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.
Co-crystals are crystalline complexes of two or more molecules bound together in crystal lattices through noncovalent interactions. The solubility and dissolution properties of co-crystals can allow to increase the bioavailability of poorly water-soluble active pharmaceutical ingredients (APIs). It is currently believed that the co-crystallization strategy should not induce changes on the pharmacological profile of the APIs, even if it is not yet clear whether a co-crystal would be defined as a physical mixture or as a new chemical entity. In order to clarify these aspects, we chose indomethacin as guest poorly aqueous soluble molecule and compared its properties with those of its co-crystals obtained with 2-hydroxy-4-methylpyridine (co-crystal 1), 2-methoxy-5-nitroaniline (co-crystal 2), and saccharine (co-crystal 3). In particular, we performed a systematic comparison among indomethacin, its co-crystals, and their parent physical mixtures by evaluating via HPLC analysis the API dissolution profile, its ability to permeate across intestinal cell monolayers (NCM460), and its oral bioavailability in rat. The indomethacin dissolution profile was not altered by the presence of co-crystallizing agents as physical mixtures, whereas significant changes were observed by the dissolution of the co-crystals. Furthermore, there was a qualitative concordance between the API dissolution patterns and the relative oral bioavailabilities in rats. Co-crystal 1 induced a drastic decrease of the transepithelial electrical resistance (TEER) value of NCM460 cell monolayers, whereas its parent mixture did not evidence any effect. The saccharin-indomethacin mixture induced a drastic decrease of the TEER value of monolayers, whereas its parent co-crystal 3 did not induce any effects on their integrity, being anyway able to increase the permeation of indomethacin. Taken together, these results demonstrate for the first time different effects induced by co-crystals and their parent physical mixtures on a biologic system, findings that could raise serious concerns about the use of co-crystal strategy to improve API bioavailability without performing appropriate investigations.
We present the results of a study on the morphology and magnetic properties of size-selected Ni nanoparticles films grown on Si/SiOx substrates. The films were produced by deposition of preformed Ni nanoparticles, using a gas aggregation nanocluster source and an electric quadrupole mass filter. The diameter d of the produced particles ranged between 3 and 10 nm. The morphology of the films, with average thickness t varying from t = 0.5 up to t = 7nm, was studied with Atomic Force Microscopy and Scanning Electron Microscopy, combining in this way information about height and lateral topography. We observed the presence of some small aggregates made of 2 o 3 particles at the early stage of film formation, probably due to some degree of cluster diffusion on the substrate, and particle average flattening. For increasing values of t, large agglomerates are formed in the films, resulting in a porous structure. Information about the magnetic properties was obtained with Field Cooled-Zero Field Cooled (FC/ZFC) magnetization curves. We observed a reversibility-irreversibility transition at temperatures 70 K < TI < 80 K, and a significant deviation from the superparamagnetic behavior at T>TI, even for the lowest coverage studied (t = 2 nm for ZFC/FC measurements,
Ferrofluids are nanomaterials consisting of magnetic nanoparticles that are dispersed in a carrier fluid. Their physical properties, and hence their field of application are determined by intertwined compositional, structural, and magnetic characteristics, including interparticle magnetic interactions. Magnetic nanoparticles were prepared by thermal decomposition of iron(III) chloride hexahydrate (FeCl3·6H2O) in 2-pyrrolidone, and were then dispersed in two different fluids, water and polyethylene glycol 400 (PEG). A number of experimental techniques (especially, transmission electron microscopy, Mössbauer spectroscopy and superconducting quantum interference device (SQUID) magnetometry) were employed to study both the as-prepared nanoparticles and the ferrofluids. We show that, with the adopted synthesis parameters of temperature and FeCl3 relative concentration, nanoparticles are obtained that mainly consist of maghemite and present a high degree of structural disorder and strong spin canting, resulting in a low saturation magnetization (~45 emu/g). A remarkable feature is that the nanoparticles, ultimately due to the presence of 2-pyrrolidone at their surface, are arranged in nanoflower-shape structures, which are substantially stable in water and tend to disaggregate in PEG. The different arrangement of the nanoparticles in the two fluids implies a different strength of dipolar magnetic interactions, as revealed by the analysis of their magnetothermal behavior. The comparison between the magnetic heating capacities of the two ferrofluids demonstrates the possibility of tailoring the performances of the produced nanoparticles by exploiting the interplay with the carrier fluid.
