Journal of Magnetism and Magnetic Materials 316 (2007) e5–e8 Different aggregation states in Cu/Co multilayers prepared by RF sputtering on rotating substrates Paolo Allia a,Ã , Federica Celegato b , Marco Coı¨sson b , Paola Tiberto b , Franco Vinai b a Dipartimento di Fisica, Politecnico di Torino, Corso Duca degli Abruzzi 24, I-10129 Torino, Italy b INRIM, Strada delle Cacce 91, I-10135 Torino, Italy Available online 25 February 2007 Abstract Co/Cu multilayers with different Co/Cu ratio were prepared by RF sputtering on Si 3 N 4 substrates in Ar atmosphere by the rotating- substrate technique. The samples were produced with five subsequent expositions of the substrate, rotating at nine angular velocities between 0.23 and 0.63 rad/s. Room temperature measurements of magnetization and magnetoresistance point to a gradual transition from a multilayered to a granular structure on increasing the angular velocity of the substrate. r 2007 Elsevier B.V. All rights reserved. PACS: 75.47.De; 75.70.Cn; 75.50.Tt Keywords: Magnetoresistance; Magnetic multilayer; Magnetic nanoparticle 1. Introduction The isotropic giant magnetoresistance (GMR) of bi- metallic granular systems containing magnetic nanoparti- cles in a metallic matrix has been extensively studied because of its interest in basic research and applications [1]. This phenomenon was first observed in magnetic multi- layers [2] and, subsequently, in metallic granular films [2,3]. Granular materials produced by sequential deposition of discontinuous magnetic layers and continuous non mag- netic layers made by immiscible elements by plasma sputter deposition have recently attracted notable interest [4]. Owing to the high surface free energy of the magnetic element, reducing its nominal thickness to a few nan- ometers makes it possible to turn uniform magnetic layers into an assembly of nanosized Co particles, thereby producing systems whose magnetic and magnetotransport properties gradually evolve from those of a multilayer into those of a granular system [4]. In this work, Cu/Co multilayers have been produced by RF sputtering using the rotating-substrate technique. Magnetic and magnetotran- sport properties of these Cu/Co multilayers have been studied as functions of the angular velocity of the substrate holder. 2. Experimental Cu/Co multilayers were prepared by RF sputtering on Si 3 N 4 substrates in Ar atmosphere (pressure: 9  10 3 mbar) by the rotating-substrate technique. Each substrate was fixed to a rotating plate whose angular velocity o was kept constant for each sample, and varied between 0.23 and 0.63 rad/s in nine subsequent depositions. In each deposition, the substrates were sequentially exposed to the two targets in order to produce a Cu/Co bilayer at each turn of the plate. All samples were produced with five subsequent expositions. The nominal thickness t of each single Cu or Co layer (assumed uniform) is reported in Table 1. Room-temperature magnetization measurements were performed by alternating-gradient magnetometry (AGFM) under a maximum field of 20 kOe. Magnetoresistance (MR) measurements at 300 K were done by the 4-contacts technique under the same magnetic field. The MR DR/R (%) is defined as [R(H)R(H ¼ 0)]/R(H ¼ 0)  100. ARTICLE IN PRESS www.elsevier.com/locate/jmmm 0304-8853/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2007.02.011 Ã Corresponding author. Tel.: +39 11 5647336; fax: +39 11 5647399. E-mail address: paolo.allia@polito.it (P. Allia).