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

J. NOGUES; A. LÓPEZ-ORTEGA; D. TOBIA; E. WINKLER; S. ESTRADÉ; I.V. GOLOSOVSKY; M. ESTRADER; J. SORT; G. SALAZAR-ALVAREZ; J. ARBIOL; F. PEIRÓ; S. SURIÑACH; R.D. ZYSLER; M. BARÓ

Uppsala

Congreso; 7th International Conference on Fine Particle Magnetism (7ICFPM); 2010

Univ. de Uppsala

The experimental and theoretical interest on bi-magnetic core/shell nanoparticles is steadily increasing due to their novel properties and prospective applications. In particular, passivated ferromagnetic (FM) nanoparticles coated with the corresponding antiferromagnetic (AFM) oxide shell have been extensively investigated [1]. However, studies of core-shell nanoparticles with AFM cores are rather scarce [2]. Here we present the study of passivated AFM MnO nanoparticles. The nanoparticles 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). The room temperature passivation of the nanoparticles 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, lager 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) [3]. Finally, a considerable enhancement of the Néel temperature, TN, and the anisotropy of the MnO core for decreasing core sizes has been observed.