Research in the Center for Molecular Spintronics focuses on switchable paramagnetic molecules in the active layer of spintronic devices. Center researchers have established theoretically and experimentally that magnetic bistability in molecules has important consequences for electrical transport. Bistability allows the possibility to externally control the spin state in a molecular system using temperature, pressure, light, or electric field using spin-crossover (SCO) complexes and valence tautomers (VTs). For VT complexes, the concerted impact of SCO and intramolecular electron transfer suggests large spin-dependent electronic structure effects that will give rise to rich spintronic behaviors. There is a clear possibility to develop direct electric control of the magnetic properties of molecules. Such control would lead to high speed, low power computing applications in spintronics including magnetoelectric gating necessary for an all-organic spin field-effect transistor.
Paramagnetic molecules are a new paradigm for molecular spintronics. They can be used as “spin filters,” (as outlined recently by Phase II PI Professor Mark Ratner), thus allowing spintronic effects without ferromagnetic electrodes. In this case, the α (“up”) and β (“down”) orbital conduction channels derive from the frontier molecular orbitals of the spin-filtering molecule, and a spin-polarized current results from preferential coupling of one of these spin-dependent orbitals with the band structure of the electrodes. When this effect is combined with switchable and bistable paramagnetic molecules the result is a unique methodology for enhancing, modulating, and controlling the propagation of spin-polarized electron currents.
Impact of Spin-crossover (SCO) Transition on Thin Film Charge Transport.
Figure 1. a) Schematic change in oribital filling for a typical Fe(II) SCO material; b) Scanning tunneling microscope imageshowing submolecular resolution of an bilayer SCO compound on Au(111); c) Experimental I/V curves for a thick SCO film across the transition; d)Theoretically predicted I/V curve for the same materials.
Spin-crossover complexes are those in which a metal ion possesses two states which differ in the number of unpaired electrons. Iron(II) SCO complexes are the most studied (Fig. 1a), and switch between a diamagnetic, S=0 complex and a paramagnetic S=4/2 complex as a function of temperature and sometimes upon irradiation. A known iron(II) SCO compound Fe[H2B(pz)2]2(bpy) prepared by the Shultz
group undergoes a spin transition from low-spin S=0 to high-spin S=2 at a transition temperature between 150 and 200 K. The Dougherty
group prepared ordered monolayer and bilayer films of Fe(II) SCO molecules by evaporation on Au(111) in ultrahigh vacuum (Fig. 1b). The bilayers display long-range ordering in a dimerized arrangement induced by the π orbital dispersion interactions between the bipyridine ligand groups. Experimental I-V measurements for thicker films agree with first principle density functional theory (DFT) calculations by the Buongiorno-Nardelli
group that show that the spin-crossover transition influences the charge transport characteristics of the system and that the high-spin configuration conducts better than the low-spin form (Fig. 1bc-d).
Tunneling spectroscopy and electronic transport in switchable valence tautomer films.
Figure 2. (a) Molecular structure and conceptual switching device geometry that exploits the CoIII(Cat)(SQ)(CN-py)2 <-> CoII(SQ)2(CN-py)2 VT transition; (b) Spin density (red isosurface) from DFT calculations that confirms the intramolecular charge transfer picture from the catecholate ligands to the cobalt ion in the low-to-high-spin magnetic transition a the polymeric variant of the VT compound.
Valence Tautomers are a class of switchable paramagnets that combine spin crossover with an intramolecular metal-to-ligand electron transfer. This combined effect suggests the possibility of dramatic and tunable spin-dependent transport effects. Tsui
studied films of a CoIII(Cat)(SQ)bpy complex (Shultz)
as a prototypical system that displays valence tautomerism (VT). Molecular films have been fabricated (Tsui, You)
that utilize CoIII(Cat)(SQ)bpy as the tunneling barrier between two Au- or two ferromagnetic (FM) electrodes (Fig. 2). Tunneling spectroscopy experiments on Au/VT-molecule/Au junctions (Fig. 2a) show clear resonances below the VT transition that can be activated or deactivated by light irradiation. (Fig. 3a,b) Changes in the differential conductance curves (and their first derivatives) confirm a variation in the electronic characteristics with light exposure (Fig. 3a,b).
