Enhanced Magnetic Properties in Bi-Magnetic Core|Shell Nanoparticles

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

Gijón,

Simposio; 18th International Symposium on Metastable, Amorphous and Nanostructured Materials (ISMANAM 2011); 2011

ISMANAM

The experimental and theoretical interest on bi-magnetic core|shell nanoparticles, where both the core and the shell are magnetic, is steadily increasing. Although most of the studied systems consist of ferromagnetic (FM) transition metal cores and the corresponding antiferromagnetic (AFM) passivation shells, new types of “inverted” structures, with AFM cores, are also being studied. Moreover, in the search for multifunctional materials with enhanced properties a new type of core|shell systems composed of different transition metal materials, with new degrees of freedom to control the magnetic properties, are emerging. In this work we present the study of two different types of bi-magnetic systems, AFM|ferrimagnetic(FiM) and soft-FiM|hard-FiM, core|shell nanoparticles: MnO|Mn3O4 and MnFe2O4|FeMn2O4. The AFM|FiM nanoparticles were obtained by the passivation of AFM (MnO) nanoparticles. On the other hand, the soft|hard nanoparticles were synthesized by following a multi-step procedure, where preformed core nanoparticles were used as seeds for the subsequent growth of the shell. By designing the synthesis parameters both processes allow us to independently control the core diameter and the shell thickness. The systems were studied by X-ray diffraction, transmission electron microscopy, electron energy loss spectroscopy (EELS) and magnetic measurements (SQUID). The structural characterization clearly demonstrates the core|shell structure of the particles. The temperature dependence of the magnetization shows that both types of core|shell nanoparticles exhibit two magnetic transitions corresponding to the dissimilar components. The hysteresis loops reveal enhanced coercivities in both cases originating from exchange bias and spring-magnet effects, respectively. Moreover, a range of novel properties such as proximity effects, enhanced Néel and blocking temperatures or size-dependent shell phase, have been found in this type of particles.