Size effects in passivated antiferromagnetic MnO nanoparticles: surface composition, exchange bias, proximity effects, enhanced Néel temperature

JOSEP NOGUÉS; ALBERTO LÓPEZ-ORTEGA; DINA TOBIA; ELIN WINKLER; SÒNIA ESTRADÉ; IGOR GOLOSOVSKY; MARTA ESTRADER; JORDI SORT; GERMAN SALAZAR-ALVAREZ; JORDI ARBIOL; FRANCESCA PEIRÓ; SANTIAGO SURIÑACH; ROBERTO ZYSLER; MARÍA DOLORS BARÓ

Uppsala, Suecia

Conferencia; 7th International Conference on Fine Particle Magnetism; 2010

The experimental and theoretical interest on bi-magnetic core/shell nanoparticles is steadilyincreasing due to their novel properties and prospective applications. In particular, passivatedferromagnetic (FM) nanoparticles coated with the corresponding antiferromagnetic (AFM) oxideshell have been extensively investigated. However, studies of core/shell nanoparticles with AFM cores are rather scarce. Here we present the study of passivated AFM MnO nanoparticles. They have been prepared by thermolysis of Mn(II) metal organic salt leading to narrowly size distributed AFM MnO nanoparticles with different sizes (5-20 nm). Their room temperature passivation leads to shells with higher Mn oxidation state and a roughly fixed thickness (~3 nm). Interestingly, it is found that the phase of the passivation shell (γ-Mn2O3 or Mn3O4) depends on the size of the nanoparticles. Structural and magnetic characterizations  coincide that while the smallest nanoparticles have a pure γ-Mn2O3 shell, larger ones have  increasing amounts of Mn3O4. The nanoparticles exhibit an “inverted”  AFM-core/ferrimagnetic(FiM)-shell structure (MnO/γ-Mn2O3 - Mn3O4). The exchange coupling at the AFM/FiM interface leads to strong exchange bias effects (e.g., large loop shift,  HE and coercivities, HC) at low temperatures which depend non-monotonically on the  AFM-core size. Remarkably, the magnetic order of the FiM shell is stable far above its TC,  exhibiting two characteristic temperatures, at T ~ 40 K TC(γ-Mn2O3 - Mn3O4)] and at T ~  120 K TN(MnO)] (i.e., a magnetic proximity effect). Finally, a considerable enhancement of  the Néel temperature, TN, and the magnetic anisotropy of the MnO core for decreasing core  sizes has also been observed.