Review: Determining effective magnetic anisotropy in magnetic nanoparticles systems using DC-magnetometry methods

The effective magnetic anisotropy constant Keff K eff controls the thermal stability, relaxation dynamics, and performance of magnetic nanoparticles in applications such as magnetic hyperthermia, sensing, and data storage. However, reliably determining Keff K eff remains difficult because interparticle interactions, surface and interface effects, and structural heterogeneities strongly affect the magnetic response of MNP ensembles. This review critically examines four DC-magnetometry-based approaches to determine Keff K eff : Néel relaxation analysis, coercive field thermal decay, remanent magnetization decay, and a modified high-field Actis–Zélis asymptotic expansion. Using representative magnetite and cobalt-doped ferrite nanoparticles, we systematically compare the assumptions, limitations, and applicability of each method. We show that conventional low-field methods intrinsically mix single-particle anisotropy with collective effects, causing systematic errors in Keff K eff . In contrast, the Actis–Zélis high-field approach exploits large Zeeman energies to largely suppress interparticle coupling, enabling a more accurate determination of the intrinsic anisotropy barrier. By defining clear applicability criteria for each method, this work provides a consistent framework for extracting reliable anisotropy values and practical guidelines for interpreting magnetometry data in complex nanoparticle systems.