Interactions and surface effects in bimagnetic CoO_core/Co0.5Ni0.5Fe2O4_shell nanoparticles

G. C. LAVORATO; E. L. WINKLER; E. LIMA JR.; D. PEDDIS; H. E. TROIANI; E. AGOSTINELLI; D. FIORANI; A. GHIRRI; D. RINALDI; R. D. ZYSLER

Barcelona

Conferencia; 20th IN TE RNATION AL CONFE REN CE ON MAGNE TISM; 2015

With the aim to investigate the surface effects on bimagnetic core/shell nanoparticles we have fabricated antiferromagnetic CoO nanoparticles encapsulated within a ferrimagnetic Co0.5Ni0.5Fe2O4 shell. The material was synthesized by seed-mediated growth high-temperature decomposition method, which leads to nanoparticles composed by ~ 2.4 nm CoO cores surrounded by a ~2 nm thick Co0.5Ni0.5Fe2O4 shell. We have analyzed the magnetic properties of two samples by ac and dc-magnetization measurements: the as-synthesized (A-sample) and the annealed in air at 573 K nanoparticles (B-sample). From the ac-susceptibility we have observed that the B-sample presents a single peak centered in the 150-200 K range, following a Vogel-Fulcher phenomenological law. On the contrary, the A-sample presents two peaks, one located in the 100-150 K range, which also follows the Vogel-Fulcher law and a low temperature maximum located at 2-5 K, which is weakly frequency sensitive. The Vogel-Fulcher dependence can be related to the progressive blocking of interacting magneticsupermoments and the fitting parameter indicates that the interactions are weaker in the assynthesized sample. The ZFC-FC measurements confirm the superparamagnetic-blocked behavior, where the mean blocking temperatures are ~64 K and ~104 K for the A and B samples, respectively. Instead, the frequency dependence of the low temperature peak of the as-synthesized sample follows a Power Law compatible with spin-glass transition. Finally, it is important to mention that the annealed sample presents an enhanced coercivity (HC) with an exchange bias field (HEB), i.e. HC~13kOe and HEB~1.5 kOe at 50 K, compared to the as synthesized sample. These results indicate that the annealed bimagnetic nanoparticles behave as an exchange coupled system while the as-synthesized system presents weaker intraparticle interactions with a lower TB. In addition, the surface of the Co0.5Ni0.5Fe2O4 shell remains magnetically disordered and thermally fluctuates and freezes into a spin-glass- like state at a much lower temperature.