Phthalocyanine based molecular spintronic devices.

Molecular spintronics is an effervescent field of research, which aims at combining spin physics and molecular nano-objects. In this article, we show that phthalocyanine molecules integrated in magnetic tunnel junctions (MTJs) can lead to magnetoresistance effects of different origins. We have investigated cobalt and manganese phthalocyanine molecule based magnetic tunnel junctions. CoPc MTJs exhibit both tunneling magnetoresistance (TMR) and tunneling anisotropic magnetoresistance (TAMR) effects of similar magnitude. However, for MnPc MTJs, a giant TAMR dominates with ratios up to ten thousands of percent. Strong features visible in the conductance suggest that spin-flip inelastic electron tunneling processes occur through the Mn atomic chain formed by the MnPc stacks. These results show that metallo-organic molecules could be used as a template to connect magnetic atomic chains or even a single magnetic atom in a solid-state device.

Hysteresis and change of transition temperature in thin films of Fe{[Me2Pyrz]3BH}2, a new sublimable spin-crossover molecule.

Thin films of the spin-crossover (SCO) molecule Fe{[Me2Pyrz]3BH}2 (Fe-pyrz) were sublimed on Si/SiO2 and quartz substrates, and their properties investigated by X-ray absorption and photoemission spectroscopies, optical absorption, atomic force microscopy, and superconducting quantum interference device. Contrary to the previously studied Fe(phen)2(NCS)2, the films are not smooth but granular. The thin films qualitatively retain the typical SCO properties of the powder sample (SCO, thermal hysteresis, soft X-ray induced excited spin-state trapping, and light induced excited spin-state trapping) but present intriguing variations even in micrometer-thick films: the transition temperature decreases when the thickness is decreased, and the hysteresis is affected. We explain this behavior in the light of recent studies focusing on the role of surface energy in the thermodynamics of the spin transition in nano-structures. In the high-spin state at room temperature, the films have a large optical gap (∼5 eV), decreasing at thickness below 50 nm, possibly due to film morphology.

Localized states in advanced dielectrics from the vantage of spin- and symmetry-polarized tunnelling across MgO.

Research on advanced materials such as multiferroic perovskites underscores promising applications, yet studies on these materials rarely address the impact of defects on the nominally expected materials property. Here, we revisit the comparatively simple oxide MgO as the model material system for spin-polarized solid-state tunnelling studies. We present a defect-mediated tunnelling potential landscape of localized states owing to explicitly identified defect species, against which we examine the bias and temperature dependence of magnetotransport. By mixing symmetry-resolved transport channels, a localized state may alter the effective barrier height for symmetry-resolved charge carriers, such that tunnelling magnetoresistance decreases most with increasing temperature when that state is addressed electrically. Thermal excitation promotes an occupancy switchover from the ground to the excited state of a defect, which impacts these magnetotransport characteristics. We thus resolve contradictions between experiment and theory in this otherwise canonical spintronics system, and propose a new perspective on defects in dielectrics.

Ultrafast angular momentum transfer in multisublattice ferrimagnets.

Femtosecond laser pulses can be used to induce ultrafast changes of the magnetization in magnetic materials. However, one of the unsolved questions is that of conservation of the total angular momentum during the ultrafast demagnetization. Here we report the ultrafast transfer of angular momentum during the first hundred femtoseconds in ferrimagnetic Co0.8Gd0.2 and Co0.74Tb0.26 films. Using time-resolved X-ray magnetic circular dichroism allowed for time-resolved determination of spin and orbital momenta for each element. We report an ultrafast quenching of the magnetocrystalline anisotropy and show that at early times the demagnetization in ferrimagnetic alloys is driven by the local transfer of angular momenta between the two exchange-coupled sublattices while the total angular momentum stays constant. In Co0.74Tb0.26 we have observed a transfer of the total angular momentum to an external bath, which is delayed by ~150 fs.

First glimpse of the soft x-ray induced excited spin-state trapping effect dynamics on spin cross-over molecules.

