Thermal hysteresis of superparamagnetic Gd nanoparticle clusters

We report a theoretical study of the impact of dipolar interactions on the magnetic phases and thermal hysteresis of superparamagnetic Gd nanoparticle clusters. We consider Gd nanoparticles with size ranging from 5.5 to 10 nm, arranged with uniform density in a few-hundred-nanometer-sized ellipsoidal clusters. We show that in densely packed systems the dipolar interaction may grant thermal stabilization of the individual nanoparticle moments at high-temperature values, leading to large thermal loop widths, tunable by external magnetic fields of moderate strength. We have found, for instance, that with an external magnetic field strength of 85 Oe one may reach thermal loop widths ranging from 88 K for 5.5-nm-diam Gd nanoparticle clusters to 660 K for 8-nm-diam Gd nanoparticle clusters.