Thickness-dependent optical band gap in one-dimensional Ca3Co2O6 nanometric films

Recent studies on the physical properties of Ca₃Co₂O₆ nanometric samples have shown that their properties are significantly different from those of the bulk samples. The origin of this change is not trivial. We have carried out optical measurements on Ca₃Co₂O₆ thin films with different thicknesses in order to characterize their electronic structure using optical spectroscopy measurements. The absorption spectra show a dependence on the film thickness that is correlated to the grain size in the polycrystalline layers. We found that the optical band gap increases from 1.3 to 1.55 eV when the thickness changes from 35 to 100 nm. The change in the band gap evolution with the film thickness is discussed in terms of both the amorphous effect and the grain size in the Ca₃Co₂O₆ thin films. Finally, we show that these results are consistent with recent measurements concerning magnetic and electrical properties of Ca₃Co₂O₆ nanometric samples.     

Thermal stability of spin valve sensors using artificial Co/Ir based ferrimagnets

Hard-soft spin valve structures have been grown by molecular beam epitaxy on MgO(0 0 1) substrates. The hard magnetic layer consists of an artificial Co/Ir/Co ferrimagnet system (AFi), while a Fe/Co bilayer from the buffer has been used as a soft detection layer. The Fe layer has been grown at 600°C giving rise to a monocrystalline layer with a BCC structure. Consequently, this layer presents a hard and a easy magnetization axis, respectively, along the BCC [1 1 0] and the [1 0 0] directions. The AFi system presents dramatic differences after successive annealing steps up to 350°C. An increase of the GMR from 3% to 3.5% is observed after annealing at 250°C while the coercive field of the AFi and the GMR plateau are strongly reduced. After further annealing at higher temperature the GMR drops down accompanied with a strong decrease in the antiparallel alignment amount in the AFi system. Rutherford back scattering measurements have been performed to investigate the changes in the interface morphology and to correlate it to the magneto-transport properties.     

Optical memory and multistep luminescence thermochromism in single crystals of K2Na[Ag(CN)2]3

Single crystals of the layered compound K2Na[Ag(CN)2]3 exhibit a dual emission with high-energy (HE) and low-energy (LE) phosphorescence bands at 313 and 402 nm, respectively. Remarkably, the crystals exhibit "optical memory", in which a new emission band with intermediate energy (IE) at 380 nm is generated upon laser irradiation (lambda ex = 266 nm) at cryogenic temperatures. The irradiated crystals reinstate their original luminescence spectrum upon heating to room temperature and then recooling. In addition to these unusual "write/read/erase" changes, the crystals also exhibit multistep luminescence thermochromism such that the LE/HE intensity ratio increases between 17 and 80 K but then decreases upon further heating. The unprecedented occurrence of both novel phenomena in one compound has been related to reversible photophysical changes instead of irreversible photochemical changes.     

Pulsed electrodeposition and magnetism of two-dimensional assembly of controlled-size Co particles on Si substrates

We present an extremely simple and inexpensive way to obtain controlled-size and density Co metallic particles on Si(1 1 1) using electrodeposition. When unpatterned substrates are used, the particle density and size can be controlled by adjusting the pulse frequency and the total deposition time. Randomly arranged cobalt particles with diameters of few tens of nanometres are obtained for short deposition times. Continuing the deposition, the particle size and density can be increased until coalescence. Magnetic force microscopy images show magnetically coupled/uncoupled particles depending on the size and distance between them. For small decoupled particles, no in-plane uniaxial anisotropy is found, in agreement with transmission electron microscopy observations which show randomly oriented single crystal particles. As the particle coalescence increases, the in-plane anisotropy evaluated from magnetization loops increases as well. When deposited on focused ion beam patterned substrates, well organized nanoparticles with adjustable magnetic anisotropy are obtained. Ferromagnetic resonance measurements performed on these samples reveal that the magnetic anisotropy originates mainly from the particle shape.     

Structural studies of lanthanide ion complexes of pure gold, pure silver and mixed metal (gold-silver) dicyanides

Crystal structures of four lanthanide complexes of La[Au(CN)2](3).3H2O, La[Ag(CN)2](3).3H2O, La[Ag(0.83)Au(0.17)(CN)2](3).3H2O, and La[Ag(0.39)Au(0.61)(CN)2](3).3H2O are reported. Studies reveal that all the structures reported are isostructural. All systems were found to be in the hexagonal crystal system, space group P6(3)/mcm. The metal-metal distance for the pure gold system is 3.332 (1) A versus 3.359(1) A for the pure silver system. The mixed-metal systems have shown no distinct differences in the location of the metal atoms, with the La[Ag(0.83)Au(0.17)(CN)2](3).3H2O complex having a metal-metal Ag-Au separation of 3.346(1) A, and 3.344(1) A for the La[Ag(0.39)Au(0.61)(CN)2](3).3H2O complex. The crystal structures of the pure and mixed La complexes have been solved to provide evidence of Ag-Au heterometallic interactions and as a basis for understanding the interesting optical properties of the systems.     

GMR enhancement in spin valves structures with nano-semiconducting layer

We report on the giant magnetoresistance enhancement in Co/Ru/Co-based spin valve structures with nano-semiconducting layer. The films were grown by ion beam sputtering on glass substrate at room temperature. The soft layer is composed of Fe/Co bilayers, while the hard layer is ensured by the Co/Ru/Co artificial antiferromagnetic subsystem (AAF) as follows: Fe_5nm/Co_0.5nm/Cu_3nm/Co_3nm/Ru_0.5nm/Co_3nm/Cu_2nm/Cr_2nm. This structure shows a giant magnetoresistance (GMR) signal of about 1.7%. To confine the electrons inside the spin valve structure, a 1.5 nm thick ZnSe semiconducting layer has been grown on the top of the AAF. This induces a strong GMR increase, up to 4%, which can be attributed to a dominant potential step at the Co/ZnSe interface.     

Multiple charge-transfer emissions from different metal-ligand pairs in ruthenium diimines

Ruthenium diimines are unique in their emissivity. Optical excitation with light of less than 500 nm leads to a strong emission in the 600-700 nm range. All emissive ruthenium complexes appear to undergo intersystem crossing from the absorptive singlet metal-to-ligand charge-transfer (MLCT) state to an emissive triplet MLCT state localized on the lowest-energy metal-ligand pair. In contrast to this currently accepted model, in which a single emissive state is populated and then equilibrates among other states based on a particular set of conditions, the excitation-wavelength dependence of the [(bpy)2RudppH]3+ emission suggests two emissive pathways. One populates an emissive MLCT state localized on a bpy-Ru pair, and the other populates a lower-energy MLCT state localized on the dpp-Ru pair.     

Two-Dimensional Antiferromagnetism in the [Mn(3+x) O(7) ][Bi(4) O(4.5-y) ] Compound with a Maple-Leaf Lattice

The modular compound [Mn(3+x) O(7) ][Bi(4) O(4.5-y) ] contains a rare example of true 2D maple-leaf lattice of edge-sharing Mn(3+/4+) . This compound displays a magnetic transition at 210?K without evidence for a Néel ordering, which indicates in-plane 2D antiferromagnetism.