Molecular systems for spintronics

Resume : Graphene has been proposed recently to be used as a transparent and conducting electrode in oganic devices such as organic light emitting diodes [1]. It is used also as a soft top contact for connecting molecules in molecular devices [2]. Electronic properties of interfaces between graphene and molecules [3] need thus to be investigated and understood in the near future. Here, we present a study concerning transport experiments performed in devices including a thin film composed of functional molecules (bis-thienyl benzene or BTB) with thichness ranging from few nanometers to tens of nanometers inserted between two electrodes. As demonstrated previously in metal/molecule/metal junctions, the electrical response of such organic devices shows a strong rectification with negative threshold voltages of around few volts [4]. Here we show that by inserting graphene between the top metallic electrode and the molecular thin film, a drastic change in the electrical response of the device is observed as charge injection occurs more symmetrically at both positive and negative bias voltages. A temperature dependence study is also presented in order to investigate charge transport and injection mechanisms. Interestingly, we observe that pure graphene top electrodes allow to inject charges at even lower bias voltages (below 100 mV). Graphene at the bottom electrode has been also tested and preliminary results obtained in Ni/graphene/BTB/Co spintronics devices are also presented. Graphene is grown directly on the Ni electrode to passivate it and preserve its spin injection properties [5] and is used as a support for grafting the molecules. [1] S. Pang, Y. Hernandez, X. Feng, and K. Müllen, Adv. Mater. 23, 2779 (2011). [2] T. Li, J. R. Hauptmann, Z. Wei, S. Petersen, N. Bovet, T. Vosch, J. Nygård, W. Hu, Y. Liu, T. Bjørnholm, K. Nørgaard, and B. W. Laursen, Adv. Mater. 24, 1333 (2012). [3] G. Hong, Q. H. Wu, J. Ren, C. Wang, W. Zhang, and S. T. Lee, Nano Today 8, 388 (2013). [4] P. Martin, M. L. Della Rocca, A. Anthore, P. Lafarge, and J.-C. Lacroix, J. Am. Chem. Soc. 134, 154 (2012). [5] B. Dlubak, M.-B. Martin, R. S. Weatherup, H. Yang, C. Deranlot, R. Blume, R. Schloegl, A. Fert, A. Anane, S. Hofmann, P. Seneor, and J. Robertson, ACS Nano 6, 10930 (2012).

Resume : Graphene has attracted great attention in spintronics due to their high potentiality for spin polarized transport due to their small spin-orbit coupling, vanishingly small hyperfine interaction and excellent charge carrier mobility. However, despite considerable efforts in recent years, it is still challenging to efficiently manipulate the spins of the conduction electrons in graphene-based devices. Proximity contacts with magnetic oxides are of current interest from the expectation of the induced spin polarization as well as weak interactions at the graphene/magnetic oxide interfaces for efficient spin polarized injection. Here, we provide a direct evidence for the magnetic proximity effect in the junctions of single layer graphene and half-metallic manganite La0.7Sr0.3MnO3 (LSMO). In the current study we have employed the spin-polarized metastable deexcitation spectroscopy technique with extremely high surface sensitivity and successfully demonstrated that at the graphene/manganite interface, a large spin polarization is induced in the graphene pi-band together with a high thermal robustness that is independent of the chemical environment around graphene. For this purpose (001) orientation of LSMO with the c-axis out-of-plane and (110) orientation of LSMO with the c-axis in-plane were selected. In these two orientations, the termination layer of the LSMO are different that results in different chemical environments and electronic wavefunctions for the first deposited layer of graphene. This allow us to clarify whether the observed spin polarization is due to chemical interaction or due to proximity effect of the magnetic oxide. The results clearly show evidence for the magnetic proximity effect in the single layer graphene (SLG)/LSMO interface. It is demonstrated that graphene pi band in the SLG/LSMO junction is spin-polarized along the spin polarization direction of LSMO in a broad energy range below the Fermi level. The spin polarization in the vicinity of the Fermi level is significantly high in SLG/LSMO junctions different from that at the LSMO surface.

