Exchange bias in inverted antiferromagnetic-core/ferrimagnetic-shell nanoparticles

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

Conferencia; Trends in NanoTechnology (TNT2009); 2009

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 inverted AFM-core/FiM-shell systems (MnO/γ-Mn2O3 and FeO/Fe3O4) as opposed to the typical FM-core/AFM-shell obtained from oxidation of transition metal cores. The nanoparticles have been prepared by thermolysis of the corresponding metal organic salt leading to the AFM-core (MnO or FeO) which is passivated under air yielding to the corresponding FiM-shell (γ-Mn2O3 or Fe3O4) [2,3]. The dependence of the magnetic properties of core-shell nanoparticles as a function of the AFM core size are systematically addressed for the first time, in contrast to the archetypical FM metal-core/AFM metal oxide-shell configuration where the magnetic properties are usually studied as a function of the FM size.Narrowly size distributed MnO/Mn3O4 nanoparticles with different core sizes (2-20 nm) and fixed shell thickness (∼3 nm) were synthesized. This system may be considered as double inverted since it is composed of a MnO-AFM core with TN = 122 K and a γ-Mn2O3-FiM shell with TC = 39 K (i.e., TC < TN, as opposed to conventional exchange biased systems). On the other hand, monodispersed 11nm FeO|Fe3O4 particles (Figure 1), with easily tuneable ratio between the core diameter and the shell thickness wereprepared by controlling the passivation conditions. In this case TC(Fe3O4) > TN(FeO), thus the system can be considered single inverted. The samples were characterized by means of X-ray and neutron diffraction, transmission electron microscopy, electron energy loss spectroscopy and magnetic measurements.In both systems the coupling at the AFM|FiM interface leads to strong exchange bias effects (e.g., large loop shift, HE and coercivities, HC) at low temperatures. Moreover, the magnetic properties depend in a complex way on the core and shell sizes. Interestingly, in both systems the temperature dependence of the exchange bias properties is mainly controlled by the counterpart with lowest critical temperature (γ-Mn2O3 or FeO).