Direct observation of a highly spin-polarized organic spinterface at room temperature

Abstract

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.

Figure 1. Direct and inverse photoemission reveals a high interface spin polarisation P using commonplace Co and phthalocyanine molecules.

(a) Spin-resolved difference spectra of direct (closed symbols; hν = 20 eV) and inverse (open symbols) photoemission (PE) spectroscopy at room temperature of Co/MnPc(2.6(2.0) ML for direct(inverse) PE) reveal a P~+80% at E_F. (b) The Pc thickness dependence of the direct PE signal (hν = 20 eV) reveals that Pc-induced intensity at low binding energies is essentially confined to the interface. (c) Spin-resolved difference spectra of direct PE spectroscopy at room temperature of Co(3 ML)/MnPc(2.6 ML) for hν = 100 eV show no sign of any Pc-induced interface state, indicating that the interface states are mainly of C or N 2p character.

Figure 2. The formation and properties of the Co/MnPc spinterface reflect distinct mechanisms in each spin channel.

As the distance between molecule and the Co surface is reduced from (a) 6.6 Å to (b) 3.5 Å and to (c) the final position of 2.1 Å, p-d hybridization with the Co spin ↓ band causes energetically sharp, spin ↓ MOs in the z-DOS to disperse (red area of panel d), leading to a monotonous spin-↓ z-DOS (black) at/near E_F (right-hand graph of panel c). In the spin ↑ channel at the vicinity of E_F, there are neither Co d band states nor MOs but simply Co surface states (panel a) that begin to hybridize as the molecule is brought closer in (panel b) and lead, at the final molecular position (panel c), to energetically sharp peaks that cross E_F. These surface-induced spinterface states (SISS) carry virtually no Co s-character (gray datasets in panels a,b,c) and involve all atomic species of the molecule (panel e). The spinterface's planar DOS (pl-DOS; magenta) near E_F is mostly featureless and adopts the spin polarization of Co (right- and left-hand graphs of panel c).

Figure 3. The Co/MnPc spinterface as a highly spin-polarised current source.

(a) Adsorption geometry of MnPc on Co(001). The spin ↑ and ↓ z-DOS within E_F−25 meV < E < E_F+25 meV : (b) SISS (BISS) lead to a sharp (monotonous) energy dependence at E_F; and (c–d) spatial charge density maps, taken along the dashed line of panel (a), show how the numerous C and N sites of MnPc exhibit a highly spin-polarised density of states at E_F that, furthermore, are hybridised with Co states and thus contribute to conduction. The maps are in umirs of e. Å⁻³.

Figure 4. Magnetic moments induced through direct exchange onto the molecular sites provide a signature of the Co/MnPc spinterface.

(a) Top view of the on-site magnetisation density of the MnPc molecule adsorbed onto Co. While the pyrrole cage around Mn is ferromagnetically coupled to Co (F, red), that of the C-based benzene rings is mostly coupled antiferromagnetically (AF, blue) to Co. x-ray magnetic circular dichroic spectra acquired for H = 5 T and a 45° angle of photon incidence to the sample surface reveal a magnetic polarisation of the N π states of MnPc for (b) Co/MnPc(0.5 ML) at T = 300 K but not (c) Cu/MnPc(1.2 ML) even at T = 8 K. This confirms that the z-DOS of N just above E_F is spin-polarised. The slight energy shift of the N edge onset when going from Cu to Co reflects an increase in chemisorption strength.