Resume : We show theoretically that the intrinsic (phonon-limited) carrier mobility in graphene nanoribbons is considerably influenced by the presence of spin-polarized edge states. In zigzag nanoribbons, when the coupling between opposite edges switches from antiferromagnetic to ferromagnetic with increasing carrier density, the current becomes spin polarized and the mean free path rises from ten nanometers to micrometers. In the ferromagnetic state, the current flows through one majority-spin channel which is ballistic over micrometers and several minority-spin channels with mean free paths as low as 1 nm. These features predicted in technology-relevant conditions could be nicely exploited in spintronic devices. Comprehensive calculations of the carrier mobility vs nanoribbon width, carrier density and chiral angle reveal the unexpectedly rich transport properties of graphene nanoribbons.

Resume : Triarylmethyls (TAMs) are stable open-shell molecules composed of three aryl rings bonded to a central methyl carbon atom, where their unpaired electron mainly resides. TAMs have played a prominent role within the research field of molecular magnetic materials and have a range of practical applications.[1] Moreover, very recently the potential of TAMs for use in molecular spintronics has been firmly established experimentally.[2] Using accurate ab initio density functional calculations we have shown that the spin localization within any TAM derivative, regardless of the chemical funtionalization and temperature, can be controlled by the torsion angles of the three aryl rings.[3] To exploit such a powerful feature, we have designed a range of chemically viable TAM-based 2D covalent organic frameworks (2D-COFs) where, in full agreement with our previous study,[3] the spin localization can be finely controlled by applying external strain. As a consequence of the strain-induced structural and spin localisation changes, other very important properties of these 2D materials (e.g. ferromagnetism, optical bandgaps, electrical conduction) can also be finely tuned in a reversible way. [1] I. Ratera and J. Veciana, Chem. Soc. Rev., 2012, 41, 303?49. [2] R. Frisenda, R. Gaudenzi, C. Franco, M. Mas-Torrent, C. Rovira, J. Veciana, I. Alcon, S. T. Bromley, E. Burzurí and H. S. J. van der Zant, Nano Lett., 2015, 15, 3109?3114. [3] I. Alcon and S. T. Bromley, RSC Adv., 2015, 5, 98593-98599.

Resume : We present a first-principles investigation of the electronic structures and magnetic properties of a double perovskite Sr2CrZrO6, by using the full-potential linearized augmented plane-wave (FP-LAPW) method based on Density Functional Theory (DFT) as implemented in the WIEN2K.The properties of double perovskite Sr2CrZrO6 were calculated using generalized gradient approximation (GGA) method. In order to take into account the strong on-site coulomb interaction that means we included the Hubbard correlation terms GGA+U approache.We have analyzed the structural parameters,band structure, total and partial densities of states. The results show a half-metallic ground state with the ferrimagnetic coupling of Cr and Zr spins. Keywords : Electronic structure, GGA+U, First-principales, Double perovskite, Half-metalic, Ferrimagnetic coupling.

Resume : 3d transition metal centered phthalocynanines are potential candidates for spintronics based on single molecule magnets. Weakly interacting 2D materials are attractive substrates for supporting these molecules for realizing functional devices. Here, we will present ab initio studies on the adsorption characteristics of iron and cobalt phthalocyanine molecules supported on 2D graphene and MoS2 substrates. Calculated structural, electronic and magnetic properties will be presented with a comparison between adsorbed and gas phase molecules. Influence of the substrate on the spin-dipole and magnetic anisotropy contribution of the transition metal centers will be shown along with the changes in work functions due to adsorption.

