Co_xMn_1-xFe₂O₄ nanoferrites with tailored size and magnetism were produced by one-pot coprecipitation. Selection of Co(II):Mn(II) ratio and alkaline agent allows precise control of the material properties.

Iron-carbon nanocomposites have been elaborated by means of a simple chemical procedure based on in situ synthesis of iron nanoparticles within the nanopores of an activated carbon. The Fe nanoparticles present a broad particle-size distribution (5\char21{}40 nm). A combined structural and magnetic study seems to suggest that most of the nanoparticles of mean size $\sim${}15$\hskip0.3em${}nm have exotic ``onionlike'' core-shell morphology of $\gamma${}-Fe nucleus surrounded by a concentric double shell of $\alpha${}-Fe and maghemitelike oxide. The true nature of Fe-oxide was successfully evidenced through room temperature x-ray absorption spectroscopy. The whole system does not reach a fully superparamagnetic regime even at 750 K, probably due to higher blocking temperatures for the largest nanoparticles. M\"ossbauer spectrometry indicates that low temperature para-to-antiferromagnetic transition for the $\gamma${}-Fe phase cannot be discarded. In addition, the external Fe-oxide shell exhibits spin-glass behavior giving rise to the freezing of its magnetic moments at low temperatures. Hence, we propose a competing double magnetic coupling: (i) the oxide shell/$\alpha${}-Fe interaction and (ii) the possible antiferromagnetic coupling between $\gamma${}-Fe nucleus and $\alpha${}-Fe layer; as being both responsible for the observed exchange bias effect at T=5$\hskip0.3em${}K (Hex$\approx${}150$\hskip0.3em${}Oe).

The interest in magnetic nanostructures exhibiting perpendicular magnetic anisotropy and exchange bias (EB) effect has increased in recent years owing to their applications in a new generation of spintronic devices that combine several functionalities. We present a nanofabrication process used to induce a significant out-of-plane component of the magnetic easy axis and EB. In this study, 30 nm thick CoO/Co multilayers were deposited on nanostructured alumina templates with a broad range of pore diameters, 34 nm ≤ D_p ≤ 96 nm, maintaining the hexagonal lattice parameter at 107 nm. Increase of the exchange bias field (H_EB) and the coercivity (H_C) (12 times and 27 times, respectively) was observed in the nanostructured films compared to the non-patterned film. The marked dependence of H_EB and H_C with antidot hole diameters pinpoints an in-plane to out-of-plane changeover of the magnetic anisotropy at a nanohole diameter of ∼75 nm. Micromagnetic simulation shows the existence of antiferromagnetic layers that generate an exceptional magnetic configuration around the holes, named as antivortex-state. This configuration induces extra high-energy superdomain walls for edge-to-edge distance >27 nm and high-energy stripe magnetic domains below 27 nm, which could play an important role in the change of the magnetic easy axis towards the perpendicular direction.