Eur. Phys. J. B 40, 463–470 (2004) DOI: 10.1140/epjb/e2004-00285-7 THE EUROPEAN PHYSICAL JOURNAL B Magnetization reversal measurements in mesoscopic amorphous magnets by magneto-optical Kerr effect R. Morales, J.I. Mart´ın a , M. V´elez, and J.M. Alameda Depto. F´ısica, Universidad de Oviedo, c/ Calvo Sotelo s/n, 33007 Oviedo, Spain Received 11 December 2003 / Received in final form 16 June 2004 Published online 24 September 2004 – c  EDP Sciences, Societ`a Italiana di Fisica, Springer-Verlag 2004 Abstract. A magneto-optical setup based on the transverse Kerr effect has been designed to study the magnetization reversal processes by vector magnetometry in arrays of magnetic nanostructures with a reduced total volume. This system allows the measurement of both the parallel and perpendicular to the field components of the magnetization. It has been used to analyze the behavior of amorphous CoxSi1−x lines fabricated by electron beam lithography that present a very well defined shape induced uniaxial anisotropy. When the field is applied near to the hard direction, coherent rotation processes are found to occur with a collapse of this reversal mode at fields very close to the hard axis that allows to estimate the very low anisotropy dispersion of these samples. The analysis of the vector hysteresis loops reveals that the magnetization switches via an incoherent process that starts prior to the Stoner-Wohlfarth instability and that can be described in terms of a localized curling-like reversal mode. PACS. 75.75.+a Magnetic properties of nanostructures – 75.60.Jk Magnetization reversal mechanisms – 75.50.Kj Amorphous and quasicrystalline magnetic materials 1 Introduction The fast development of different lithography methods has allowed the fabrication of high quality ordered mag- netic nanostructures in the last few years. The combi- nation of some techniques, as electron beam lithography, X-ray lithography or laser interference lithography, with several lithography procedures, as lift-off, etching or elec- troplating, has led to the preparation of patterned na- noelements made of a wide variety of magnetic materials and with different structural properties, such as poly- crystalline, single-crystalline or amorphous [1–5]. These small nanostructures are very interesting for applica- tion purposes in magnetic recording and are also inten- sively studied from the basic point of view because they can reveal novel properties related with their mesoscopic dimensions. On the other hand, the study of the magnetic proper- ties of these small nanostructures can become difficult due to the reduced volume of the nanoelements and, therefore, to their small magnetic moment. Thus, the conventional magnetization measurement techniques are often not suit- able to characterize them, unless the nanostructures are patterned in arrays over very large areas. To address this issue, different measurement methods have been designed and developed. Some of these techniques that allow the a e-mail: jmartin@condmat.uniovi.es analysis of arrays of particles with small dimensions are magnetic force microscopy (MFM) [6], electron hologra- phy [7], Lorentz microscopy [8], Hall magnetometry [9], magneto-optical Kerr effect [10,11] and micron super- conducting quantum interference device (µ-SQUID) [12]. However, important characteristics of the magnetization reversal processes remain unsolved as these measurements do not usually give a complete description of the magne- tization in the magnetic nanostructures when a hysteresis loop is measured. One interesting approach to analyze the domain structure of periodic nanostructures is the analysis and modeling of the magnetooptical response of the light diffracted by the samples [13]. On the other hand, in order to make a clear determination of the presence of rotation processes in the magnetization reversal in bulk materials and unpatterned thin films, it is often very helpful to mea- sure not only the usual hysteresis loop of the component of the magnetization that is parallel to the applied mag- netic field (i.e., the longitudinal component M  ), but also the hysteresis loop of the component of the magnetization that is perpendicular to the applied field (i.e., the trans- verse component M ⊥ ) [14–17]. Vector magnetometry has also been used to study the magnetic behavior of contin- uous magnetic films with arrays of submicrometric holes patterned over extended areas [18] but, up to our knowl- edge, this kind of technique has not been implemented yet to analyze magnetic nanostructures such as lines or dots with a reduced total volume.