Imaging magnetic domains in Ni nanostructures

Imaging magnetic domains in Ni nanostructures

ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 310 (2007) e936–e938 www.elsevier.com/locate/jmmm Imaging magnetic domains in Ni nanost...

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ARTICLE IN PRESS

Journal of Magnetism and Magnetic Materials 310 (2007) e936–e938 www.elsevier.com/locate/jmmm

Imaging magnetic domains in Ni nanostructures A. Asenjoa,, M. Jaafara, E.M. Gonza´lezb, J.I. Martinc, M. Va´zqueza, J.L. Vicentb a

Instituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco, Madrid 28049, Spain Departmento Fı´sica de Materiales, Facultad de CC Fı´sicas, Universidad Complutense de Madrid, Madrid 28040, Spain c Departmento Fı´sicas, Facultad de Ciencias, Universidad de Oviedo, Oviedo 33007, Spain

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Available online 20 November 2006

Abstract The study of nanomagnets is the subject of increasing scientific effort. The size, the thickness and the geometric shape of the elements determine the magnetic properties and then the domain configuration. In this work, we fabricated by electron-beam lithography the three different arrays of Ni nanostructures keeping the size, the thickness and also the distance constant between the elements but varying the geometry: square, triangular and circular. The domain structure of the nanomagnets is studied by magnetic force microscopy. r 2006 Elsevier B.V. All rights reserved. PACS: 75.75.+a; 75.70.Kw Keywords: Magnetic nanostructures; Magnetic force microscopy; Magnetic domains; Electron-beam lithography; Thin films

1. Introduction In the last years, a great effort was made in the miniaturization of the magnetic structures and devices. These so-called micro or nanomagnets present novel magnetic behavior associated to low-dimensional effects that can be used in a variety of applications [1] (magnetic storage, semiconductor industries, etc.). Reproducible patterning methods such as optical, ion-beam or electronbeam lithography can be used to fabricate elements with different shapes and dimensions [2]. The effective anisotropy of these small dimension features depends on the balance between magnetocrystalline and shape anisotropy [3]. Moreover, their domain configuration depends on the nanostructure’s shape. The characterization of these devices cannot be performed by standard magnetometers like vibrant sample magnetometer (VSM) or superconducting quantum interference device (SQUID). Since local information on magnetic state of individual nanoelements is required, so that new characterization systems must be used. In particular, magnetic force microscope (MFM) Corresponding author. Tel.: +34 91 3348990; fax: +34 91 372 0623.

E-mail address: [email protected] (A. Asenjo). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.10.991

imaging [4,5] reveals as a useful technique to characterize magnetic nanostructures due to its high-spatial resolution (about 30 nm) and sensitivity. In this work, three arrays of Ni (1 1 1) nanostructures with different shapes (squares, triangles and dots) were grown by electron-beam lithography. All the nanostructures present similar lateral and vertical dimensions, the distance between structures is also kept constant. The domain configuration was studied in several remanence states and under in situ magnetic fields by a conveniently modified MFM [6].

2. Experimental results and discussion The Ni (1 1 1) nanostructures arrays were prepared by a combination of electron-beam lithography, sputtering deposition and lift-off process. The nanoelements, fabricated on Si (1 0 0) substrates, have lateral dimensions of about 500 nm, the thickness is 50 nm and the distance between nanomagnets is around 800 nm. The nanostructures were characterized by a AFM–MFM combined system from Nanotec Electronica S.L. [7]. In order to discard the tip influence [8], low moment-MFM

ARTICLE IN PRESS A. Asenjo et al. / Journal of Magnetism and Magnetic Materials 310 (2007) e936–e938

probes (LM-MESP) are used. Fig. 1 shows the topographic images of the three arrays. The MFM images reveal the dependence of the domain structure with the nanomagnet geometric shape. In the case of the triangles or squares (see Fig. 2), every nanostructure present a number of in-plane domains: three in the triangles and four in the squares. The bright and dark contrast corresponds to the extremes of the domain where high positive or negative poles concentrate. In both cases, the domains structure tries to decrease the magnetostatic energy. A strong in-plane anisotropy from shape origin is assumed. In addition, for each nanoelement shape a different in-plane easy axis are considered. In

Fig. 1. AFM images corresponding to an array of (a) Ni nanosquares, (b) Ni nanotriangels and (c) Ni nanodots.

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Fig. 3. MFM images of the same area of the magnetic dots array recorded with the fast scan direction parallel to (a) x-direction or (b) y-direction. In the inset, see the scheme of the vortex proposed as magnetic moment configuration.

order to decrease the pole density the magnetic moment aligns with the nearest edge of the nanostructure. The size of the elements is near the threshold for the development of walls in the small particles since typically the wall size in Ni is about 100 nm. The domain configuration proposed for this situation is presented schematically in Fig. 2c. In addition to the dipolar contrast, bright or dark contrast appears in the center of every element that can be interpreted like a kind of vortex. The domain configuration of the circular nanostructures differs from the above examples. Imaging the Ni nanodots, a double minimum is found in the MFM images. The interpretation of these images considers that we have a delocalized vortex. The movement of the vortex could be induced by the tip field since its position depends on the acquisition parameters, in particular on the fast scan direction and tip–sample distance. The effective anisotropy for Ni (1 1 1) thin films is in-plane direction since the magnetocrystalline anisotropy is much lower than the shape anisotropy. The magnetization in these nanostructures must be in plane. MFM experiments are in agreement with this prediction since the MFM images correspond to in-plane domains with the magnetic moment oriented parallel to edges of the elements. In the case of the circular nanomagnets, Cowburn and coworkers [3] proposed that the external field deforms the vortex by pushing its core away from the center. This metastable situation can be induced by the tip field (about 300 Oe) (Fig. 3).

Acknowledgments

Fig. 2. MFM images of the array of (a) square nanostructures, (b) triangular nanostructures and (c) scheme of the proposed domain configuration.

This work has been supported by the following Grants PTR95-0935, NAN2004-09183-C10-04, NAN2004-9087, FIS2005-7392, MAT2005-23924E, UCM-Santander and CAM S-0505/ESP/0337. M.J. thanks Comunidad Auto´noma de Madrid for the postgraduate fellowship.

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