Effect of molecular ordering on spin and charge injection in rubrene

Abstract

Studies have shown that interfaces play a crucial role for efficient spin injection and transport. Here, we address the complex role of interface in spin and charge injection into organic materials by various interfacesensitive characterization tools. Inelastic tunneling spectroscopy and polarized neutron reflectometry were mainly adopted to explore the interfaces of high mobility organic semiconductor rubrene sandwiched by two ferromagnetic electrodes. The dramatic difference in the reported magnetotransport properties and charge injection characteristics in trilayer magnetic junctions has been attributed to the different growth morphology of rubrene molecules at the interface dictated by the presence or absence of a 0.6 nm alumina seed layer. The magnetic contribution of the top ferromagnetic electrode is also influenced by the morphology of the rubrene layer underneath, directly affecting the spin injection efficiency. This work highlights the importance of interface engineering in the development of organic-based spintronics devices. DOI: 10.1103/PhysRevB.80.195212 PACS number͑s͒: 72.25.Dc, 72.25.Mk, 72.80.Le, 75.47.Ϫm Spin transport in -conjugated organic semiconductors ͑OSs͒ is necessarily a vast and complex area that is just beginning to be explored.1 Recent demonstrations of spin transport in organic materials [2][3][4][5][6] have been promising, giving impetus to explore the potential of OSs in spintronics applications. Although their study can be extremely challenging, it is expected to be rich in knowledge. For instance, unlike inorganic semiconductors, OSs have shown significantly complex charge injection and transport mechanisms. To date, several theories have dealt with the transport phenomena: modeling the strong electron-phonon coupling ͑polarons͒ and the structural disorder of the molecules to account for the observed transport characteristics in different organic systems.7-9 Growth-related structural changes in OS films have also been known to influence the transport properties 10 and, consequently, the spin injection and transport, as observed in this work. Due to the complexity associated with organic systems, appropriate characterization methods need to be implemented to understand the spin transport.In our recent work, we demonstrated a large tunneling magnetoresistance ͑TMR͒ of 6% at room temperature, showing a spin-diffusion length of 13 nm in thin amorphous rubrene films ͑C 42 H 28 ͒.11 The study was performed in the tunnel junction structure for hybrid ͑rubrene film grown on alumina as tunnel barrier͒ and rubrene ͑rubrene film grown without alumina seed layer͒ barriers. In this work, we address the role of interfaces, their influence on the growth morphology of the OS, and hence on the transport properties by performing interface-sensitive characterization by inelastic tunneling spectroscopy ͑IETS͒ and polarized neutron reflectometry ͑PNR͒ measurements. Charge and spin transport measurements were extended to thick rubrene barriers ͑Ͼ20 nm͒ to obtain information on the evolution of t...

Strong asymmetrical bias dependence of magnetoresistance in organic spin valves: the role of ferromagnetic/organic interfaces

“…Again owing to the shortest bonding and thus the strongest interfacial hybridization, the O atom shows the highest LPDOS and the largest SP around E F than the C and N atoms. Therefore, the O-Co bond should dominate the electron injection from Co to Alq 3 since the hopping integral from Co to Alq 3 is directly correlated with the hybridization strength [35].…”

Interface-assisted molecular spintronics

“…Orbital symmetry and overlap at the interface lead to a stronger interaction of the out-of-plane d-and p-orbitals creating spin-resolved hybridized p-d interface electronic states. Understanding these interactions is a complicated study and its origin depends on a number of parameters such as, not limiting to, molecular structure and morphology, 38 chemical sensitivity of the molecule, 39 surface property, image-potential, 36 and van der Waals (vdW) interaction 39 at the interface. As a general rule, one may assume that due to steric hindrance, non-planar molecules would bind less strongly with the magnetic surface than planar molecules.…”

Magnetoresistance and Spinterface of Organic Spin Valves Based on Diketopyrrolopyrrole Polymers

“…[1][2][3][4][5][6][7][8][9] This offers the possibility to retain the spin information over long distances and perform advanced spin manipulations, such as in the potential prospects of increased processing speed, reduced power consumption, and nonvolatility. [28] Actually, much more organic small molecules were applied in the OSV devices, including tris(8-hydroxyquinoline)aluminum (Alq 3 ), [21,[29][30][31][32][33] fullerene (C 60 and C 70 ), [34][35][36][37][38] rubrene, [39] pentacene, [40] bathocuproine, [41,42] and lanthanide molecules. [21,27] After that, Majumdar et al prepared polymers spin valve with LSMO/P3HT/Co structure, and analyzed that P3HT and LSMO electrodes occurred certain chemical interaction leading to a spin-selective tunneling interface.…”