Demonstrates effective easy-axis alignment in aggregated nanoparticles, suggesting implications for design and application.
Understanding dynamic magnetization in aggregated magnetic nanoparticles remains a critical yet unexplored aspect of nanoparticle design. This study introduces an analytical framework based on static and dynamic magnetization measurements of easy-axis-aligned samples to quantitatively investigate the effective easy axis formed via dipole interactions during aggregation. Direct-current magnetization measurements revealed two-dimensional distributions of anisotropy energy and magnetic moment, reflecting the degree of alignment of easy axes within the aggregates. Furthermore, analysis of the measured relaxation process yielded the magnetic relaxation-time spectrum. The results demonstrate that magnetic relaxation is governed by an effective easy axis formed within the aggregates via dipolar interactions and proceeds via two distinct steps: (i) Néel relaxation of individual nanoparticles within the aggregates and (ii) relaxation induced by the effective magnetization of the aggregates. These findings identify easy-axis alignment within aggregates as a key structural factor governing magnetization behavior. The proposed evaluation methods provide valuable insight into the magnetic behavior of aggregated nanoparticles relevant to biomedical applications.
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Goto et al. (2026) studied this question.
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