Figure 3. (a) Temperature-dependent differential conductance (dI/dV) and (b) derivative of differential conductance (d2I/dV2) vs. bias voltage and with visible light exposure (coincident with LMCT band). The traces are shifted for clarity. The resonances are peaks/dips in d2I/dV2 (indicated by the blue arrows) and are switched-off by light (red traces in (a) and (b)), thus returning to the high temperature behavior (orange traces at 150K). (c) Theoretical density of states plot that confirms the difference in minority and majority spin density conducive to the measured spin-valve effect. (d) Spin valve effect in permalloy/VT molecule/Co trilayer structure (Fig. 2(a)) – tunneling magnetoresistance (TMR = (RAP-RP)/RP) at two temperatures.
Moreover, the resistance of the magnetic tunnel junction (permalloy/VT-molecule/Co) device (Tsui)
exhibits a spin valve effect at room temperature (Fig. 3d): note the higher resistance for antiparallel alignment between the magnetization of the two ferromagnetic layers, RAP, and the lower resistance for parallel alignment, RP, underscoring that it is possible to investigate spin-dependent states in VT molecules via spin-polarized resonant tunneling spectroscopy. These results correlate strongly with first principles DFT calculations (Buongiorno-Nardelli)
, (Fig. 3c), where a marked asymmetry in the density of states for minority and majority spins supports the spin valve effect, and demonstrate that valence tautomers can form the basis for a new type of molecular photoswitchable spintronic device
Strong Coupling of Paramagnetic Molecules to Graphene.
Figure 4. (a) STM image of FePc on epitaxial graphene: large FePc-FePc distance suggests a strong FePc-graphene interaction; (b) The HOMO and LUMO for FePc on graphene indicate significant interfacial electronic interactions; (c) Differential conductance measurement where a mixed orbital state is observed at 0.5 eV (blue arrow); (d) theoretical density of states (DOS) and its projection into the individual contribution of the molecule and the Fe center. The existence of mixed FePc and graphene states is confirmed by these results.
Graphene is one of the most promising electronic materials, due to its high carrier mobilities and its robust chemical and mechanical nature. A crucial goal in ongoing graphene research is to find ways to control its physical properties by chemical doping.
groups have demonstrated that paramagnetic iron phthalocyanine (FePc) molecules couple strongly to graphene surfaces, and can be used to control charge and spin injection in potential graphene spin transistor devices that feature graphene epitaxially grown on SiC. STM measurements (Dougherty, Rowe)
evidence the stability of FePc adsorbed on graphene in contrast to the weak bonding observed on bulk graphite, which indicates stronger bonding with graphene (Fig. 4a). Thus, graphene functionalization can be exploited to enhance spin injection and device transport properties. These results have been explained by first principles DFT calculations (Buongiorno-Nardelli) that predict a high level of orbital mixing between the molecule and the substrate (Fig. 4b). In-depth analyses of the electronic characteristics of the system based on experimental differential conductance measurements (Fig. 4c) and theoretical evaluation of the density of states of the system (Fig. 4d), confirm this physical picture.
Spin-polarized Scanning Tunneling Microscopy of Alq3.
Spin-polarized scanning tunneling microscopy (SPSTM) experiments has been developed by Dougherty
as a crucial characterization tool to advance center research. Preliminary experiments using this technique have provided insight into the role of the metal-molecule interface in spin injection into Alq3 (tris(8-hydroxyquinoxolinato)Al(III)). This organic semiconductor has been one of the most intensively-investigated spintronic materials due to multiple reports of Giant Magnetoresistive effects. Fig. 5a shows the highly disordered interface that is created in the first monolayer of Alq3 on layered antiferromagnetic Cr(001) surfaces. Figure 5b shows a spin-polarized conductance map of the clean metal surface where high and low conductances alternate between adjacent (001) terraces as their local magnetizations vary from aligned to anti-aligned with the probe tip magnetization.
Figure 5. (a) STM image of disordered monolayer of Alq3 on Cr(001); (b) Spin-polarized conductance map of Cr(001) layered antiferromagnetic surface; (c) Spin-polarized conductance map of submonolayer Alq3/Cr(001). Circle indicates a single molecule.
Figure 5c shows a spin-polarized conductance map after the depositing a submonolayer of Alq3 on Cr(001). Molecules appear as high conductance peaks on each terrace. The conductance asymmetry measured locally for more than 500 different Alq3 molecules is dramatically smaller than expected based on recent Alq3 thin film tunnel barrier measurements. This demonstrates strong interfacial spin depolarization effects that must be carefully considered in the interpretation of Alq3-based spin valve measurements.
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