Resume : Spin dependent properties of molecules/ferromagnetic metal interfaces are known to be very sensitive to the nature of the interaction (chemisorption versus physisorption) between the two materials [1]. Here a two-step electrochemical procedure to covalently graft molecules over ferromagnetic surfaces like Co and Ni is presented. It is based on an in-solution electrochemical process [2] allowing to graft radicals (diazonium chemistry) over a conducting surface. The deposited molecular thin film is then very robust even against aggressive fabrication processes like photolithography. An other key aspect comes from the presence of undesired oxyde at the interface which might alter the electronic and magnetic properties of the magnetic material. This electrochemical process could easily remove oxyde by reducting it and passivate the Co film. The magnetic properties of functionalized thin magnetic films of Si//Pt (40 nm)/Co (8 nm)/nitro-benzene diazonium (NBD) have been investigated by Superconducting Quantum Interfermetry Device (SQUID) magnetometry. For a 8 nm thin Co film deposited over Pt, a fully in-plane magnetization is found where as for a functionalized 8 nm thin Co film an out-of-plane component emerges. Atomic force microscopy and X-ray photoelectron spectrocopy characterizations of the functionnalized surface is also presented. The roles of the different parameters of the procedure are finally discussed. [1] M. Galbiati, S. Tatay, C. Barraud, A. V. Dediu, F. Petroff, R. Mattana, and P. Seneor, MRS Bull. 39, 602 (2014). [2] T. Fluteau, C. Bessis, C. Barraud, M. L. Della Rocca, P. Martin, J.-C. Lacroix, and P. Lafarge, J. Appl. Phys. 116, 114509 (2014).

Resume : In dye-sensitized solar cells (DSSC) a crucial role for high photoconversion efficiency play electron transfer processes proceeding at the oxide semiconductor/dye/electrolyte interfaces. The exact nature of ultrafast electron injection and further charge carrier separation processes involving intermediate stage of electron-hole (e-h) pairs is so far not fully understood. In this work we propose a novel mechanism of photocurrent generation in DSSCs examined by magnetic field effect (MFE) technique. During dissociation processes singlet 1(e-h) and triplet 3(e-h) pairs are created where the electron occupies the conduction level of TiO2 and the hole is localized on an oxidized dye molecule. The external magnetic field of hundreds mT strength affects the intersystem crossing (ISC) between 1(e-h) and 3(e-h) pairs and this way change the generated photocurrent. We have observed that the magnitude of the small negative MFE on photocurrent in DSSCs is controlled by the radius and spin coherence time of e-h pairs which are modified by the photoanode morphology (TiO2 nanoparticles or nanotubes) and electronic orbital structure of various dyes (ruthenium N719, dinuclear ruthenium B1 and fully organic squarine SQ2 dyes). The observed MFE is explained by the different values of Lande factor for electron and hole entities forming e-h pairs (?g mechanism). This work was supported by the Polish Ministry of Science and Higher Education under ?Diamond Grant? 0228/DIA/2013/42.

Resume : The recent discovery of 2D materials has opened up novel exciting opportunities in terms of functionalities and performances for spintronics devices. One of them is to address the issue of the oxidation of ferromagnetic materials which considerably limit the use of air/wet processes in the fabrication of devices. The recent progress on the growth of graphene and h-BN by chemical vapour deposition on top of ferromagnetic catalysts like Ni[1] or Fe[2] has permitted to circumvent this issue. We will first show how a thin graphene passivation layer can prevent the oxidation of a ferromagnetic electrode of Ni, enabling its use in in novel humide/ambient low-cost processes for spintronics devices while adding a new interesting spin filtering property[3]. We will then highlight the passivation properties of a monolayer of h-BN on top of Fe and show the resulting successful integration of h-BN as a tunnel barrier in magnetic tunnel junctions[4]. These different experiments unveil promising uses of 2D materials for spintronics. [1] Weatherup et al. ACS Nano 6, 9996 (2012) [2] Caneva et al. Nanoletters 15, 1867 (2015) [3] Martin et al. ACS Nano 8(8) 7890 (2014), Appl. Phys. Lett. 107, 012408 (2015) [4] Piquemal et al. submitted

