Tunable coercivity and exchange bias in CoO@ZnxCo1‐xFe2O4 (x = 0‐1) bimagnetic core@shell nanoparticles

G.C. LAVORATO; E. LIMA JR.; H.E. TROIANI; R.D. ZYSLER; E.L. WINKLER

Gaithersburg

Conferencia; 9th International Conference on Fine Particle Magnetism (ICFPM2016); 2016

NIST

Exchange-anisotropy phenomena are still not fully understood and numerous systems where a ferrimagnetic (FiM) material is pinned by an antiferromagnetic (AFM) one are being studied in the form of lhin films and core@shell nanoparticles. In this sense, the ability to tune the exchange anisotropy by modifying systematically the composition of the FiM phase is relevant to the study of new nanoparticlebased materíals. In this work we show that the effective anisotropy and the exchange-bias of Zn5ubslÍtuted Co-ferrite spinel can be tuned by the intra-particle magnetic interactions in AFM@FiM C00 gZnxCo1-xFe204 bimagnetic nanoparticles. CoO seeds with a mean diameter of ;:::5 nm were fabricated by thermal decomposition of acetylacetonates in diphenylether assisted by oleylamine and oleic acid as surfactants and ;:::3 nm-thick Zn-Co spinel ferrites were subsequently grown on the CoO nanocrystals by a seed-mediated process_ A systematic study on the structural and magnetic properties was perfonned for a variable Zn concentration (x=O-I). At 5 K, the coercivity decreases continuously from :: 1j kOe to 7.1 kOe when increasing the Zn concentration, but the exchange-bias field shows a nonmonotonically behavior. The absence of exchange bias for CoO@CoFe204 NPs is interpreted through a high interface exchange-coupling energy that would be overcoming the anisotropy energy ofthe amiferromagnetic coreo However, su eh interface exchange-coupling energy is presumably being reduced tor Zn-!'Ubstituted Co-ferrite spinel shells, leading to exchange-biased loops as shown, for example, in Figure l. The results are discussed in the light ofthe phenomenology ofexchange-coupled magnetic malerials.