Bimodal properties of a patterned magnetic nanostructure: Separation of individual components’ contributions I. Sveklo a,c , Z. Kurant a , A. Maziewski a , E. Sieczkowska b , A. Petroutchik b , L.T. Baczewski b , A. Wawro b,n a Faculty of Physics, University of Bialystok, ul. Lipowa 41, 15-424 Bialystok, Poland b Institute of Physics Polish Academy of Sciences, Al. Lotników 32/46, 02-668 Warszawa, Poland c Kupala State University, Ozheshko str. 22, Grodno 230023, Belarus article info Article history: Received 17 April 2013 Received in revised form 24 May 2013 Available online 10 June 2013 Keywords: Magnetic nanodots Magnetization reversal mechanisms Magnetic hysteresis Magnetic force microscopy Magneto-optic Kerr effect abstract We study the properties of magnetic islands with perpendicular magnetization (dots), hundred nanometres in lateral size, embedded in a magnetic matrix, by magneto-optical method exploiting Kerr effect and magnetic force microscopy. Due to a complex structure the investigated patterned system displays a bimodal character. In order to gain an insight into behaviour of the components we apply a simple approach for decomposition of bimodal hysteresis loops obtained from magneto-optical measurements. The hysteresis loops of subsystems are determined from the field dependent rotation of a plane polarization and ellipticity of a laser beam reflected from the sample surface. Comparison with the adequate reference samples reveals differences in the behaviour of the patterned system resulting from confined size of the dots and their dipole coupling with the matrix. Magnetization reversal mechanisms of the individual components are discussed in details. & 2013 Elsevier B.V. All rights reserved. 1. Introduction A need of fundamental research of magnetism in nanoscale objects causes that complex magnetic systems such as multilayers or patterned nanostructures are of great scientific interest [1–3]. They are of high technological relevance, too. However, grown in nanometre scale, these systems cannot be considered usually as composed of two or more non-interacting mutually magnetic subsystems. Thus their properties are not a simple superposition of the features describing the individual isolated components. The properties of such systems exhibit a multimodal character. This behaviour can be observed in layered systems with interlayer coupling [4] or bias exchange [5] and in patterned structures [6–11]. Several methods for decomposition of the hysteresis loops obtained from magneto-optical investigations have been proposed up to date [12–14]. Obtained results depend, however, on the wavelength and angle of beam incidence or need an implementation of additional optical phase compensators to extract a signal from a selected layer. Moreover, these methods are sophisticated and require enhanced instrumentation. In this work we investigate the properties of a complex magnetic system patterned in the nanoscale. This system is induced in a layered structure containing a Co layer deposited on a structured buffer in a form of Au islands self-assembled on a Mo layer surface (Fig. 1). The magnetic patterning is obtained due to a strong dependence of the thin Co layer magnetic anisotropy on a buffer layer type [15]. In consequence a system of elevated islands with perpendicular magnetization (Co layer grown on top of the Au islands) embedded in a trench magnetic matrix (Co layer deposited on the Mo surface, between the Au islands) was created. Due to self- assembly the Au islands may take non-regular shapes. Moreover, they display a distribution in height. Detailed investigations of the Au islands grown on a Mo layer surface in various conditions are reported in Ref. [16]. Their height is much larger than the thickness of the deposited Co layer. Despite irregular shapes of the islands with perpendicular magnetization they are called further in this work as dots, for brevity. The measurements were carried out by magneto- optical methods exploiting Kerr effect in polar configuration (P-MOKE) and magnetic force microscopy (MFM). Depending on the Co layer thickness, d Co , the dots and the matrix exhibit different alignments of mutual magnetization (Fig. 1): P–P (configuration (I)), P–I (configuration (II)), and I–I (configuration (III)), where P denotes magnetization perpendicular to the film plane, whereas I—magne- tization oriented in the sample plane [17]. In this paper we confine our considerations to the most interesting configurations, where the dots are magnetized perpendicularly, i.e. to (I) and (II). In order to gain an insight into behaviour of the individual components, i.e. the dots and the matrix, we present a simple method to separate their Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/jmmm Journal of Magnetism and Magnetic Materials 0304-8853/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmmm.2013.05.061 n Corresponding author. Tel.: +48 22 116 3236. E-mail address: wawro@ifpan.edu.pl (A. Wawro). Journal of Magnetism and Magnetic Materials 345 (2013) 82–88