Magnetic anisotropy enhancement by interface exchange coupling in bimagnetic nanoparticles

E. WINKLER; E. LIMA JR.; D. TOBIA; R.D. ZYSLER

Buenos Aires

Workshop; X Latin American Workshop on Magnetism, Magnetic Materials and their Applications (X LAW3M); 2013

LAW3M

The size reduction of magnetoelectronic devices, as magnetic storage media and permanent magnets, has motivated the search of new materials. Nevertheless, this challenge encountered the superparamagnetic limit, which imposes a lower threshold in the material grain size to maintain the thermal stability of the magnetization.Recently, it has been shown that the magnetization thermal stability could be enhanced in nanostructures with ferromagnetic (FM)/ antiferromagnetic (AFM) interface. In these compounds, by means of the exchange coupling at the interface, the antiferromagnetic phase exerts a pinning action on the magnetization of the adjacent ferromagnetic one. The possibility of tuning the magnetic anisotropy through interface exchange coupling between materials with different magnetic structures and different magnetic anisotropies has driven in the last year to the research on bimagnetic systems like nanoparticles embedded in magnetic matrix and nanoparticles with core/shell structure.In this context we present the study of two different bimagnetic systems consisting in ~1.5 nm Co nanoparticles embedded in Cr2O3 AFM particulate matrix, and ~7 nm bimagnetic CoO core/CoFe2O4 shell nanoparticles.  In the first system, the magnetic measurements have shown large irreversibility in field-cooling/zero-field-cooling magnetization curves and much larger coercivity, even up to room temperature, compared to the Co nanoparticles of similar size. Besides, the core/shell nanoparticles show a remarkable coercivity enhancement and high squareness, defined as the remanence (Mr) normalized to the saturation (MS), with respect to the single phase of ferrite counterpart. At T = 5 K the coercive field (HC) results HC = 27.8 kOe and the squareness is Mr/MS = 0.79.  The origin of the effective anisotropy enhancement observed in both systems is analyzed.