This paper concerns with giant magnetoresistance (MR) effects in organic spin valves, which are realized as layered (La,Sr)MnO3 (LSMO)-based junctions with tris-(8, hydroxyquinoline) aluminum (Alq3)-spacer and ferromagnetic top layers. The experimental work was focused on the understanding of the transport behavior in this type of magnetic switching elements. The device preparation was carried out in an ultrahigh vacuum chamber equipped with a mask changer by evaporation and sputtering on SrTiO3 substrates with LSMO stripes deposited by pulsed laser technique. The field and temperature dependences of the MR of the prepared elements are studied. Spin-valve effects at 4.2K have been observed in a broad resistance interval from 50Ω to MΩ range, however, without systematic dependence on spacer layer thickness and device area. In some samples, the MR changes sign as a function of the bias voltage. The observed similarity in the bias voltages dependences of the MR in comparison with conventional magnetic tunnel junctions with oxide barriers suggests a description of the found effects within the classical tunneling concept. This assumption is also confirmed by a similar switching behavior observed on ferromagnetically contacted carbon nanotube devices. The proposed model implies the realization of the transport via local Co chains embedded in the Alq3 layer and spin dependent tunneling over barriers at the interface Co grains∕Alq3∕LSMO. The existence of conducting Co chains within the organics is supported by transmission electron microscopic∕electron energy loss spectroscopic studies on cross-sectional samples from analogous layer stacks.

as compared to the dielectrics. [1][2][3] The enhanced nonlinear optical properties of metals are extensively explored either as metal-dielectric (MD) nanocomposites or plasmonic structures. [4][5][6][7][8][9][10][11] The inherent properties of strong resonant absorption and considerable local-field enhancement of metals are observed due to huge optical polarization associated with the free electron oscillations. These MD composites show many interesting applications in ultrafast and nonlinear photonic domains. However, accessing of optical activities from metals or metal composites in the visible region is rather difficult due to thickness restricted high optical attenuation. All dielectric 1D photonic crystals become attractive for enhanced optical field confinement, thus produce several orders of magnitude changes in both linear and nonlinear optical properties. [12,13] Unlike dielectric-dielectric wavelength ordered multilayers, MD multilayer shows strong optical response due to high refractive index contrast between the metal and dielectric layers. [14][15][16] Therefore, a transparent metal can be realized when thin metal layer is sandwiched between the wavelength-ordered dielectric layers to make 1D photonic bandgap structure. [17][18][19][20] Novelty of these MD structure is that the total amount of metal is increased to several times larger than the skin depths in net thickness and still remains transparent for optical pulse propagation. In such Bragg photonic structures, the resonant Fabry-Perrot (FP) cavity modes produce a tailor-made photonic stopband and the transmission split into several minibands on both sides of the stopband. [17,21] As a result, optical fields can easily be propagated and manipulated to much deeper of these structures than the normal skin depth restricted bulk metals in both resonant and nonresonant optical regions. [14,22] Thus, the propagation of high intense light can be hugely altered in such MD structures and can be designed for high optical nonlinearities, while retaining the photonic transmission/reflection features similar to dielectric photonic structures. [23][24][25][26][27] Such novel MD structures have many exciting applications such as optical switching, laser optical limiters for human eye and sensor protection, and transparent conducting device technology. [28,29] The nonlinear enhancement of these 1D photonic bandgap structures is generally explained in the context The giant nonlinear optical responses of photonic minibands of (Ag/SiO 2 ) 4 metal-dielectric multilayers are reported using high intense femtosecond laser pulses. Ag and SiO 2 alternative stack of layers form a series of coupled Fabry-Pérot resonators (Ag-SiO 2 -Ag) and the cavity modes are split into transmission minibands in the metal reflective spectral region. The strong saturation of two-photon absorption associated with multiphoton absorption (MPA) is observed at photonic miniband minimum (≈700 nm), whereas MPA is the strong dominant nonlinearity at peak maximum (≈725 nm). The metal-cavity indu...

We have determined the energy level alignment at interfaces between La0.7Sr0.3MnO3 and two typical organic semiconductors, copper-phthalocyanine and α-sexithiophene. La0.7Sr0.3MnO3 thin films have been grown using pulsed laser deposition and subsequently ex situ cleaned before the organic materials have been deposited. This procedure is often applied in the fabrication of organic devices. We show that under these conditions the interfaces are free from chemical interaction and characterized by a short range interface dipole and large charge injection barriers.

