Aggregates of monodisperse iron-based nanoparticles were investigated by small-angle neutron scattering. The field dependence of the scattering intensity showed marked differences for particles depending on size and degree of oxidation. The angular dependence of the intensity indicated magnetic regions within an oxidized sample with spins perpendicular to the applied field, which dominated the scattering at the diffraction peak. The unexpected results are interpreted in terms of an iron core that is exchange coupled to an iron oxide shell.

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