Size Dependent Magnetism in FeO/Fe3O4 Core/Shell Nanoparticles

ALEJANDRO G. ROCA; MARTA ESTRADER; ALBERTO LÓPEZ-ORTEGA; SÒNIA ESTRADÉ; GERMAN SALAZAR-ALVAREZ; DINA TOBIA; ELIN WINKLER; IGOR GOLOSOVSKY; W. A. MACEDO; MARIANNA VASILAKAKI

Barcelona

Conferencia; 20th International Conference on Magnetism; 2015

The proposed use of exchange bias to overcome the superparamagnetic limit triggered a renewed in bi-magnetic magnetic core/shell nanoparticles. However, most of the studies are being carried out in ferromagnetic (FM) transition metal core and the corresponding antiferromagnetic (AFM) shell, (FM/AFM systems). In this work we present the study of the structural and magnetic properties of "inverted" antiferromagnetic/ferrimagnetic (AFM/FiM) core/shell nanoparticles as opposed to the typical FM/AFM. The AFM/FiM nanoparticles were prepared by thermal decomposition of iron organic compounds leading to the AFM-core (FeO) which is passivated under air resulting in a FiM-shell (Fe3O4). Two extreme sizes of the AFM/FiM nanoparticles, small (9 nm) and large (45 nm), are investigated. The results show that while both systems exhibit some similar features, like exchange bias, there are also important differences. The detailed characterization by neutron diffraction, Mössbauer, and electron spin resonance evidences that while the core in the large particles is antiferromagnetic and has a more bulk-like behavior, in the smaller ones the core is highly non-stoichiometric and strained, displaying no significant antiferromagnetism. Namely, while the large particles exhibit clear a transition at the Néel temperature, (TN) of the core (with an enhanced TN = 250 K) and a clear Verwey transition of the shell (despite its reduced thickness), the core in the small ones seems to be non-magnetic and the shell does not exhibit any Verwey transition. The results evidence that the spread of magnetic results in the literature for FeO/Fe3O4 nanoparticles stem from the strong size effects exhibited by this system and highlight the importance of ample characterization to fully understand the properties of nanostructured metal oxides.