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Illustration of energy level alignment of Alq3 and metal contacts: (a) transition ferromagnetic metals have a work-function closer to HOMO of Alq3 facilitating hole injection. In contrast, (b) Ca inserts between Alq3 and ferromagnetic metals have a work function closer to LUMO facilitating electron injection.
Schematic view of each device, (a) Co/Alq3/NiFe and (b) Co/Ca/Alq3/Ca/NiFe, and the electrical and MR measurement geometry are illustrated. Typical I–V curves for Co/Alq3/NiFe and Co/Ca/Alq3/Ca/NiFe devices measured at 4.5 K are plotted in (c) and (d), respectively. MR curves measured at 4.5 K are presented in (e) for Co/Alq3/NiFe device and in (f) for Co/Ca/Alq3/Ca/NiFe and the corresponding junction resistance was indicated in the right axis of each graph. For the measurements, 0.6 V in (e) and 0.2 V in (f) of external bias voltages were applied. Arrows indicate the sweep direction of the external magnetic field. MR(%) was defined by 100 × (R(B) − R(P))/R(P) where R is the measured resistance at the applied magnetic field B and R(P) is the resistance when two ferromagnetic electrodes have a parallel magnetization configuration.
Schematic energy diagrams and probable spin carrier injection/extraction routes of (a) Co/Ca/Alq3/Ca/NiFe and (b) Co/Alq3/NiFe systems. When the Fermi level of metal contacts are close to the LUMO of Alq3, electrons with majority spins are injected and extracted as described in (a). If the Fermi level of metal contacts are close to the HOMO, the extraction of holes with minority spins below the Fermi level can be more efficient as shown in (b).
I–V curves of Co/Ca/Alq3/Ca/NiFe device measured at (a) 77 K and (b) 4.5 K and MR plots of the same device at (c) 77 K and (d) 4.5 K, respectively. For the MR measurements, 0.1 V in (c) and 0.13 V in (d) of external bias voltages were applied. Initially, the measurement was carried out at 77 K for a few hours and later, the device was investigated at 4.5 K.
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