Eur. Phys. J. B (2017) 90: 77
https://doi.org/10.1140/epjb/e2017-70534-9

Regular Article

Individual-collective crossover driven by particle size in dense assemblies of superparamagnetic nanoparticles^*

¹ Laboratoire Léon Brillouin, UMR12 CEA-CNRS, 91191 Gif-sur-Yvette, France
² Institut de Chimie Moléculaire et des Matériaux d’Orsay, CNRS, Université Paris Sud 11, 91405 Orsay, France

^a e-mail: gregory.chaboussant@cea.fr

Received: 13 September 2016
Received in final form: 9 February 2017
Published online: 26 April 2017

Abstract

Prussian blue analogues (PBA) ferromagnetic nanoparticles Cs^I_xNi^II[Cr^III(CN)₆ ]_z·3(H₂O) embedded in CTA⁺ (cetyltrimethylammonium) matrix have been investigated by magnetometry and magnetic small-angle neutron scattering (SANS). Choosing particle sizes (diameter D = 4.8 and 8.6 nm) well below the single-domain radius and comparable volume fraction of particle, we show that the expected superparamagnetic regime for weakly anisotropic isolated magnetic particles is drastically affected due to the interplay of surface/volume anisotropies and dipolar interactions. For the smallest particles (D = 4.8 nm), magnetocrystalline anisotropy is enhanced by surface spins and drives the system into a regime of ferromagnetically correlated clusters characterized by a temperature-dependent magnetic correlation length L_mag which is experimentally accessible using magnetic SANS. For D = 8.6 nm particles, a superparamagnetic regime is recovered in a wide temperature range. We propose a model of interacting single-domain particles with axial anisotropy that accounts quantitatively for the observed behaviors in both magnetic regimes.