The magnetic moment of magnetic nanoparticles (MNPs) is one of the key parameters for various applications. We have experimentally studied the effective magnetic moment of multi-core MNPs and especially their dependence on the applied magnetic field. In contrast to single-core MNPs, the situation is considerably more complex since the effective magnetic moment depends on the size of individual crystallites, their packing density, core configuration, and, thus, the interaction between them. Different approaches to determine the effective magnetic moment are applied for two multi-core MNP systems (FeraSpin XL and BNF Starch). The effective magnetic moment at low magnetic fields is obtained from the ac susceptibility spectra measured at ac field amplitudes up to 9 mT. The obtained values are compared with the high-field values measured at 7 T. In the small-field range (up to 9 mT), a linear field dependence of the effective magnetic moment is found for FeraSpin XL while the value of BNF Starch was found to be nearly constant. The low-field values of both MNP systems are smaller than the values at larger fields, proving the magnetic field dependence of the effective magnetic moment of multi-core MNPs. The estimated values are discussed within a simple model. A consistent picture was found for BNF Starch while the model fails for FeraSpin XL. The different behaviors of both studied multi-core MNP systems are related to the magnetic interactions between the individual crystallites inside a multi-core structure, which are strong for FeraSpin XL due to the high packing density of the crystallites.

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