The dynamics of the soft x-ray induced excited spin state trapping (SOXIESST) effect of Fe(phen)2(NCS)2 (Fe-phen) powder have been investigated by x-ray absorption spectroscopy (XAS) using the total electron yield method, in a wide temperature range. The low-spin (LS) state is excited into the metastable high-spin (HS) state at a rate that depends on the intensity of the x-ray illumination it receives, and both the temperature and the intensity of the x-ray illumination will affect the maximum HS proportion that is reached. We find that the SOXIESST HS spin state transforms back to the LS state at a rate that is similar to that found for the light induced excited spin state trapping (LIESST) effect. We show that it is possible to use the SOXIESST effect in combination with the LIESST effect to investigate the influence of cooperative behavior on the dynamics of both effects. To investigate the impact of molecular cooperativity, we compare our results on Fe-phen with those obtained for Fe{[Me2Pyrz]3BH}2 (Fe-pyrz) powder, which exhibits a similar thermal transition temperature but with a hysteresis. We find that, while the time constant of the dynamic is identical for both molecules, the SOXIESST effect is less efficient at exciting the HS state in Fe-pyrz than in Fe-phen.

Direct observation of a highly spin-polarized organic spinterface at room temperature.

Organic semiconductors constitute promising candidates toward large-scale electronic circuits that are entirely spintronics-driven. Toward this goal, tunneling magnetoresistance values above 300% at low temperature suggested the presence of highly spin-polarized device interfaces. However, such spinterfaces have not been observed directly, let alone at room temperature. Thanks to experiments and theory on the model spinterface between phthalocyanine molecules and a Co single crystal surface, we clearly evidence a highly efficient spinterface. Spin-polarised direct and inverse photoemission experiments reveal a high degree of spin polarisation at room temperature at this interface. We measured a magnetic moment on the molecule's nitrogen π orbitals, which substantiates an ab-initio theoretical description of highly spin-polarised charge conduction across the interface due to differing spinterface formation mechanisms in each spin channel. We propose, through this example, a recipe to engineer simple organic-inorganic interfaces with remarkable spintronic properties that can endure well above room temperature.

Simple characterisation of a deformable mirror inside a high numerical aperture microscope using phase diversity.

We present a simple and versatile scheme for characterising amplitude and phase modulation by an active element, such as a deformable mirror, in the pupil plane of a high NA microscope. By placing a mirror in the vicinity of the focal plane of the objective and recording images of the reflected focal spot on a camera, we show that reliable measurements of the influence function of the mirror actuators in the pupil plane of the objective can be obtained using an iterative electric field retrieval algorithm. Compared to direct wavefront sensors, the proposed method allows characterisation for a variety of objectives with different NA and pupil sizes without modification of the setup, requires minimal space inside the microscope, and can be used with pulsed sources such as used for multiphoton microscopy. In order to validate our method, we compare our data to the results obtained with a Shack-Hartmann wavefront sensor, and show that comparable precision is achieved.

Impact on interface spin polarization of molecular bonding to metallic surfaces.

We have studied the repercussion of the molecular adsorption mechanism on the electronic properties of the interface between model nonmagnetic or magnetic metallic surfaces and metallo-organic phthalocyanines molecules (Pcs). Our intertwined x-ray absorption spectroscopy experiments and computational studies reveal that manganese Pc (MnPc) is physisorbed onto a Cu(001) surface and retains the electronic properties of a free molecule. On the other hand, MnPc is chemisorbed onto Co(001), leading to a dominant direct exchange interaction between the Mn molecular site and the Co substrate. By promoting an interfacial spin-polarized conduction state on the molecule, these interactions reveal an important lever to tailor the spintronic properties of hybrid organic-metallic interfaces.

Distinguishing the ultrafast dynamics of spin and orbital moments in solids.

For an isolated quantum particle, such as an electron, the orbital (L) and spin (S) magnetic moments can change provided that the total angular momentum of the particle is conserved. In condensed matter, an efficient transfer between L and S can occur owing to the spin-orbit interaction, which originates in the relativistic motion of electrons. Disentangling the absolute contributions of the orbital and spin angular momenta is challenging, however, as any transfer between the two occurs on femtosecond timescales. Here we investigate such phenomena by using ultrashort optical laser pulses to change the magnetization of a ferromagnetic film and then probe its dynamics with circularly polarized femtosecond X-ray pulses. Our measurements enable us to disentangle the spin and orbital components of the magnetic moment, revealing different dynamics for L and S. We highlight the important role played by the spin-orbit interaction in the ultrafast laser-induced demagnetization of ferromagnetic films, and show also that the magneto-crystalline anisotropy energy is an important quantity to consider in such processes. Our study provides insights into the dynamics in magnetic systems as well as perspectives for the ultrafast control of information in magnetic recording media.

