Magnetic properties of spinel ferrite based nanoparticle systems

DINO FIORANI; KALLIOPI N. TROHIDOU; MARIANNA VASILAKAKI; S. G. MARGARIS; R. MATHIEU; GABRIEL LAVORATO; ELIN WINKLER; ROBERTO ZYSLER; ENIO LIMA JR.; HORACIO TROIANI; DINA TOBIA; ELISABETTA AGOSTINELLI; D. PEDDIS

Paris

Conferencia; 4th Internationa Conference on Nanoscience, Nanotechnology ans Nanobiotechnology; 2017

SÉMINAIRES INTERNATIONAUX DE RECHERCHE DE SORBONNE UNIVERSITÉS

The magnetic properties of an assembly of nanoparticles are determined by the interplay between intra-particle (size, shape, surface and interface coupling in bimagnetic systems) and inter-particle effects.Spinel ferrites nanoparticles are particularly attractive in this context as they provide a further tool to tune the anisotropy by the control (trough the composition, particle size, and preparation method) of the cation distribution between the tetrahedral and octahedral sublattices.In this work, we have investigated the magnetic properties of two different types of nanoparticle systems: 1) a dense system of MnFe2O4 nanoparticles, characterized by strong dipole-dipole interactions and exchange interactions between the spins at the surface of neighbouring particles in contact; 2) core/shell nanoparticle systems, ZnO/CoFe2O4 and CoO/CoFe2O4,where surface and interface exchange coupling effects provide a dominant contribution to the resulting magnetic anisotropy.The main features of the MnFe2O4 nanoparticles ( = 2nm ) are a superspin glass (SSG) behaviour (Tg = 45 K) and the Exchange Bias (EB) effect (below 25 K), due to interface exchange coupling between the ferrimagnetic core and the disordered surface shell. The static and dynamical equilibrium properties of the SSG state and the temperature dependence of the EB exchange field were satisfactorily reproduced by Monte Carlo simulations (1) accounting for surface effects, intra-particle and inter-particle interactions. A mesoscopic model was used, based on the reduction of the amount of simulated spins to the minimum necessary to describe the structure of the core/surface particles.The core/shell particles ( core = 4 nm; shell thickness: 2 nm) ZnO/CoFe2O4 and CoO/CoFe2O4 showed very different coercive field values: at 5 K, Hc = 8 kOe and 28 kOe, respectively (2). The much larger value of Hc for CoO/CoFe2O4 comes from the interface exchange coupling between the antiferromagnetic core and the ferrimagnetic shell.1) M. Vasilakaki, G. Margaris, D. Peddis, R.Mathieu, N.Yaacoub, D. Fiorani and K. Trohidou, submitted to Physical Review B2) G. Lavorato, D. Peddis, E. Lima, Jr, H.E. Troiani, E. Agostinelli, D. Fiorani, R. Zysler and E. L. Winkler, J. Phys. Chemistry C 119, 15755 (2015)