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Epitaxial clusters of chromium and chromium-vanadium oxides are studied by tunnel magneto-resistivity measurements, x-ray absorption spectrometry and circular magnetic circular dichroism. They turn out to carry a small magnetic moment that follows a super-paramagnetic behavior. The chromium ion contribution to this magnetization is mainly due to an original magnetic Cr

Epitaxial clusters of chromium and chromium–vanadium oxides are studied by tunnel magneto-resistivity measurements, x-ray absorption spectrometry and circular magnetic circular dichroism. They turn out to carry a small magnetic moment that follows a super-paramagnetic behavior. The chromium ion contribution to this magnetization is mainly due to an original magnetic Cr2O3-like phase, whereas usual Cr2O3 is known to be anti-ferromagnetic in the bulk. For mixed clusters, vanadium ions also contribute to the total magnetization and they are coupled to the chromium ion spins. By measuring the dichroic signal at different temperatures, we get insight into the possible spin configurations of vanadium and chromium ions: we propose that the magnetic dipoles observed in the clusters assembly could be related to ionic spins that couple at a very short range, as for instance in short one-dimensional spins chains.

International audience; The metal-insulator transition and the original magnetic phase that appear in vanadium oxide nanoclusters epitaxially embedded in an MgO matrix are studied via electron transport, magneto-resistance, and X-ray circular dichroism measurements. The metal-insulator transition temperature is observed to be in the range of 1.5-27 K. We observe, from 0.3 K to at least 125 K, a superparamagnetic behavior, whereas vanadium oxide, in its bulk phase, is reported to be anti-ferromagnetic (AF) at low temperatures. This striking feature is consistent with either ferromagnetic spin pairing or spin canting in the AF spin order along the 1D vanadium ion chains of the Magn eli phase or for the VO 2 (A)-like phase. Finally, the observed magneto-resistive effect, which reached up to 8% at low temperatures, indicates ferromagnetic behavior for some of the nanoparticles. This enables their integration in spintronic devices that could be combined with metal-insulator switching and magneto-resistive effects.

We study electron transport in an assembly of epitaxial Cr2O3 nanoparticles embedded in a MgO tunnel barrier: an unusual variation in the Coulomb blockade charging energy is observed as a function of the size of the clusters. In striking contrast to the expected increase in charging energy when decreasing the cluster size, an almost constant behavior is observed. We argue here that the spontaneous superparaelectric moment carried by the cluster core is the origin of this unusual behavior since it drives the dielectric constant in this cluster assembly. This phenomenon could be exploited to fabricate devices with single valued Coulomb blockade energy despite a statistical dispersion in the cluster size.We study electron transport in an assembly of epitaxial Cr2O3 nanoparticles embedded in a MgO tunnel barrier: an unusual variation in the Coulomb blockade charging energy is observed as a function of the size of the clusters. In striking contrast to the expected increase in charging energy when decreasing the cluster size, an almost constant behavior is observed. We argue here that the spontaneous superparaelectric moment carried by the cluster core is the origin of this unusual behavior since it drives the dielectric constant in this cluster assembly. This phenomenon could be exploited to fabricate devices with single valued Coulomb blockade energy despite a statistical dispersion in the cluster size.