Temperature dependence of the pre-edge structure in the Ti K-edge x-ray absorption spectrum of rutile.

The temperature dependence of the pre-edge features in x-ray absorption spectroscopy is reviewed. Then, the temperature dependence of the pre-edge structure at the K-edge of titanium in rutile TiO(2) is measured at low and room temperature. The first two peaks grow with temperature. The fact that these two peaks also correspond to electric quadrupole transitions is explained by a recently proposed theory.

FePt and CoPt nanoparticles co-deposited on silicon dioxide-a comparative study.

We compare CoPt and FePt nanoparticles grown under identical conditions on oxidized Si substrates by electron beam co-evaporation. Growth was performed under high vacuum conditions at substrate temperatures of 1023 K and was immediately followed by an annealing step. This process forms CoPt and FePt nanoparticles with mean diameters between ∼17 and ∼22 nm. In particular, the annealing step results in grain size enlargement for all samples and in a progressive magnetic hardening of the nanoparticles which reach maximum perpendicular coercivities of ∼6.6 kOe (for the CoPt) and ∼10.2 kOe (for the FePt nanoparticles). We show that, during this annealing step, a progressive transition towards the hard magnetic L1(0) ordered phase takes place in both materials. In contrast to FePt, CoPt nanoparticles must be annealed in order to crystallize in this phase.

Engineering of FePt nanoparticles by e-beam co-evaporation.

Fe(50)Pt(50) nanoparticles were deposited on thermally oxidized Si substrates by electron-beam co-evaporation of Fe and Pt, at substrate temperatures T(s) between 300 and 700 degrees C. The co-deposition led to the formation of drop-like, coalesced nanoparticles, chain-like structures or continuous films, the morphology being dependent on T(s) or the nominal thickness of the layer, f. The nanoparticles have a mean diameter D(p) between 3 and 45 nm, which increases with increasing f. The degree of crystallization in the ordered face centred tetragonal (fct) phase of the samples depends strongly on the growth conditions and increases with increasing T(s) and f. Nanoparticles with a higher proportion of the fct phase exhibit higher coercivity, with a maximum value of approximately 10.3 kOe (for the specimens prepared at 600 degrees C with f = 8.5 nm). Conversely, samples with a high proportion of the cubic phase are either superparamagnetic or ferromagnetically soft. The thermal annealing performed on selected samples resulted in structural transformation as well as magnetic hardening that depended on f and D(p).

Second harmonic imaging and scoring of collagen in fibrotic tissues.

We compare second harmonic generation (SHG) to histological and immunohistochemical techniques for the visualization and scoring of collagen in biological tissues. We show that SHG microscopy is highly specific for fibrillar collagens and that combined SHG and two-photon excited fluorescence (2PEF) imaging can provide simultaneous three-dimensional visualization of collagen synthesis and assembly sites in transgenic animal models expressing GFP constructs. Finally, we propose several scores for characterizing collagen accumulation based on SHG images and appropriate for different types of collagen distributions. We illustrate the sensitivity of these scores in a murine model of renal fibrosis using a morphological segmentation of the tissue based on endogenous 2PEF signals.

Damped precession of the magnetization vector of superparamagnetic nanoparticles excited by femtosecond optical pulses.

The ultrafast magnetization and electron dynamics of superparamagnetic cobalt nanoparticles, embedded in a dielectric matrix, have been investigated using femtosecond optical pulses. Our experimental approach allows us to bypass the superparamagnetic thermal fluctuations and to observe the trajectory of the magnetization vector which exhibits a strongly damped precession motion. The magnetization precession is damped faster in the superparamagnetic particles than in cobalt films or when the particle size decreases, suggesting that the damping is enhanced at the metal dielectric interface. Our observations question the gyroscopic nature of the magnetization pathway when superparamagnetic fluctuations take place as we discuss in the context of Brown's model.

Micrometer scale ex vivo multiphoton imaging of unstained arterial wall structure.

