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The low-field complex susceptibility curves of ferrofluids have in general a Debye-type profile with a loss component which peaks at a frequency from which it is possible to estimate the average relaxation time of the particles in the ferrofluid. The authors report on magnetic measurements on a water-based ferrofluid whose loss curve displayed two distinct peaks. It is concluded that one peak is due to Brownian relaxation and the other is due to a combination of Brownian and Neel relaxation.

Multifunctional nanoparticles that actively target specific tissues are studied for cancer diagnosis and treatment. Magnetically and optically active particles are of particular interest because they enable multiple imaging modalities and physically modulated therapies, such as

The short-circuit coaxial transmission line technique has been used to investigate ferromagnetic resonance in magnetic fluids over the frequency range 0.1-6 GHz. Measurements of the complex susceptibility, chi ( omega )= chi '( omega )-i chi "( omega ), of magnetic fluid samples of magnetite and cobalt particles, of approximate median diameters 10 and 5.8 nm are presented. The presence of loss peaks in the chi "( omega ) components at approximate frequencies of 1.2 and 2.5 GHz, respectively, coupled with the transition of the chi '( omega ) components from positive to negative values at slightly higher frequencies, is indicative of ferromagnetic resonance. Appropriate equations for the calculation of the complex susceptibility are presented.

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Nanometre-sized particles of ferrite, commonly used in magnetic fluids, are single-domain. The direction of magnetic moment of these small, uniaxial, ferromagnetic particles is known to fluctuate due to thermal agitation, and can relax through the Neel-type relaxation mechanism. The relaxation time of such fluctuations is usually determined by means of Brown's equations for high and low barrier heights. More recently, modified equations catering for a continuous range of barrier heights have been proposed. Comparison of these equations shows that, even in the most extreme case only a factor of approximately 1.7 distinguishes the corresponding eigenvalues (which represent the inverse of the relaxation time). It is concluded that the major source of error in predicting the relaxation time arises, not primarily due to the particular equations used, but because of the large uncertainty in obtaining precise experimental data needed to determine the components, f0 and sigma , of these equations. For example, for a small change in anisotropy constant K by a factor of 2.5 (typical values for the system considered here are (2-5)*104 J m-3), the calculated values of Neel relaxation times using Brown's equation differ by a factor of about 37, corresponding to times of 1.6*10-7 to 4.3*10-9. An experimental value of 5*10-9 s determined from the frequency of the maximum of the loss-peak of the imaginary part of the complex susceptibility is at the outer limit of these calculated values.