Maite Insausti

Chemically Induced Permanent Magnetism in Au, Ag, and Cu Nanoparticles: Localization of the Magnetism by Element Selective Techniques

We report a direct observation of the intrinsic magnetization behavior of Au in thiol-capped gold nanoparticles with permanent magnetism at room temperature. Two element specific techniques have been used for this purpose: X-ray magnetic circular dichroism on the L edges of the Au and 197Au Mössbauer spectroscopy. Besides, we show that silver and copper nanoparticles synthesized by the same chemical procedure also present room-temperature permanent magnetism. The observed permanent magnetism at room temperature in Ag and Cu dodecanethiol-capped nanoparticles proves that the physical mechanisms associated to this magnetization process can be extended to more elements, opening the way to new and still not-discovered applications and to new possibilities to research basic questions of magnetism.

Influence of Fe in giant magnetoresistance ratio and magnetic properties of La0.7Ca0.3Mn1−xFexO3 perovskite type compounds

In this work polycrystalline perovskite with composition La0.7Ca0.3Fe0.05Mn0.95O3 has been produced by standard ceramic method. A considerable change occurs in magnetic, transport and magnetoresistance properties with respect to the classic composition without Fe. The introduction of a different metal, Fe, in Mn-O layer causes a decrease of about 50 K in the value of TC. In the same way, a decrease of about 10–15% in the average magnetic moment measured at 1 T is also observed. On the other hand, the introduction of Fe does not cause any appreciable change in the value of the lattice parameter. This new compound presents 60% of giant magnetoresistance ratio at 200 K. Magnetization vs. temperature measurements in zero field cooling and field cooling show clear differences at applied fields below 80 kA m−1. Such a behavior, also observed in La0.7Ca0.3MnO3 sample, is not recognizable as a simple ferromagnetic one. A qualitative discussion of the effect in the magnetic and transport properties of these compou...

Spectroscopic and magnetic properties of copper(II) complexes derived from pyridine-2-carbaldehyde thiosemicarbazone. Structures of [Cu(NO3)(C7H8N4S)(H2O)](NO3) and [{Cu(NCS)(C7H7N4S)}2]

Abstract Complexes with the formula CuX(L) (X=N3 1, NCO 2 and NCS 3) and [Cu(NO3)(HL)(H2O)](NO3) 4, where HL=C7H8N4S, (pyridine-2-carbaldehyde thiosemicarbazone), have been characterised. Single-crystal X-ray diffraction studies on compounds 3 and 4 have been carried out. The structure of compound 4 consists of monomeric distorted square pyramidal copper(II) species. The copper(II) ions are coordinated to the NNS atoms from the tridentate thiosemicarbazone ligand and one oxygen atom of a nitrate group in the equatorial position. The oxygen atom of the water molecule occupies the apical position. The structure of compound 3 consists of non-centrosymmetric {Cu2(μ-SR)2} entities in which the copper(II) ions exhibit five-coordinate square–pyramidal geometry. The thiosemicarbazone ligand and one nitrogen atom from the thiocyanate ion are in a basal position. The sulfur atom of the tridentate ligand acts as a bridge occupying the apical position. Structural and spectroscopic results suggest the presence of relevant σ ligand-to-metal charge transfer and metal-to-ligand π-backdonation character in these compounds. The ESR spectra of compounds 3 and 4 show rhombic symmetry. For complexes 1 and 2 the ESR spectra exhibit axial signals. Magnetic measurements on compounds 1, 2 and 3 show antiferromagnetic couplings. The susceptibility data were fitted by the Bleaney–Bowers’ equation for copper(II) dimers. The obtained J/k values are −4.22, −6.10 and −7.33 K for compounds 1, 2 and 3, respectively.

