The exchange bias effect has been studied in Ni/ NiO nanogranular samples prepared by mechanical milling and partial hydrogen reduction of NiO; the Ni weight fraction varied between 4% and 69%. In this procedure, coarse-grained NiO powder has been ball milled in air for 20 h and subsequently subjected to annealing in H 2 ͑at a temperature ranging between 200 and 300°C͒ to induce the formation of metallic Ni. The structural properties of the samples have been studied by x-ray diffraction, electron microscopy, and extended x-ray absorption fine structure. The magnetic properties have been extensively investigated by carrying out hysteresis loops and magnetization measurements in the 5 -300 K temperature range, in zero-field-cooling and fieldcooling conditions. The results indicate that both in the as-milled NiO powder and in the hydrogenated samples, the NiO phase is composed of nanocrystallites ͑having a mean size of ϳ20 nm, structurally and magnetically ordered͒ and of highly disordered regions. The samples with low Ni content ͑up to 15%͒ can be modeled as a collection of Ni nanoparticles ͑mean size of ϳ10 nm͒ dispersed in the NiO phase; with increasing Ni content, the Ni nanoparticles slightly increase in size and tend to arrange in agglomerates. In the Ni/ NiO samples, the exchange field depends on the Ni amount, being maximum ͑ϳ600 Oe͒, at T = 5 K, in the sample with 15% Ni. However, exchange bias is observed also in the as-milled NiO powder, despite the absence of metallic Ni. In all the samples, the exchange bias effect vanishes at ϳ200 K. We propose a mechanism for the phenomenon based on the key role of the disordered NiO component, showing a glassy magnetic character. The exchange bias effect is originated by the exchange interaction between the Ni ferromagnetic moments and the spins of the disordered NiO component ͑in the as-milled NiO powder, the existence of ferromagnetic moments has been connected to chemical inhomogeneities of the NiO phase͒. The thermal dependence of the exchange bias effect reflects the variation of the anisotropy of the NiO disordered component with temperature.

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.

The synthesis and characterization of gold nanoparticles (AuNPs-1) stabilized by a novel bifunctional thiolate organometallic complex containing Pt(II) centers, that is, trans,trans-[(CH(3)COS) (PBu(3))(2)Pt-C equivalent to C-C(6)H(4)-C(6)H(4)-C equivalent to C Pt(PBu(3))(2)(SCOCH(3))] (complex 1), has been carried out. As a comparison, gold nanoparticles stabilized with an organic thiol, allylmercaptane, that is, AuNPs-2, and self-assembled monolayers (SAMs) of both thiols were also prepared and investigated. The AuNPs-1 show a direct link between Pt(II) and Au nanoparticles through a single S bridge and are candidates for the achievement of 2D or 3D networks. The size control of the Au nanoparticles was achieved by careful control of synthesis parameters, and the hybrids were characterized by means of high-resolution transmission electron microscopy (HR-TEM) and synchrotron radiation induced X-ray photoelectron spectroscopy (SR-XPS). SR-XPS measurements allowed the assessment of the anchoring of the organic or organometallic thiols onto gold substrates as well as onto gold nanoparticles. AuNPs-2 with diameters in the range from 1.6 to 3.9 nm were obtained. AuNPs-1 with an average diameter in the range of 4.5-3.6 nm were obtained, and linkage between the nanoparticles can be envisaged with the formation of dyads supported by SR-XPS measurements. In fact, S2p core-level data indicate that both sulfur atoms of the organometallic thiol chemically interact with gold grafting vicinal natioparticles

In this paper, we present a systematic study of the effect of growth parameters on the structural and optical properties of InAs quantum dot ͑QD͒ grown under Stranski-Krastanov mode by molecular beam epitaxy. The dot density is significantly reduced from 1.9ϫ10 10 to 0.6ϫ10 10 cm Ϫ2 as the growth rate decreases from 0.075 to 0.019 ML/s, while the island size becomes larger. Correspondingly, the emission wavelength shifts to the longer side. By increasing the indium fraction in the InGaAs capping layer, the emission wavelength increases further. At indium fraction of 0.3, a ground state transition wavelength as long as 1.4 m with the excited state transition wavelength of around 1.3 m has been achieved in our dots. The optical properties of QDs with a ground state transition wavelength of 1.3 m but with different growth techniques were compared. The QDs grown with higher rate and embedded by InGaAs have a higher intensity saturation level from excitation dependent photoluminescence measurements and a smaller intensity decrease from temperature dependent measurements. Finally, single mirror light emitting diodes with a QD embedded in InGaAs have been fabricated. The quantum efficiency at room temperature is 1.3%, corresponding to a radiative efficiency of 21.5%.

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