Probing the atomistic origins of magnetic heating with colloidal solutions of ferrimagnets.

Understanding the physics of alternating magnetic field-induced heating in magnetic materials is important for optimizing the performance of systems, such as electrical transformers, spintronic devices, and nanomaterials used for controlling biological signaling and cancer ablation. Heating in these systems can be accurately explained by theories where magnetization of the material is uniform and the only relevant energy barrier is magnetic anisotropy. In these models, total heating is proportional to hysteresis loop area and scales with the total magnetization. However, these models assume uniform magnetization, which does not necessarily hold in nanostructured materials. A more accurate model should consider all nanomagnetic energy dynamics and show that heating is a result of the rapid movement of individual spins. Here, colloidal solutions of near-compensation GdCo ferrimagnets are used to probe the atomistic energetic losses that occur in non-uniform magnetic systems. Although near-compensation ferrimagnets should not heat according to uniform-magnetization-based theories, we find that heating occurs in a manner that is not dependent on the total hysteresis loop area, but rather on the movement of spins in the Gd and Co sublattices. The atomistic magnetic spin dynamics in ferrimagnets can be connected to uniform-magnetization-based heating theories by considering the energetic losses of the individual sublattices. This modeling predicts that magnetic field-induced heating is optimized in systems where exchange-driven dynamics drive rapid movement of spins, an insight that could allow for optimization of alternate magnetic field induced heating in systems with a non-uniform magnetization.