Journal of Magnetism and Magnetic Materials 289 (2005) 118–121 Experimental evidence of surface effects in the magnetic dynamics behavior of ferrite nanoparticles E.C. Sousa a , C.R. Alves a , R. Aquino b , M.H. Sousa b , G.F. Goya c , H.R. Rechenberg c , F.A. Tourinho b , J. Depeyrot a,Ã a Complex Fluids Group, Instituto de Fı´sica, Universidade de Brası´lia, Caixa Postal 04455, 70919-970 Brası´lia (DF), Brazil b Complex Fluids Group, Instituto de Quı´mica, Universidade de Brası´lia, Caixa Postal 04478, 70919-970 Brası´lia (DF), Brazil c Instituto de Fı´sica Universidade de Sa˜o Paulo, Caixa postal 66318, 05315-970 Sa˜o Paulo (SP), Brazil Available online 26 November 2004 Abstract The magnetic dynamics behavior of copper and nickel ferrite nanoparticles used in the magnetic fluid elaboration and with mean sizes between 3.5 to 10.4 nm is investigated by measurements of magnetic hysteretic properties and zero field cooling (ZFC) susceptibility. The dependence of the irreversibility field, inversely proportional to the particle size, clearly indicates that the magnetic anisotropy of our particles finds its origin on the surface layer. r 2004 Elsevier B.V. All rights reserved. PACS: 75.50.Mm; 75.40.Gd; 75.30.Pd Keywords: Magnetic liquids; Magnetic naoparticles; Spinel ferrites; Surface effects; Magnetic dynamics 1. Introduction Magnetic ferrite nanoparticles are currently very promising new materials in technological applications and also in the design of new magneto-pharmaceuticals [1]. From a more fundamental point of view, these applications require the knowledge of how the proper- ties of magnetic systems differ from bulk ones when the size decreases to the nanometric range. At this scale, the spatial confinement induces finite size effects of the magnetic core and the existence of an interface leads to an incomplete and distorted atomic surrounding in the surface shell [2]. As an example, significant deviations of the Bloch law were predicted and observed due to finite size effects [3]. Moreover, surface spin canting, non- saturated magnetization curves, freezing of surface spin in a disordered state and surface anisotropy are one of the most current experimental manifestations of surface effects [4]. In this work, we investigate the magneto- dynamic properties of copper- and nickel-based spinel ferrite nanoparticles used in the elaboration of electric double layered magnetic fluids (EDL-MF) [5]. Our measurements point the main role played by surface spins in the thermal activated relaxation and in high field irreversibility. 2. Experimental details The investigated spinel CuFe 2 O 4 and NiFe 2 O 4 nanoparticles are prepared by hydrothermal coprecipi- tation, in alkaline medium, of a ferrite stoichiometric mixture of aqueous solutions of CuCl 2 and NiCl 2 with ARTICLE IN PRESS www.elsevier.com/locate/jmmm 0304-8853/$-see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2004.11.035 Ã Corresponding author. Tel.: +55613072900; fax: +55613072363. E-mail address: depeyrot@fis.unb.br (J. Depeyrot).