Exchange bias, size and proximity effects in inverted, antiferromagnetic (AFM)/ferrimagnetic (FiM), core/shell nanoparticles

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

Atlanta

Congreso; 55th Annual Conference on Magnetism and Magnetic Materials; 2010

AIP-IEEE

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/Mn3O4 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 (Mn3O4 or Fe3O4) [2,3]. Narrowly size distributed MnO/Mn3O4 nanoparticles with different core sizes (2-20 nm) and fixed shell thickness (~3 nm) were synthesized. Interestingly, it is found that the phase of the passivation shell (g-Mn2O3 or Mn3O4) depends on the size of the nanoparticles. Structural and magnetic characterizations coincide that while the smallest nanoparticles have a pure g-Mn2O3 shell, larger ones have increasing amounts of Mn3O4 [4]. This system may be considered as double inverted since it is composed of a MnO-AFM core with TN = 122 K and a Mn3O4 (or g-Mn2O3)-FiM shell with TC = 39-42 K (i.e., TC < TN, as opposed to conventional exchange biased systems). On the other hand, monodispersed 11 nm FeO/Fe3O4 particles, with easily tuneable ratio between the core diameter and the shell thickness were prepared by controlling the passivation conditions. In this case TC(Fe3O4) > TN(FeO), thus the system can be considered single inverted. 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 (Mn3O4 or FeO).  Remarkably, in the case of MnO 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) [5].