Functionalized Fe3O4@Au superparamagnetic nanoparticles: in vitro bioactivity

The interaction of nanoparticles with cells has been a focus of interest during the past decade. We report the fabrication and characterization of hydrosoluble Fe₃O₄@Au nanoparticles functionalized with biocompatible and fluorescent molecules and their interaction with cell cultures by visualizing them with confocal microscopy. Gold covered iron oxide nanoparticles were synthesized by reducing metal salts in the presence of oleylamine and oleic acid. The functionalization of these particles with an amphiphilic polymer provides a water soluble corona as well as the possibility to incorporate different molecules relevant for bio-applications such as poly(ethylene glycol), glucose or a cadaverine derived dye. The particle size, and the presence of polymer layers and conjugated molecules were characterized and confirmed by transmission electron microscopy, thermogravimetric measurements and infrared spectroscopy. A complete magnetic study was performed, showing that gold provides an optimum coating, which enhances the superparamagnetic behaviour observed above 10-15 K in this kind of nanoparticle. The interaction with cells and the cytotoxicity of the Fe₃O₄@Au preparations were determined upon incubation with the HeLa cell line. These nanoparticles showed no cytotoxicity when evaluated by the MTT assay and it was demonstrated that nanoparticles clearly interacted with the cells, showing a higher level of accumulation in the cells for glucose conjugated nanoparticles.

Preparation and characterisation of Cu-Co heterogeneous alloys by potentiostatic electrodeposition

Abstract Electrodeposition in the potentiostatic mode has been used to produce Cu–Co metastable alloys. The influence of different plating parameters, such as bath composition, working electrode voltage (vs. reference) and intensity of agitation, on the composition and properties of the deposit has been investigated. A broad range of compositions (2–26 at.% Co) are obtainable. In particular, the Co content in the samples is determined mainly by the Co concentration in the bath and the electrode voltage applied during the deposition. The lack of agitation produces a deposition with a higher Co content. Experimentally obtained magnetic loops have been fitted successfully taking into account a superparamagnetic cluster size distribution. From these fits we conclude that the as deposited samples consist of superparamagnetic clusters and diluted Co atoms which do not contribute to magnetisation. These results demonstrate that electrodeposition is a promising and inexpensive preparation method for obtaining Cu–Co heterogeneous alloys with controllable and adequate properties for technological applications.

Structural phase transitions in the ordered double perovskite Sr2MnTeO6

The crystal structure of the ordered double perovskite Sr2MnTeO6 has been refined at ambient temperature from high resolution neutron and x-ray powder diffraction data in the monoclinic space group P 121/n 1 with a = 5.7009(1) A, b = 5.6770(1) A, c = 8.0334(1) A and β = 90.085(1)°. This represents a combination of in-phase (+) and out-of-phase (−) rotations of virtually undistorted MnO6 and TeO6 octahedra in the (−−+) sense about the axes of the ideal cubic perovskite. High temperature x-ray powder diffraction shows three structural phase transitions at approximately 250, 550 and 675 °C, each corresponding to the disappearance of rotations about one of these axes. The first transition was analysed by differential scanning calorimetry and showed a thermal hysteresis with an enthalpy of 0.55 J g−1. We propose the () sequence of structural transitions which has not been previously reported for a double perovskite oxide.

Magnetic and transport properties of Pb perovskites and Fe containing giant magnetoresistance perovskites

The magnetic and magnetoresistive properties of the Ln0.7Pb0.3Mn1−xFexO3 (Ln=La, Nd, and x=0, 0.1) perovskites, prepared by the solgel low temperature method, have been studied. In all cases the exact stoichiometry is different from the nominal one probably due to the presence of cation vacancies. All the phases exhibit ferromagnetic behavior with Tc values ranging form 345 K observed for La0.7Pb0.3MnO3 to 75 K for Nd0.7Pb0.3Mn0.9Fe0.1O3. The 10% Fe contribution lowers the Tc by about 130 K in the La compound and 90 K in the Nd one. These results can be interpreted in terms of antiferromagnetic coupling between Mn and Fe. However, the influence of the Nd ions is not well known although it seems not ordered down to 10 K. The magnetoresistence [MR=ΔR/R(0)] measured at 6 T in pellets of pressed powder reaches values of about 80% around the magnetic transition.

