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.

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.

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.

Novel Oscillation Period of the Interlayer Exchange Coupling in Fe/Cr/Fe Due to MgO Capping.

A novel period of the interlayer exchange coupling as a function of Cr thickness is observed in epitaxial Fe/Cr/Fe (001) sandwiches capped with MgO. This additional period, equal to 3 chromium atomic layers, vanishes when the capping is Cr. A strong oscillation of the magnetic coupling is also observed as a function of the thickness of the Fe layer next to the MgO capping layer. This effect is attributed to the formation of quantum well states in this Fe layer. It is believed that this confinement modifies the reflection coefficient at the Cr/Fe interface for electrons of a particular symmetry and leads to the new coupling period which is linked to the Fermi surface topology of chromium.

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).

Detection of an Alu insertion in the POMT1 gene from three French Walker Warburg syndrome families.

Walker Warburg syndrome (WWS) is the most severe of a group of multiple congenital disorders known as lissencephaly type II ( LIS Type II) associated with congenital muscular dystrophy and eye abnormalities. The POMT1 gene is the most frequently affected found in 20% of patients with WWS. We describe five fetuses with WWS in three non-related families carrying a same mutation in the POMT1 gene. All fetuses presented with tetra ventricular hydrocephaly, and arachnoidal neuroglial ectopia and cortical dysplasia characteristic of LIS type II. We performed sequencing of the POMT1 gene on fetal DNA. The five fetuses were found to share an insertion of an inversed Alu repeated DNA element within exon 3 of the POMT1 gene, all at the heterozygous state except one at the homozygous state. This mutation was associated with a common transition c.2203 C > T (p.Arg735Cys) in exon 20 on the same allele and similar intragenic haplotype, suggesting that the three families could be related or indicating a possible founder effect in France. Insertions of Alu sequences, which are rarely found in coding regions, have occasionally been reported to cause other genetic diseases. However, this is the first report of a retrotransposon insertion in the POMT1 gene associated with WWS.