Dipolar field-induced asymmetric magnetization hysteresis of immobile superparamagnetic nanoclusters

Despite the critical contribution of dipolar interparticle interactions to the hysteretic magnetization curve of superparamagnetic nanoclusters, the quantitative analysis of dipolar field remains a major challenge at room temperature. To investigate such demagnetizing effect due to a close core-to-core distance within a nanocluster, we evaluate the symmetry of ac minor magnetization hysteresis in the presence of static magnetic field. While the nanoclusters are immobilized in a gelatin matrix, we record their dynamic magnetizations for a given 100-kHz sinusoidal magnetic field perpendicular to the static field and further correlate the changes in the coercive fields and remanences with the dipolar field and its demagnetization ratio, respectively. In this work, we also show that the dipolar field-induced shift of the magnetization loop is size-dependent. Under high static bias field, larger core size proportionally enhances the resulting dipolar field. Meanwhile, the cluster size appears to be influential to the dipolar magnetism triggered at low bias field. This study conclusively highlights the distinguishable dynamics between randomly oriented and collaterally aligned magnetic moments.