Two Methods for the Experimental Determination of the Relaxation Time and SPA of Magnetic Nanoparticles under RF Fields: Implications for Nanowarming and Hyperthermia

In applications such as hyperthermia and nanowarming, power dissipation arises when the time-dependent magnetization M ( t ) of an out-of-equilibrium system of nanoparticles lags behind the applied magnetic field H ( t ). The key parameter governing this process is the relaxation time τ, which induces a phase shift ϕ n between H ( t ) and each n- th harmonic component of M ( t ). In this work, we present two methods to obtain the effective value of τ from radiofrequency field (RF) magnetization measurements. One method derives the result directly from ϕ n, while the other fits the M ( H ) cycle using a time-delayed superparamagnetic response. We compare these methods applied to the variation of τ for magnetic nanoparticles under an RF field in two experiments: the solid-to-liquid transition of an aqueous suspension of particles and the effect of increasing concentration of the suspension.