The exchange bias (EB) effect has been studied in Ni/NiO nanogranular samples obtained by annealing in H2, at different temperatures (200 ⩽ Tann ⩽ 300 °C), NiO powder previously ball-milled for 20 h. Typically, the samples consist of Ni nanoparticles (mean size of 10–18 nm) embedded in a nanocrystalline NiO matrix. With increasing Tann, the Ni fraction varies from 4% up to 69%. The exchange field depends on the Ni amount, being maximum (∼600 Oe), at T = 5 K, in the sample with 15% Ni. In all the samples, the EB effect vanishes at T = 200 K. The structural features of the samples have been investigated by x-ray diffraction, electron microscopy and extended x-ray absorption fine structure and the low-temperature magneto-thermal behaviour has been thoroughly analyzed. The results show the existence of a structurally and magnetically disordered NiO component, which plays the key role in the EB mechanism.
A comprehensive description of the exchange bias phenomenon in an antiferromagnetic/ferromagnetic IrMn[10 nm] / NiFe[5 nm] continuous film and in arrays of square dots with different size (1000 nm, 500 nm and 300 nm) is presented, which elucidates the temperature dependence of the exchange field Hex and coercivity HC, in conjunction with spatial confinement effects. To achieve this goal, samples prepared by electron beam lithography and liftoff using dc-sputtering were subjected to structural investigations by electron microscopy techniques and to magnetic study, through SQUID and magneto-optic magnetometry measurements coupled to micromagnetic calculations. In particular, we have observed that at T = 300 K Hex decreases with reducing the size of the dots and it is absent in the smallest ones, whereas the opposite trend is visible at T = 10 K (Hex ~ 1140 Oe in the dots of 300 nm). The exchange bias mechanism and its thermal evolution have been explained through an exhaustive phenomenological model, which joins spatial confinement effects with other crucial items concerning the pinning antiferromagnetic phase: the magnetothermal stability of the nanograins forming the IrMn layer (mean size ~ 10 nm), assumed as essentially non-interacting from the magnetic point of view; the proven existence of a structurally disordered IrMn region at the interface between the NiFe phase and the bulk of the IrMn layer, with a magnetic glassy nature; the stabilization of a low-temperature (T < 100 K) frozen collective regime of the IrMn interfacial spins, implying the appearance of a length of magnetic correlation among them.
The trinuclear [Cu3(RCOO)4(H2TEA)2] copper(II) complexes, where RCOO(-) = 2-furoate (1), 2-methoxybenzoate (2), and 3-methoxybenzoate (3, 4), as well as dimeric species [Cu2(H2TEA)2(RCOO)2]·2H2O, have been prepared by adding triethanolamine (H3TEA) at ambient conditions to hydrated Cu(RCOO)2 salts. The newly synthesized complexes have been characterized by elemental analyses, spectroscopic techniques (IR and UV-visible), magnetic susceptibility, single crystal X-ray structure determination and theoretical calculations, using a Difference Dedicated Configuration Interaction approach for the evaluation of magnetic coupling constants. In 1 and 2, the central copper atom lies on an inversion center, while in the polymorphs 3 and 4, the three metal centers are crystallographically independent. The zero-field splitting parameters of the trimeric compounds, D and E, were derived from high-field, high-frequency electron paramagnetic resonance spectra at temperatures ranging from 3 to 290 K and were used for the interpretation of the magnetic data. It was found that the dominant interaction between the terminal and central Cu sites J12 is ferromagnetic in nature in all complexes, even though differences have been found between the symmetrical or quasi-symmetrical complexes 1-3 and non-symmetrical complex 4, while the interaction between the terminal centers, J23, is negligible.
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