Journal of Magnetism and Magnetic Materials 221 (2000) 63}79 Surface-related properties of -Fe  O  nanoparticles E. Tronc*, A. Ezzir, R. Cherkaoui, C. Chane H ac, M. Nogue`s, H. Kachkachi, D. Fiorani, A.M. Testa, J.M. Grene`che , J.P. Jolivet Laboratoire de Chimie de la Matie % re Condense & e (LCMC), UMR 7574 CNRS-Universite & Pierre et Marie Curie, T54-E5, 4 Place Jussieu, 75252 Paris cedex 05, France LMOV, UMR 8634 CNRS-Universite & de Versailles-Saint Quentin, 78035 Versailles cedex, France ICMAT, CNR, P.B. 10, 00016 Monterotondo Stazione, Roma, Italy LPEC, UPRESA 6087 CNRS, Universite H du Maine, 72085, Le Mans, Cedex 9, France Abstract Surface/interface e!ects on static magnetic properties of -Fe  O  nanoparticles of average diameter 3}10 nm, have been investigated by magnetisation measurements (0}55 kOe, 4.2}300 K) as a function of the "eld and temperature and by in-"eld (60 kOe) Mo K ssbauer spectroscopy experiments. The comparison of the results between particle dispersions with di!erent concentrations in polyvinylic alcohol and phosphate-coated particles provides evidence for the interplay between surface anisotropy, intraparticle exchange coupling between surface and core, and interparticle interactions. Enhanced local surface anisotropy and surface spin disorder at low temperature have been found in both systems. In the dispersions, the existence of a canted surface spin structure below +85 K is evidenced by in-"eld Mo K ssbauer experiments and by magnetisation measurements showing a strong increase of the high-"eld magnetisation with decreasing temperature as the "eld increases, more and more important with decreasing particle size. In the phosphate-coated particles, consisting of a -Fe  O  core surrounded by a disordered phosphated iron shell, the surface spins are paramagnetic down to +30 K, below which they freeze in a speromagnetic-like structure.  2000 Elsevier Science B.V. All rights reserved. PACS: 75.50.T; 75.50.G; 75.50.L; 75.30.G; 75.60.E; 76.80 Keywords: Fine particles; Magnetisation; Anisotropy; Surface; Dipolar interactions 1. Introduction The properties of "ne magnetic particles [1,2] are dominated by "nite-size e!ects and interaction e!ects. The competition between surface and core magnetic properties determines the spin structure * Corresponding author. Tel.: #33-1-44-27-5556; fax: #33- 1-44-27-4769. E-mail address: elt@ccr.jussieu.fr (E. Tronc). in the particle, which may thus be altered with respect to that in the bulk material [3}6]. Surface e!ects become more and more important as the particle size decreases, because the surface-to-vol- ume ratio increases, but they are inevitably mixed with volume e!ects and thus generally blurred. Surface magnetic properties result basically from the breaking of symmetry of the lattice [7,8], which leads to site-speci"c, generally unidirectional, surface 0304-8853/00/$ - see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 0 3 6 9 - 3