INSTITUTE OF PHYSICS PUBLISHING JOURNAL OF PHYSICS D: APPLIED PHYSICS J. Phys. D: Appl. Phys. 36 (2003) 1227–1235 PII: S0022-3727(03)55409-2 The effect of colloidal stabilization upon ferrimagnetic resonance in magnetic fluids in the presence of a polarizing magnetic field P C Fannin 1 , C N Marin 2,5 , V Socoliuc 3 , G M Istr ˘ atuc ˘ a 4 and A T Giannitsis 1 1 Department of Electronic and Electrical Engineering, Trinity College, Dublin 2, Ireland 2 West University of Timi¸ soara, Faculty of Physics, B-dul V. Pˆ arvan, no. 4, 1900 Timi¸ soara, Romania 3 National Institute for Research and Development in Electrochemistry and Condensed Matter, Tˆ ırnava, no. 1, 1900 Timi¸ soara, Romania 4 Institute of Chemistry from Timi¸ soara, Romanian Academy, M. Viteazu, no. 24, 1900 Timi¸ soara, Romania E-mail: cmarin@physics.uvt.ro Received 31 October 2002, in final form 4 February 2003 Published 14 May 2003 Online at stacks.iop.org/JPhysD/36/1227 Abstract The complex magnetic susceptibility of two magnetic fluids, with different degrees of colloidal stabilization, has been measured over the frequency range 100 MHz to 6 GHz. The colloidal stabilization of the magnetic fluids has been investigated using magneto-optical measurements. Based on complex magnetic susceptibility measurements, χ(ω) = χ ′ (ω) − iχ ′′ (ω), the dependence of the maximum absorption frequency at resonance, f max , and of line width, f , on an external magnetic polarizing field, H , over the range 0–1.45 kOe, has been examined for both magnetic fluids. The experimental results have been interpreted in terms of magnetic interparticle interactions and particle agglomeration. 1. Introduction Magnetic fluids are stable colloidal systems consisting of magnetic single domain particles dispersed in a carrier liquid. In order to preserve the colloidal stabilization, the particles are coated with a surfactant [1]. Particle agglomeration may occur within magnetic fluids [1] depending on a number of factors including the type of stabilization, particle size distribution, temperature and the strength of an applied magnetic field. Since magnetic resonance measurements are very sensitive to changes in the local magnetic field, these can be used for the investigation of structural changes in magnetic fluids. As is well known, two experimental arrangements are used for magnetic resonance measurements. In the conventional 5 Author to whom correspondence should be addressed. magnetic resonance technique, the sample is placed within a resonant cavity, the frequency of the microwave field remains constant and the static magnetic field increases slowly over a fixed range and in a settled time interval. The recorded signal is the power absorbed by the sample as a function of the polarizing magnetic field. In the second experimental arrangement, the magnetic resonance phenomenon is determined from measurements of complex magnetic susceptibility, χ(ω), at a constant polarizing field [2], resonance being indicated by a transition in the value of real part of complex magnetic susceptibility, χ ′ (ω), from a positive to a negative quantity at the resonance frequency. Investigations concerning the effect of interparticle interactions within magnetic fluids on the magnetic resonance line have been performed in papers [3–6] based on conventional ferromagnetic resonance techniques. Sharma 0022-3727/03/111227+09$30.00 © 2003 IOP Publishing Ltd Printed in the UK 1227