Crystal structure, spectroscopic and magnetic properties of two unusual compounds: [Cu(terpy)(N3)Cl] and [{Cu0.75Ni0.25(terpy)(N3)2}2]·2H2O (terpy = 2,2′ : 6′,2″-terpyridine)

The crystal structures of two unusual complexes [Cu(terpy)(N3)Cl]1 and [(Cu0.75Ni0.25(terpy)-(N3)2}2]·2H2O 2(terpy = 2,2′:6′,2″-terpyridine) have been determined at room temperature: 1. space group P21c. a= 10.586(5), b= 8.572(1), c= 16.396(3)A, β= 100.69(3)°, Z = 4, R=R′= 0.036, for 2818 reflections with I 3σ(I); 2, space group P21/c, a= 10.195(5), b= 9.996(3), c= 15.866(8)A, β= 91.10(5)°, Z= 4, R=R′= 0.055 for 3134 reflections with I 3σ(I). Complex 1 possesses the first known co-ordination polyhedron including both halide (Cl) and pseudohalide (N3) ligands. It shows a square-pyramidal topology for the copper(II), with the three nitrogen atoms of the terpy ligand and one terminal nitrogen atom of the azide group in the basal positions while the chloride atom is in the apical one. Compound 2 contains 25% of [{Ni(terpy)(N3)2}2]·2H2O and 75% of [{Cu-(terpy)(N3)2}2]·2H2O enclosed in the global lattice. It possesses (end-on) azide bridges and terminal azide ligands. The co-ordination polyhedron of the metal ion is a slightly distorted octahedron with the nitrogen atoms of the terpy ligand and a nitrogen atom of one of the azide bridging groups in the equatorial positions, while the nitrogen atoms of both the bridge and terminal azide groups occupy the axial positions. The pure copper phase related to compound 2 exhibits a discrete molecular topology; however, the pure nickel phase shows the same dimeric structure, which is useful for comparison. The magnetic properties of compound 2 can be explained by contributions from the isolated Cu2 and Ni2 entities, giving rise to global ferromagnetic interactions characterised by the parameters J= 20.1 cm–1, D=–12.5 cm–1, gNi= 2.26 and ρ= 0.76 corresponding to the copper proportion. The EPR measurements for both compounds reveal rhombic symmetries for the g tensors. Single-crystal EPR measurements on 2 show the presence of two magnetically different copper entities, with an antiferrodistortive ordering, in the lattice.

Synthesis, crystal structure and transport properties of a new non-stoichiometric CaVO3 +δ phase

A non-stoichiometric calcium vanadium(IV) oxide with formula CaVO3 +δ has been synthesized via the thermal decomposition of the [Ca(VO)(±-C4H2O6)(H2O)4]2 metallo–organic precursor at 1 073 K. The morphology of the samples obtained by using this method shows the influence of the transport vapour phase formed during the ligand pyrolysis. The oxide contains an excess of oxygen, δ=+0.23, which implies that the vanadium atom exhibits an oxidation state between VIV and VV. The crystal structure of CaVO3 +δ is perovskite-type with orthorhombic distortion. This distortion modifies the symmetry of the [VO6] groups as was observed from the crystal refinement. The oxide presents a transition from semiconductor to metallic character at 78 K.

Thiol-capped ferromagnetic Au nanoparticles investigated by Au L3 x-ray absorption spectroscopy

Different dodecanethiol capped Au nanoparticles (NP) with similar sizes (∼2nm) but different ferromagnetic signals at room temperature have been investigated by means of x-ray absorption spectroscopy at the Au L3-edge. The reversion of the x-ray magnetic circular dichroism signal with the change of sign of the external applied magnetic field confirms the location of the magnetism at the Au atoms. In comparison with the Au foil, all the samples present accentuated white lines at the x-ray absorption near-edge structure (XANES) indicating generation of 5d holes in the Au atoms located at surface of the NPs as consequence of a localized charge transfer from the Au surface atoms to the S atoms of the capping agent. XANES spectra reflect differences among the electronic structure of the Au NPs which are compared with the observed different macroscopic magnetic signals.