We present a study of the magnetic properties of oxidized Co nanoparticles with an average grain size of 3nm, embedded in an amorphous Al2O3 matrix. These nanoparticles can be considered as imperfect Co-core CoO-shell systems. Magnetization measurements after magnetic field cooling show a vertical shift of the hysteresis loop, while no exchange bias is observed. With a simple model, we show that there is a critical grain size for hybrid ferromagnetic-antiferromagnetic particles, below which exchange bias is absent for any ratio of ferromagnetic and antiferromagnetic constituents. The reason is that the interfacial exchange energy dominates over other energies in the system due to a large surface-to-volume ratio in the nanoparticles.

Analysis of published data gathered on a sample of Na(2)IrO(3), held deep inside the antiferromagnetic phase at 1.58 K, shows that iridium magnetic dipole moments, measured in resonant x-ray Bragg diffraction, lie in the a-c plane of the monoclinic crystal and enclose an angle ≈118° with the c-axis. These findings, together with bulk measurements, are united in a plausible magnetic ground state for an iridium ion constructed from a Kramers doublet. A magnetic space group, derived from the chemical space group C2/m (unique axis b), possesses an anti-translation, to accommodate antiferromagnetic order, and an odd, two-fold axis of rotation symmetry on the b-axis, C'(2b), placing Ir magnetic dipoles perpendicular to the b-axis. Anapoles (toroidal dipoles) are predicted to be likewise confined to the a-c plane, and magnetic charges forbidden.

International audienceThe exchange-bias properties of a Co/CoO/Co sandwich structure are examined. The Co/CoO bottom bilayer was obtained by oxidation of a Co layer and the top bilayer was formed by subsequent deposition of Co on top of CoO. The strength of the interfacial coupling is shown to be stronger at the bottom Co/CoO interface than at the top interface. A number of original properties characterize exchange bias of the top bilayer. This includes weak temperature dependence of the bias field, weak coercivity due to exchange bias, and lack of the training effect. These properties suggest that the antiferromagnetic moment configuration is frozen during magnetization reversal of the top Co layer

International audienceWe show that the generally accepted demagnetizing field corrections are not applicable to the broad class of modern granular magnetic materials, where magnetization reversal occurs via discrete switching of the moments of individual grains, and thus the self-demagnetizing field of a given grain is proportional to the spontaneous magnetization Ms, not to the sample magnetization M. This leads to the fact that the self-demagnetizing field must be considered as a contribution to the coercive field and that the slope of the magnetization variation is higher than the classical demagnetizing field slope. This description of demagnetization processes is confirmed by numerical modeling