 (1997) 188.