Resume : The study of the spin properties of ferromagnetic metal-organic (MO) interfaces has received considerable attention in the past, because of the prospect of developing new hybrid and multi-functional molecular devices. A number of studies could evidence in such systems the presence of interface states (IS) in the gap region of the organic semiconductor. If such IS were to show in addition a high spin polarization, they could be of great interest. In fact, highly spin-polarized IS close to the Fermi level are of utmost importance for the spin injection efficiency across ferromagnetic MO interfaces and thus for the performance of future organic spintronics devices. Such highly spin-polarized IS at the Fermi energy could indeed be observed in a very recent spin-resolved photoemission study of phthalocyanine (Pc) films on ferromagnetic Co(001). Having evidenced highly spin-polarized IS, the question is raised whether this property is confined to Pc molecules, or whether these results are indicative of a broader pattern. We deploy topographical and spectroscopic techniques to show that a strongly spin-polarized interface arises already between ferromagnetic Co and mere carbon atoms. Scanning tunneling microscopy and spectroscopy show how a dense amorphous semiconducting carbon film with a low band gap of about 0.4 eV is formed atop the metallic interface. Spin-resolved photoemission spectroscopy reveals a high degree of spin polarization at room temperature of carbon-induced states.

Resume : One very efficient route to tailor the spin properties of surfaces and interfaces, which was so far only applied to ferromagnetic metals, is the creation of customized hybrid interfaces between inorganic and organic materials. The interest in the spin properties of such systems was originally stimulated by the observation of magneto-resistive effects in spin-valve structures prepared with an organic-based spacer layer [1, 2]. Today, it is known that the performance of such organic spintronics devices is mostly determined by the spin-dependent properties of the hybrid interface formed between the organic molecules and the ferromagnetic electrodes: the so-called spinterface [3]. Crucially, the spin properties of a specific spinterface such as spin polarization, spin filtering, and coercive field can be in principle tuned by any external stimulus that will either modify the electronic structure of the organic molecules forming the spinterface, or change the strength of the interaction between the molecules and the ferromagnetic substrate. This concept was demonstrated, for example, for doping of the organic molecules with electron donors [4] and for chemical synthesis of tailored molecules [5, 6]. In this talk I will show that the extreme multi-functionality of organic molecules can be used to functionalize the spin properties of the more general class of spin-textured materials, opening new and exciting routes for controlling spin at the atomic scale [7]. In particular, I will present a rational design approach to customize the spin-texture of surface states of the prototypical topological insulator (TI) Bi2Se3 [8]. For the rational design we use theoretical calculations to guide the choice and chemical synthesis of appropriate molecules that customize the spin-texture of Bi2Se3. The theoretical predictions are then verified in angular-resolved photoemission experiments. We show that by tuning the strength of molecule-TI interaction, the surface of the TI can be passivated, the Dirac point can energetically be shifted at will, and Rashba-split quantum-well interface states can be created. These tailored interface properties ? passivation, spin-texture tuning, and creation of hybrid interface states - lay a solid foundation for interface-assisted molecular spintronics in spin-textured materials. [1] V. A. Dediu, L. E. Hueso, I. Bergenti, and C. Taliani, Nature Materials 8, 707 (2009); [2] M. Cinchetti et al., Nature Materials 8, 115 (2009); [3] S. Steil, et al., Nature Physics 9, 242 (2013); [4] M. Cinchetti, et al., Physical Review Letters 104, 217602 (2010); [5] A. Droghetti, et al., Physical Review B 89, 094412 (2014); [6] S. Müller, et al., New Journal of Physics 15, 113054 (2013); [7] M. Cinchetti, Nature Nanotechnology 9, 965 (2014); [8] S. Jakobs, A. Narayan, B. Stadtmüller, A. Droghetti, I. Rungger, Y. S. Hor, S. Klyatskaya, D. Jungkenn, J. Stöckl, M. Laux, O. L. A. Monti, M. Aeschlimann, R. J. Cava, M. Ruben, S. Mathias, S. Sanvito, and M. Cinchetti, Nano Letters 15, 6022 (2015).

