Determination of the effective anisotropy constant of magnetic nanoparticles – Comparison between two approaches

In this paper we investigate the effective magnetic anisotropy constant K_eff of single-core magnetic nanoparticles (MNPs) as a function of temperature applying two approaches, which both are based on the Stoner-Wohlfarth model: First, the coercive field is used to determine the anisotropy field and second the descending branch of the magnetic moment-vs.-magnetic field (m-H) curve measured on immobilized MNPs is fitted with an empirical model. In particular, we compare the analysis with and without considering the distribution of core size, which is estimated from fitting the m-H curve measured on MNP suspensions. Good agreement between the K_eff(T) curves estimated with both methods is found if the size distribution is included while significant deviations are observed especially at high temperatures if the size distribution is ignored. Additionally, the blocking temperature was obtained from the maximum of the temperature derivation of the difference between the field-cooled (FC) and zero-field-cooled (ZFC) magnetization curve. The K_eff value determined from the blocking temperature showed reasonable agreement with those obtained from coercive field and m-H curve. We also found that the magnetic moment of both studied single-core MNP systems exhibits a significant paramagnetic-like contribution even at 7 T.