Magnetocaloric Effect of Uniformly Packed Ferromagnetic Gd Nanoparticle Clusters

We report a theoretical discussion on the magnetocaloric effect (MCE) of ferromagnetic Gd nanoparticles arranged in large eccentricity ellipsoidal clusters. We focus on micrometer-sized clusters of uniformly packed Gd nanoparticles, with the external field parallel to the major axis of the ellipsoidal cluster. We have replicated the bulk results for small nanoparticle spatial density. However, for large nanoparticle density, the MCE is strongly influenced by the nanoparticle's dipolar interaction. At low temperatures, the dipolar field strengths can reach a few Teslas, and there are two distinct regimes. For high external field strengths, the MCE is normal, and the dipolar interaction causes an upshift of the temperature for the peak of the isothermal entropy change \Delta S . For low external field strengths, the MCE is normal for temperatures above the cluster Curie temperature. In contrast, we found an inverse MCE (IMCE) for low temperatures due to the nucleation of an antiferromagnetic pattern, where neighboring lines along the ellipsoid major axis are magnetized in opposite directions.