The processes for obtaining crystalline and smooth poly(vinylidene fluoride) (PVDF) thin films using 2-butanone solvent are explored. The in-situ substrate temperature has been systematically controlled to observe the crystallization process. The in-situ substrate temperature is manipulated to control the rate of evaporation of 2-butanone solvent and is found to have played a vital role in the crystallization of PVDF thin films. Further, X-ray diffraction and Raman microscope were utilized to understand the crystalline phase of PDVF thin films, while atomic force microscopy and scanning electron microscopy have been utilized to investigate the surface morphology and surface roughness of the films.

The ultrafast absorption dynamics of photonic modes and enhanced features of electronic states have been demonstrated in a barium titanate (BaTiO 3 , BTO)-embedded onedimensional photonic crystal. The photonic structure has been realized as an optical microcavity, where a BTO central layer is sandwiched between two SiO 2 /TiO 2 -distributed Bragg reflectors. Angle-dependent transient absorption behavior reveals the excitedstate absorption dynamics of cavity-tuned BTO defect energies. Furthermore, the dynamic evolution of photonic minibands of both sides of the photonic cavity mode demonstrates the enormous cavity field confinement effect. The temporal evolution of nonlinear absorption dynamics is specific to cavity angle tuning and is evidenced by seven orders of two-photon absorption enhancement in BTO mid-bandgap energies. Overall, the results highlight the impact of strong optical field confinement within the BTO central layer. The simulations of spatial and angledependent localized optical field and energy deposition within the photonic structure further support the experimental findings. The enhanced features of ultrafast absorption dynamics and strong optical nonlinearities of the BTO-based active photonic structure offer a better understanding of electron−photon interaction, which paves the way for many novel nonlinear, hybrid optoelectronic, and photonic device applications.

Équipe 101 : Nanomagnétisme et électronique de spinInternational audienceSpin filtering effects in nano-pillars of Fe-MgO-Fe single crystalline magnetic tunnel junctions are explored with two different sample architectures and thin MgO barriers (thickness: 3-8 monolayers). The two architectures, with different growth and annealing conditions of the bottom electrode, allow tuning the quality of the bottom Fe/MgO interface. As a result, an interfacial resonance states (IRS) is observed or not depending on this interface quality. The IRS contribution, observed by spin polarized tunnel spectroscopy, is analyzed as a function of the MgO barrier thickness. Our experimental findings agree with theoretical predictions concerning the symmetry of the low energy (0.2 eV) interfacial resonance states: a mixture of Delta(1)-like and Delta(5)-like symmetries

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The photonic cavity-mediated precise control of femtosecond optical nonlinearity of several orders of magnitude enhancement is demonstrated in a novel nonlinear one-dimensional (1D) photonic crystal. The demonstrated photonic structure contains a highly nonlinear metal oxide, Bi 2 O 3 as a central defect layer within two SiO 2 /TiO 2 distributed Bragg reflectors. The nonlinear optical interactions of the electronic states of Bi 2 O 3 with the cavity mode and adjacent photonic minibands are closely monitored by femtosecond Gaussian laser beam propagation over a wide-range of spectral wavelengths, 350−1600 nm. Abnormal cross-over from positive (reverse saturation) nonlinear absorption (RSA, β = (+)12 × 10 −10 m W −1 ) to negative (saturation) nonlinear absorption (SA, β = (−) 11× 10 −10 m W −1 ) is witnessed when the confined optical fields are strongly coupled to the excitation laser and mid-band gap energies of Bi 2 O 3 , during effective cavity length tuning. The femtosecond laser pulse propagation at different wavelengths effectively probed the multiphoton-induced optical nonlinearities, which are distinctly different from lowand high-energy minibands compared to the cavity resonance and are manifold-enhanced relative to pristine Bi 2 O 3 . The photonic mode density-dependent pronounced two-/multiphoton absorptions are systematically analyzed with experiments and simulations. The novel photonic architecture can be utilized in optical switches, optical limiters, and ultrafast photonic device applications.