An Analysis of Minor Hysteresis Loops of Nanoparticles for Hyperthermia

In this work, we have undertaken a study of the hysteresis loops of two sets of nanoparticles as a function of the maximum applied field (H_SAT) used to generate the loop. The particle sizes chosen were ~13 nm and ~30 nm. These two sizes represent particles which are single domain and particles which lie close to the single-domain/multidomain boundary. As expected the small 13-nm particles show virtually no hysteresis at room temperature and hence no thermal effect from hysteresis loss would be expected for these materials. In contrast, the 30-nm particles show a significant hysteresis with a saturation coercivity of 83 Oe. However, the area of the hysteresis loop diminishes significantly with reduced applied saturating field and a consequent reduction in the coercivity. Theoretical calculations predict the experimental data well for the 13-nm particles using an effective anisotropy constant of 5 × 10⁵ ergs/cm³ gives a good fit to the experimental data. For 30-nm particles, the effective anisotropy constant that gives a fit to the data is 1.8 × 10⁵ ergs/cm³. This decrease in the effective anisotropy constant is due the different reversal mechanism in the particles.