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(AST)) used to generate the loop. The particle sizes chosen were similar to 13 nm and similar to 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 x 10(5) ergs/cm(3) 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 x 10(5) ergs/cm(3). This decrease in the effective anisotropy constant is due the different reversal mechanism in the particles.