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.