Journal of Magnetism and Magnetic Materials 272–276 (2004) e1251–e1252 Magnetic properties of Co–Ni alloy nanoparticles prepared by the sol-gel technique C. Sangregorio a, *, C. de Juli ! an Fern ! andez b , G. Battaglin c , G. De d , D. Gatteschi a , G. Mattei b , P. Mazzoldi b a INSTM-Dip. di Chimica, Univ. di Firenze, 50019 Sesto Fiorentino (FI), Italy b INFM - Dip. di Fisica, Univ. di Padova, 35131 Padova, Italy c INFM -Dip. di Chimica-Fisica, Univ. Venezia, 30123 Venezia, Italy d Central Glass & Ceramic Research Institute 196, Kolcata 7000032, India Abstract The magnetic properties of Co, Ni and Co x Ni 1x alloy nanoparticles with different composition (0 o x o 1), prepared by the sol-gel route, were investigated. ZFC and FC magnetization measurements show that the blocking temperature increases with the Co content, while a maximum in the anisotropy constant was found for x=0.7. Room temperature FMR measurements, suggest that in samples with larger Co content (xX0.66) interparticle interactions play a relevant role in determining their magnetic properties. r 2004 Published by Elsevier B.V. PACS: 75.50; 76.50; 36.40.c Keywords: Magnetic nanoparticle; Alloy; Ferromagnetic resonance; Superparamagnetism Materials formed by magnetic nanoparticles dispersed in dielectric matrix have novel magnetic, optical, transport, and catalytic properties which are appealing for technological applications [1]. In particular, alloy- based nanocomposites constitute an extremely intri- guing class of materials for designing new devices, since their magnetic properties can be tailored by combining size effects and alloy composition. In this paper, we report the characterization of the magnetic properties of Co x Ni 1x alloy nanoparticles with different composition ranging from pure Co (x=1) to pure Ni (x=0) dispersed in silica films. The particles were prepared by the sol-gel dip-coating technique [2]. TEM analysis revealed that in all samples well-defined spherical particles, homogeneously dispersed in a silica layer of about 130–150 nm thickness from the surface, are present. The average particle size varies with the Co– Ni ratio, increasing from 3.171.1nm, for pure Ni, to 6.571.1nm, for pure Co. The volume particle concen- tration, e; is 3% for all the samples. The nanoparticles have a FCC structure in all the composition range, contrary to what observed in the bulk alloy where the HCP structure is the most stable for Co content larger than 70% [3]. The magnetic behavior was investigated by zero field cooled (ZFC) and field cooled (FC) magnetization measurements which are shown in Fig. 1. The ZFC– FC curves of all samples display the typical features of assemblies of single domain particles with a narrow energy barrier distribution. The only exception was observed for the pure Co sample where the ZFC exhibits a broad maximum near room temperature which indicates a large magnetic anisotropy energy barrier distribution. The average blocking temperature in- creases with the Co content from 7 K for pure Ni to ca. 70K for x=0.80, while its distribution becomes progressively larger. A rough evaluation of the average anisotropy constant, K, was made from the relationship K=25k B T max /V, where k B is the Boltzmann constant, ARTICLE IN PRESS *Corresponding author: Tel.: +39-0554573338; fax: +39- 0554573272. E-mail address: claudio.sangregorio@unifi.it (C. Sangregorio). 0304-8853/$-see front matter r 2004 Published by Elsevier B.V. doi:10.1016/j.jmmm.2004.01.062