Electronic and magnetic properties of the interface between metal-quinoline molecules and cobalt

Abstract

It was recently established that spin injection from a ferromagnetic metal into an organic semiconductor depends largely on the formation of hybrid interface states. Here we investigate whether the magnetic properties of the interface between cobalt and tris(8-hydroxyquinolinato)-Al(III) (Alq 3 ), the most prominent molecular candidate for organic spin-valve devices, can be modified by substituting the aluminum atom with either gallium or indium. The electronic structure of Alq 3 , Gaq 3 , and Inq 3 and the properties of their interfaces with ferromagnetic cobalt are probed experimentally, by using different photoemission spectroscopy methods, and theoretically, through density functional theory calculations. For all cases, the results highlight the presence of spin-polarized interface states. However no striking difference between the properties of the various molecules and interfaces is observed. This is a consequence of the fact that the molecules frontier orbitals are mainly localized on the ligands and they show only a negligible contribution coming from the metal ion.

Ferromagnetic-organic interfacial states and their role on low voltage current injection in tris-8-hydroxyquinloline (Alq3) organic spin valves

“…This is in disagreement with the interpretation, by Droghetti and coworkers, of the photoemission results obtained in Ref. 15 which requires a negative FM with respect to the organic. It is not clear what the cause of this discrepancy is but one obvious difference is the UPS results are all obtained with the FM layer grown under UHV conditions and hence with no thin native oxide on the surface.…”

“…15 This trapping is also consistent with our proposed interfacial dipole mechanism, resulting from charge transfer between the FM and the organic, which can explain the observed low bias operation of OSVs. 2 There is one very important discrepancy, however.…”

“…It must be stressed that the NiFe layer was deposited first, onto a glass substrate, and the subsequent device deposition was identical for both the ITO and NiFe devices. Therefore, the HINTS are intrinsic to the FM-OSC interface, as was observed by Steil et al and Droghetti et al, 14,15 and not formed due to penetration of FM into the OSC during evaporation. The HINTS must thus be due to the coupling between the FM and the molecules in contact with it, as was suggested by Barraud et al …”

“…7,[11][12][13] Recent work by Barraud and co-workers suggests that the presence of spin-hybridization-induced polarised states in the first monolayer at the interface may play a role in charge injection, but again it is unclear how these interface states couple to the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels in the bulk to allow for efficient charge injection. 4 These spin dependent interface states were also observed using different spectroscopy methods by Steil et al and Droghetti et al 14,15 Their work demonstrated that the chemical interaction between an OSC layer, aluminium tris (8-hydroxquinoline) (Alq 3 ), and the ferromagnet (FM) cobalt, produced a pair of hybrid-interface states (HINTS) at the interface. This reaction between FM and Alq 3 has been further confirmed by Morley et al using X-ray photoelectron spectroscopy, atomic force microscopy, and Magneto-optic Kerr effect (MOKE) magnetometry.…”

Of interfaces and spin transport across nanoscale layers

“…In phenomenological models for the observed spin transport effects, the electronic structure, in particular the spin-polarization, of the MSC/FM interfaces plays a pivotal role in spin injection into the MSC [132]. This has prompted the suggestion that highly spin-polarized currents in spintronic devices may be obtained by exploiting such interface interactions, which has been dubbed "spinterface science" [137], and has motivated research into the role played by the interfaces [165,166,167,168,169,170,171].…”