Although spin injection in molecular materials has been demonstrated recently by two-photon photoemission and muon spin rotation, none of the techniques demonstrates that the magnetoresistance of a molecular spin valve is originated in spin-polarized currents in the molecular layer.
A priori, it is assumed that the magnetoresistance originates from the scattering that the spin-polarized current overcomes when entering the spin collector electrode from the molecular material, i.e., the devices show GMR. But the MR could be the consequence of the scattering that the carriers encounter at the second electrode when tunneling from the spin injector without spin transport in the molecular layer, i.e., devices showing tunneling magnetoresistance (TMR). Both effects, GMR and TMR, give rise to the same type of magnetoresistance curves and are beforehand indistinguishable.
Even though the molecular layer thicknesses often exceed the hundreds of nanometers and thus the tunneling regime is hostile, the nominal thicknesses are in general much higher than the real thicknesses due to the formation of ill-defined layers. The top electrode is usually deposited by evaporation techniques and impacts violently on the soft molecular layer causing interdiffusion or metallic inclusions as shown in the figure below. In an ill-defined layer, carriers can tunnel between electrodes through thin regions giving rise to TMR in a device that was intended to show GMR. Some possible strategies to overcome this problem are to evaporate the first metallic nanometers at low deposition rates or to insert insulating barriers between the molecular layer and the top electrode although in most cases, these do not provide the desired results.
Illustration of an ill-defined layer. The top electrode penetrates in the molecular layer creating metallic inclusions. Thus, the charge carriers may travel through pinholes or tunnel through the thin zones.
The Hanle effect consists of forcing the spins to precess once they are inside the molecular material by applying a small non-collinear magnetic field to randomize the spins. It is considered the litmus test of spin injection in molecular spin valves since it will demonstrate that their magnetoresistance is caused by spin-polarized currents in the molecular layer.
The Hanle effect has been measured in inorganic spin valves, where the transport is coherent and each precession is reflected in an oscillation in the value of the resistance and also in graphene-based spin valves. In both cases, the carrier mobilities are orders of magnitude higher than the characteristic mobilities of molecular materials. Contrary, in molecular semiconductors the transport is incoherent. In the case of molecular spin valves, the Hanle effect is measured by applying a magnetic field perpendicular to the magnetization of the electrodes (Hz).
When the spins are injected into the molecular layer, they precess around the magnetic field Hz at the Larmor frequency so that the resistance is no longer the resistance of the antiparallel state. An analogous situation is encountered by the parallel state.
Unfortunately, the scientific community is not keen on publishing negative results and only a few works reported MSVs where no Hanle effect could be measured. The reasons for the absence of Hanle effect in MSVs may be several. In the worst case, transport is not occurring through the molecular material and therefore the spins cannot precess in it since there is no spin current in the molecular layer. But if indeed, there is transport in the molecular material, the absence of Hanle effect may arise from the difference in terms of spin transport mechanisms between molecular and inorganic materials. In the former, the spin can travel much faster decoupled from the electric charge through exchange. In consequence, the pure spin currents can preclude the quenching of the MR and the observation of the Hanle effect.
REFERENCES
[1] M. Cinchetti, K. Heimer, J.-P. Wüstenberg, O. Andreyev, M. Bauer, S. Lach, C. Ziegler, Y. Gao, and M. Aeschlimann. “Determination of spin injection and transport in a ferromagnet/organic semiconductor heterojunction by two-photon photoemission”. Nature Materials 8.2 (2009), pp. 115–119.
[2] Z. G. Yu. “Suppression of the Hanle effect in organic spintronic devices”. Physical Review Letters 111.1 (2013), p. 016601.
[3] R. C. Roundy and M. E. Raikh. “Spin transport with dispersive traps: Narrowing of the Hanle curve”. Physical Review B 90.24 (2014), p. 241202.
[4] S. Jiang, S. Liu, P. Wang, Z. Luan, X. Tao, H. Ding, and D. Wu. “Exchange-Dominated Pure Spin Current Transport in Alq3 Molecules”. Physical review letters 115.8 (2015), p. 086601.
[5] Z. G. Yu. “Spin transport and the Hanle effect in organic spintronics”. Nanoelectronics and Spintronics 1.1 (2015), pp. 1–18.
[2] Z. G. Yu. “Suppression of the Hanle effect in organic spintronic devices”. Physical Review Letters 111.1 (2013), p. 016601.
[3] R. C. Roundy and M. E. Raikh. “Spin transport with dispersive traps: Narrowing of the Hanle curve”. Physical Review B 90.24 (2014), p. 241202.
[4] S. Jiang, S. Liu, P. Wang, Z. Luan, X. Tao, H. Ding, and D. Wu. “Exchange-Dominated Pure Spin Current Transport in Alq3 Molecules”. Physical review letters 115.8 (2015), p. 086601.
[5] Z. G. Yu. “Spin transport and the Hanle effect in organic spintronics”. Nanoelectronics and Spintronics 1.1 (2015), pp. 1–18.
REFERENCES
