Spin injection and transport in a solution-processed organic semiconductor at room temperature

S. Mooser, J. F. K. Cooper, K. K. Banger, J. Wunderlich, and H. Sirringhaus

Phys. Rev. B 85, 235202 – Published 14 June 2012

Abstract

Spin injection and transport in solution-processed 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-pentacene) are investigated using vertical CoPt/TIPS-pentacene/AlOx/Co spin valve architectures. The morphology and the molecular orientation of TIPS-pentacene spin-coated from a chloroform solution as grown in the spin valve devices are characterized by means of atomic force microscopy and x-ray diffraction, where the $π$ - $π$ stacking is found to be in planes oriented parallel to the substrate plane on CoPt (not $L 1_{0}$ -ordered). The magnetization hysteresis loops recorded with a superconducting quantum interference device show an individual switching of the magnetization of the two ferromagnetic layers. The antiparallel state of the relative orientation of CoPt and Co is achieved due to their different coercive fields. A spin valve effect is observed at room temperature. The scaling of the magnetoresistance (MR) with the bulk mobility of TIPS-pentacene as a function of temperature together with the dependence of the MR on the interlayer thickness clearly indicates spin injection and transport in TIPS-pentacene. From organic semiconductor-spacer thickness-dependent MR measurements, we estimate a spin relaxation length of TIPS-pentacene of $λ_{s} = (24 \pm 6)$ nm and a spin relaxation time of approximately $τ_{s} = 3.5$ $μ$ s at $T = 300$ K, taking the measured bulk mobility of holes into account.

Received 8 February 2012

DOI:https://doi.org/10.1103/PhysRevB.85.235202

Authors & Affiliations

S. Mooser¹, J. F. K. Cooper¹, K. K. Banger¹, J. Wunderlich^2,3, and H. Sirringhaus^1,*

¹Cavendish Laboratory, University of Cambridge, Cambridge CB3 0HE, United Kingdom
²Hitachi Cambridge Laboratory, Cambridge CB3 0HE, United Kingdom
³Institute of Physics ASCR, v.v.i., Cukrovarnickà 10, 162 53 Praha 6, Czech Republic

^*hs220@cam.ac.uk

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Vol. 85, Iss. 23 — 15 June 2012

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Images

Figure 1
Schematic of a typical spin valve device. The two ferromagnetic layers CoPt and Co are separated by the organic semiconductor TIPS-pentacene and a thin AlOx tunnel barrier. Pt and Al serve as adhesive buffer layer and capping layer, respectively.Reuse & Permissions
Figure 2
(a) AFM image of a TIPS-pentacene film spin-coated from a 15 g/l anhydrous chloroform solution on Si/SiO $_{2}$ (300 nm)/Pt (5 nm)/CoPt (10 nm). (b) Low-angle x-ray diffraction scan of a 150 nm thin TIPS-pentacene film spin-coated from a 25 g/l chloroform solution on the identical substrate as in (a). (c) Schematic of the molecular orientation of TIPS-pentacene on Si/SiO $_{2}$ (300 nm)/Pt (5 nm)/CoPt (10 nm).Reuse & Permissions
Figure 3
Magnetization hysteresis loops of ( $▴$ ) a spin valve with 35 nm TIPS-pentacene, as well as ( $•$ ) a 10 nm thin Co film and ( $▪$ ) a 10 nm CoPt film deposited under the same conditions as the respective ferromagnetic electrodes in the actual spin valve devices with 35 nm TIPS-pentacene, measured at $T = 300$ K.Reuse & Permissions
Figure 4
(a) Magnetoresistance (MR) loop of a Pt (5 nm)/CoPt (10 nm)/TIPS-pentacene (75 nm)/AlOx (2 nm)/Co (10 nm)/Al (5 nm) spin valve measured at $T = 300$ K and $V_{b} = 1$ V. The blue ( $▴$ ) and red ( $\circ$ ) curves denote the MR response for increasing and decreasing magnetic fields, respectively. The insets show the relative orientation of the ferromagnetic electrodes for parallel (P) and antiparallel (AP) states, respectively. (b) Bias voltage dependence of the MR value for the device in (a) measured at $T = 300$ K. Positive bias refers to hole injection into TIPS-pentacene via the CoPt layer.Reuse & Permissions
Figure 5
(a) $I$ ( $V$ ) characteristics of a Si/SiO $_{2}$ (300 nm)/Pt (5 nm)/CoPt (10 nm)/TIPS-pentacene (180 nm)/MoO $_{3}$ (10 nm)/Au (40 nm) device measured at $T = 300$ K. Positive bias refers to hole injection into TIPS-pentacene from the MoO $_{3}$ /Au electrode. The red curve shows the fit to the space-charge-limited current (SCLC, Mott-Gurney equation: $I \propto V^{2}$ ). (b) Bulk mobility of holes in TIPS-pentacene as a function of temperature extracted from the space-charge-limited regime of the device displayed in (a). (c) $I$ ( $V$ ) characteristics of a spin valve device with 40 nm thin TIPS-pentacene for different temperatures. (d) Magnetoresistance as a function of temperature for the device in (c) measured at $V_{b} = 1$ V.Reuse & Permissions
Figure 6
(a) ( $▴$ , left axis) Magnetoresistance and ( $\circ$ , right axis) absolute change of the resistance of the spin valves as a function of the TIPS-pentacene thickness, measured at $T = 300$ K and $V_{b} = 1$ V. (b) Resistance of the spin valves as a function of the TIPS-pentacene thickness at $T = 300$ K and $V_{b} = 1$ V.Reuse & Permissions

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