Resume : Recently, there has been increasing interest in investigations of fundamental magnetic interactions in low-dimensional materials [1,2]. One such fundamental interaction is the Kondo effect responsible for the screening of the local moment [3], while another one, the Ruderman-Kittel-Kasuya-Yoshida (RKKY) coupling gives rise to a long-range spin ordering via the conduction electrons [4-6]. Spin-bearing smolecules such as porphyrins and phthalocyanines deposited on metallic substrates present a material to study the interplay of these two opposing interactions and can lead to even more intriguing phenomena. Here we present the first direct observation of long-range ferrimagnetic order in a 2-dimensional (2D) material created by the co-assembly of paramagnetic iron fluorinated (FeFPc) and manganese (MnPc) phthalocyanines molecules on single-crystalline Au(111) evidenced by Scanning Tunneling Microscopy/Spectroscopy (STM/STS), X-ray Magnetic Circular Dichroism (XMCD) and the Density Functional Theory (DFT+U) first-principle calculations. At the first glance 2D and 1D ferromagnets are in conflict with the classical theory of magnetism. [7] In the present system, however, the substrate is of key importance as it introduces an antiferromagnetic nearest-neighbor coupling between the Mn and Fe centers which is mediated by the surface states of Au(111) making the ordering possible. References [1] Zhao, A., Li, Q., Chen, L., Xiang, H., Wang, W., Pan, S., Wang, B., Xiao, X., Yang, J., Hou, J. G. & Zhu, Q. Controlling the Kondo effect of an adsorbed magnetic ion through its chemical bonding. Science 309, 1542–1544 (2005). [2] Franke, K. J., G. Schulze, G. & Pascual, J. I. Competition of superconducting phenomena and 2 Kondo screening at the nanoscale. Science 332, 940-944 (2011). [3] Hewson, A. C. The Kondo Problem to Heavy Fermions. (Cambridge University Press, 1993). [4] Wahl, P., Simon, P., Diekhöner, L., Stepanyuk, V. S., Bruno, P., Schneider, M. A. &. Kern, K. Exchange interaction between single magnetic adatoms. Phys. Rev. Lett. 98, 056601 (2007). [5] Tsukahara, N., Shiraki, S., Itou, S., Ohta, N., Takagi, N. & Kawai, M. Evolution of Kondo resonance from a single impurity molecule to the two-dimensional lattice. Phys. Rev. Lett. 106, 187201 (2011). [6] Prüser, H., Dargel, P. E., Bouhassoune, M., Ulbrich, R. G., Pruschke, T., Lounis S. & Wenderoth M. Interplay between the Kondo effect and the Ruderman–Kittel–Kasuya–Yosida interaction. Nat. Commun. 5, 5417 (2015). [7] Mermin, N. D. & Wagner, H. Absence of ferromagnetism or antiferromagnetism in one- or two-dimensional isotropic Heisenberg models. Phys. Rev. Lett. 17, 1133–1136 (1966).

Resume : Molecular spintronics and molecular magnetism are merging, leading to the development of hybrid molecular-inorganic devices where magnetic molecules plays a crucial role in spin and electronic transport. In this contest the use of Single-Molecule Magnets, the magnetic molecule by definition, but also of light element-based paramagnetic systems like organic radicals (e.g Nitronyl Nitroxide radicals) discloses new exciting possibilities linked to the richness the magnetic properties of these systems as well as to versatility of chemical synthesis that can allows to tune electronic and magnetic properties of these building blocks. However playing with molecules is not trivial: stability of the molecules as well as their interaction with the substrates must be taken in to account and fruitfully used. Here we will present our recent results based on wet chemistry and UHV-compatible strategies to modify the interface between a standard spin injecting electrode (La0.7Sr0.3MnO3) and an organic semiconductors (OSC). This allowed to introduce a molecular paramagnetic layer in the standard OSC-based spin-valve devices. Using a multi-technique approach we verified the intactness of molecules, the occurrence of specific interaction between the molecules and the magnetic substrate and then to test the performances of the hybrid spin valves demonstrating that the presence of this molecular paramagnetic layer does not hamper the observation of a significant magnetoresistance.

