Size-Dependent Relaxation Properties of Monodisperse Magnetite Nanoparticles Measured Over Seven Decades of Frequency by AC Susceptometry

Abstract

Magnetic relaxation is exploited in innovative biomedical applications of magnetic particles such as magnetic particle imaging (MPI), magnetic fluid hyperthermia, and bio-sensing. Relaxation behavior should be optimized to achieve high performance imaging, efficient heating, and good SNR in bio-sensing. Using two AC susceptometers with overlapping frequency ranges, we have measured the relaxation behavior of a series of monodisperse magnetic particles and demonstrated that this approach is an effective way to probe particle relaxation characteristics from a few Hz to 10 MHz, the frequencies relevant for MPI, hyperthermia, and sensing.

Keywords: AC susceptometry; magnetic particle imaging; magnetic particles; magnetization reversal.

Fig. 1

Relaxation frequency (1/(2πτ)) calculated according to (3), (4), and (5). For calculations, the hydrodynamic layer thickness, δ, was assumed to be 10 nm, and the magnetocrystalline anisotropy, K, was the bulk value, 1.1 × 10⁴ Jm⁻³, for magnetite at 298 K. The blue shaded area represents the frequency range probed by the Acreo HFAC Susceptometer, and the orange shaded area the frequency range probed by the DynoMag system.

Fig. 2

a–c) Measurements of relaxation by ac susceptibility (filled diamonds) for magnetite nanoparticles listed in Table I. Solid lines are fits to the data using the single-core susceptibility model. d–f) Particle size determined from relaxation measurements, assuming a log-normal distribution as in (1).

Fig. 3

Size determination of UW3 by a) Chantrell fitting to M(H) data and b) TEM counting. A volume-weighted log-normal distribution as in (1), with median radius, r0_V, and distribution shape parameter, σ_V, was assumed.