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

Abstract

We report a highly asymmetric magnetoresistance (MR) bias dependence, with the inverse MR peaking at a negative bias and a sign reversal occurring at a positive bias in prototypical La 0.7 Sr 0.3 MnO 3 (LSMO)/Alq 3 /Co organic spin valve (OSV) with a tunnel barrier between LSMO and Alq 3 . This behavior is in strong contrast with the commonly found inverse MR in entire bias range for LSMO/Alq 3 /Co OSVs. The MR bias voltage dependence is independent on the type of the tunnel barrier, either SrTiO 3 or Al 2 O 3 . Together with first-principle calculations, we demonstrate that the strongly hybridized Co d-states with Alq 3 molecules at the interface are responsible for the efficient d-states spin injection and the observed MR bias dependence is originated from the energy dependent density of states of Co d-states. These findings open up new possibilities to engineer interfacial bonding between ferromagnetic materials and a wide variety of molecule selections for the desired spin transport properties.

Impurity-band transport in organic spin valves

“…The bottlenecks of electrical transport are at the two electrode/organic interfaces, where electrons tunnel from (into) the electrode into (from) the impurity band of the organic. The impurity band is also present in OSVs fabricated by using 'soft' evaporation techniques, as evidenced by the same unusual I-V characteristics observed in those OSVs 27 . This impurity band transport explains the following features of I-V observed in the OSVs.…”

“…In particular, the theoretical I-V characteristic is in good agreement with the experimental data of a LaSrMnO/Alq 3 /Co device at T ¼ 15 K in ref. 27.…”

Large magnetoresistance at high bias voltage in double-layer organic spin valves

“…3(a). The sequential spin transport mechanism is supposed to dominate in this bias window [22][23][24]. In contrast, the coherent spin transport mechanism dominates when bias voltage is beyond this window [25,26].…”

“…Therefore, the physics behind the spin transport in the DOSV structure is considerably rich. There are two dominant mechanisms that have been used to explain the spin dependent transport properties in the DMTJ: (i) coherent spin transport, where the MR magnitude of the DOSV is expected to be larger than the MR in the corresponding OSV [22][23][24]; and (ii) spin sequential transport, where the MR magnitude in the DOSV cannot exceed that in the OSV [25,26]. Since the MR in the DOSV is smaller than the MR in the OSV at À20 mV applied voltage, we conclude that the spin transport through the Co middle layer might be governed by the spin sequential transport at this applied voltage.…”

“…However, the MR seriously decays at large applied voltages and almost disappears at above 1 V bias voltage [4,13,22]. The MR dependence on the current is much less serious suggesting that the applied voltage might change the electrode Fermi energy at the OSEC/electrode interface, and hence modifying the interfacial spin polarization [23]. This is not an optimal condition for the high quality spin-polarized organic light emitting diodes (spin-OLED) or bipolar-OSVs that always operates at voltages larger than the band gap of the OSECs [24][25][26][27][28].…”