BACKGROUND: We characterize the application of multiphoton microscopy to the observation of the extracellular matrix of fresh unstained vessels. METHOD: Combined two-photon-excited fluorescence (2PEF) and second harmonic generation (SHG) imaging of large arteries reveals the architecture of elastin and collagen fibers in the vessel wall with remarkable specificity. RESULTS: We present elastin/collagen imaging in unstained rat vessels at both micrometer and whole vessel scales, and we characterize the optical properties of rat carotid artery and aorta walls. We apply this method to evidence deleterious effects of residual doses of a pesticide on the vessel wall. CONCLUSION: This study illustrates the potential of 2PEF/SHG microscopy for pharmacological studies in unlabeled arteries.

Real space trajectory of the ultrafast magnetization dynamics in ferromagnetic metals.

We have measured the real space trajectory of the ultrafast magnetization dynamics in ferromagnetic metals induced by femtosecond optical pulses. Our approach allows the observation of the initial change of the modulus and orientation of the magnetization, occurring within a few hundreds of femtoseconds, as well as its subsequent precession and damping around the effective field. The role of the magnetocrystalline anisotropy shows up in the magnetization reorientation occurring during the electron-lattice relaxation. In addition, we propose a model which takes into account the initial demagnetization in the Bloch formalism describing the magnetization dynamics.

Femtosecond spectrotemporal magneto-optics.

A new method to measure and analyze the time and spectrally resolved polarimetric response of magnetic materials is presented. It allows us to study the ultrafast magnetization dynamics of a CoPt3 ferromagnetic film. The analysis of the pump-induced rotation and ellipticity detected by a broad spectrum probe beam shows that magneto-optical signals predominantly reflect the spin dynamics in ferromagnets.

Two-photon microscopy in brain tissue: parameters influencing the imaging depth.

Light scattering by tissue limits the imaging depth of two-photon microscopy and its use for functional brain imaging in vivo. We investigate the influence of scattering on both fluorescence excitation and collection, and identify tissue and instrument parameters that limit the imaging depth in the brain. (i) In brain slices, we measured that the scattering length at lambda=800 nm is a factor 2 higher in juvenile cortical tissue (P14-P18) than in adult tissue (P90). (ii) In a detection geometry typical for in vivo imaging, we show that the collected fraction of fluorescence drops at large depths, and that it is proportional to the square of the effective angular acceptance of the detection optics. Matching the angular acceptance of the microscope to that of the objective lens can result in a gain of approximately 3 in collection efficiency at large depths (>500 microm). A low-magnification (20x), high-numerical aperture objective (0.95) further increases fluorescence collection by a factor of approximately 10 compared with a standard 60x-63x objective without compromising the resolution. This improvement should allow fluorescence measurements related to neuronal or vascular brain activity at >100 microm deeper than with standard objectives.

Odor-evoked calcium signals in dendrites of rat mitral cells.

Mitral cell dendrites do more than passively integrate and convey synaptic potentials to the soma, they release transmitter onto local interneurones to mediate recurrent and lateral inhibition. Several mechanisms may control the level of dendritic intracellular calcium ([Ca(2+)]) and define timing for dendritic release. Here we investigated in vivo, how odor controls calcium dynamics in mitral cell dendrites by combining intracellular recording and two-photon microscopy imaging of [Ca(2+)]. During odor stimulation, two types of [Ca(2+)] changes accompany membrane potential oscillations that are phase-locked with the respiratory cycle: (i) one is graded and parallels the membrane potential, even below the threshold for action potential firing; (ii) a second is transient, triggered by sodium action potentials that invade the entire dendritic tree. These results indicate that mitral cell dendritic compartments are synchronized by action potentials and suggest that the efficacy of dendritic synapses is finely tuned by odor-evoked graded changes in [Ca(2+)].

Combined scanning optical coherence and two-photon-excited fluorescence microscopy.

We demonstrate simultaneous imaging by optical coherence microscopy (OCM) and two-photon-excited (TPE) fluorescence microscopy. A mode-locked Ti:sapphire laser is focused and scanned in three dimensions through a fixed sample, generating both backscattered light and fluorescence light, which are independently detected. Both imaging modes provide rapid en-face imaging with submicrometer resolution. High-power delivery into the sample yields an OCM sensitivity in excess of 130 dB at 100-kHz pixel rates. Simultaneous imaging of cell nuclei with OCM and TPE is demonstrated in live drosophila embryos.