Resume : The combination of spin valve magnetoresistance and electrical memory effects (bistability) in hybrid devices including ferromagnetic electrodes and an organic spacer has opened up new possibilities for devices, in particular for the realization of multi-state memories [1]. It is well known that the interface effects between organic materials and electrodes are critical for setting the electric and magnetic behavior of these hybrid devices [2]. However, the literature concerning the relation between the bistable I-V characteristics and the magnetoresistance properties of organic devices is limited. In our study, we compare the magnetic and electrical behavior of magnetic tunnel junctions La0.7Sr0.3MnO3 (LSMO)/SrTiO3 (STO)/)/Co including epitaxial STO layer and of LSMO/STO/C60 (2 nm)/Co for which an organic C60 layer is inserted between STO and Co. Our results showed that both devices present typical tunneling magnetoresistance (TMR) in addition to a resistive switching behavior. In case of fully inorganic MTJ, the TMR is consistent with previous results [3]. In the case of C60 insertion, TMR ratio is tuned by electrical behavior of the device, in particular it is dependent on the resistive state in which is set. Data are interpreted in terms of interfacial effects, in particular taking into account hybridization between C60 and Co. [1] Prezioso, M. et al (2011) Adv. Mater., 23, 1371. [2] Barraud C. et al., Nat. Phys., 6, 615. [3] De Teresa J. M. et al., (1999) Science, 286, 507.

Resume : Molecular spintronics is an emerging research field at the frontier between organic chemistry and the spintronics. Compared to traditional inorganic materials molecules are flexible and can be easily tailored by chemical synthesis. However, due to their expected very long spin lifetime opportunity, they were first only seen as the ultimate media for spintronics devices and it was only very recently that new spintronics tailoring opportunities could arise from the chemical versatility brought by molecules and molecular engineering were unveiled. The hybridization between a ferromagnet and molecules induces a spin dependent broadening and energy shifting of the molecular orbitals leading to an induced spin polarization on the first molecular layer. We will show how tunneling anisotropic magnetoresistance measurements (TAMR) can be used to probe the spin dependent hybridization at the ferromagnetic/molecules interface. TAMR effects have been observed in Co/Alq3/Co and Co/Alq3/Cu organic spin valves. These experiments have shown that the TAMR effect comes from the bottom interface suggesting a stronger coupling at the bottom interface. This was confirmed by TMR experiments where a tunnel barrier has been inserted at the bottom or top interface in order to suppress the spin dependent hybridization. The study of the bias voltage dependence of the TAMR and TMR allows determining the spin dependent broadening and energy shifting of the molecular levels at the Co/Alq3 interfaces.

Resume : Spin-based electronics is one of the emerging branches in today?s nanotechnology and the most active area within nanomagnetism. So far spintronics has been based on conventional materials like inorganic metals and semiconductors. Still, an appealing possibility is that of using molecule-based materials, as components of new spintronic systems [1]. In particular, by taking advantage of a hybrid approach one can integrate molecular materials showing multifunctional properties into spintronic devices. In this talk we illustrate the use of this approach to fabricate multifunctional molecular devices combining light and spin-valve properties (i.e., Spin-OLEDs). So far only one report has been published which is based on the fabrication of an organic light emitting diode (OLED) with ferromagnetic electrodes [2]. Our new approach leads to a robust organic luminescent device in which light emission can be enhanced and modulated upon application of an external magnetic field. Furthermore, Hanle measurements [4] has been performed in our Spin-OLEDs probing for the first time no Hanle effect in a Spin-OLEDs organic spintronic.

Resume : Controlling the dynamics of spins on surfaces is pivotal to the design of spintronic and quantum computing devices. Proposed schemes involve the interaction of spins with graphene to enable surface-state spintronics, but several challenges remain unsolved: how can molecular spins be assembled into hybrid structures? What is the influence of the graphene environment on the spin? Can molecules be used to control coherent currents in graphene devices? Here we answer these questions,1 exploring spin-graphene interactions by using molecular magnetic materials. We detail the assembly process and showcase the relevance of dynamic scaling theory and of graphene surface defects. We then show that, while the static spin response remains unaltered, the quantum spin dynamics and associated selection rules are profoundly modulated. The couplings to graphene phonons, to other spins, and to Dirac fermions are quantified using a newly-developed model. Coupling to Dirac electrons introduces a dominant quantum-relaxation channel that, by driving the spins over Villain's threshold, gives rise to fully-coherent, resonant spin tunneling. Eventually we show how molecular spins can be used to introduce bistable behavior into coherent spin currents. These findings provide the tenets for spin-manipulation in graphene nanodevices.