c-Fe 2 O 3 /SiO 2 nanocomposites for magneto-optical applications: Nanostructural and magnetic properties D. Ortega a, * , J.S. Garitaonandia b , C. Barrera-Solano a , M. Ramı ´rez-del-Solar a , E. Blanco a , M. Domı ´nguez a a Departamento de Fı ´sica de la Materia Condensada, Facultad de Ciencias, Universidad de Ca ´ diz, Campus Universitario del Rı ´o San Pedro, 11510 Puerto Real (Ca ´ diz), Spain b Departamento de Fı ´sica Aplicada II, Facultad de Ciencias, Universidad del Paı ´s Vasco,Apartado 644, 48080 (Bilbao), Spain Received 22 July 2005; received in revised form 3 March 2006 Available online 9 June 2006 Abstract Monolithic nanocomposites consisting in c-Fe 2 O 3 (maghemite) nanoparticles embedded in a silica gel have been prepared as potential magneto-optic materials, suitable for magnetic field sensing. The effects of several processing parameters on the structural and magnetic properties of the samples were evaluated to optimize their performance. Their structural properties have been studied by means of X-ray diffraction, thermogravimetric analysis and transmission electron microscopy, while the magnetic behavior has been characterized by Mo ¨ ssbauer spectroscopy and Faraday magnetometry. Our results indicate that maghemite crystallization conditions must be carefully controlled to obtain nanoparticles of an adequate size and to avoid subsequent evolution to other undesired ferric oxide phases. Ó 2006 Elsevier B.V. All rights reserved. PACS: 81.20.Fw; 81.07.Bc; 81.40.Rs; 76.80.+y Keywords: Magnetic properties; Mossbauer effect and spectroscopy; TEM; Nanocomposites; Nanoparticles; Sol–gels (xerogels); Thermal properties; X- ray diffraction 1. Introduction The study of maghemite (c-Fe 2 O 3 )/silica nanocompos- ites has been undertaken from several points of view, regarding either their synthetic route [1] or their final tech- nological applications [2]. We have specifically focused our investigation on xerogels containing ferrimagnetic maghe- mite nanoparticles, not only for their optical [3,4], magnetic [5–8] and magneto-optic [9,10] properties, but also because of their particular structure, which acts like a modifier of the properties that the nanoparticles would have out of the gel matrix, either in a suspension or in a bulk form. Maghemite/silica composites, as previously reported by other authors [11,12], exhibit magneto-optical Faraday effect (MOFE), which is a desirable feature to develop mag- netic field sensors from these materials [13,14]. Maghemite is a ferrimagnetic material which represents the low temperature phase of iron oxide, and it easily evolves into the more stable phase, hematite (a-Fe 2 O 3 ), when submitted to high temperatures. Thus, special care must be taken to avoid the appearance of hematite while iron oxide xerogels are heat treated at temperatures above 500 °C, since its antiferromagnetic nature can seriously affect the desired ferrimagnetic behavior of the final nano- composite. In addition, the silica matrix magnetic dilution effect, the superparamagnetic behavior of the smallest par- ticles and the coexistence of more than one iron oxide phase give rise to a complex magnetic response. The pres- ent work is aimed to investigate the influence of processing conditions in the tailoring of magnetic properties of these 0022-3093/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2006.03.056 * Corresponding author. Tel.: +34 956 016 569; fax: +34 956 016 288. E-mail address: daniel.ortega@uca.es (D. Ortega). www.elsevier.com/locate/jnoncrysol Journal of Non-Crystalline Solids 352 (2006) 2801–2810