EN
The field of organic spintronics has taken off
since the discovery of large magnetoresistance in thick organic
spin valves (OSVs). The thickness of organic films
(~100 nm) in these OSVs should preclude direct electron
tunneling between the ferromagnetic electrodes and suggests
spin injection into the organic. To definitively prove
genuine spin injection, it is necessary to show that the injected
electron spins inside the organic precess around a
transverse magnetic field – the Hanle effect, which has
served as a litmus test of spin injection in inorganic devices.
Because of the low carrier mobility in organics, the
Hanle effect is expected to be seen at a magnetic field as
small as 10−6 mT. However, no Hanle signal has been detected
up to 10 mT in OSVs, which has been the greatest
puzzle shadowing the field of organic spintronics. In this
review, first I will give an overview of the Hanle effect and
its use in inorganic spintronics. Then I will summarize the
Hanle-effect measurements in OSVs. I will show how we
can reconcile the absence of Hanle effect in OSVs with the
apparent spin injection into the organic detected by other
spin probes. The key distinction of organic materials is that
carriers are localized and exchange between them can facilitate
efficient spin transport. Since the exchange does
not affect charge motion, spin and charge motions in organics
can be well separated. This spin-charge separation
does not occur in inorganic spintronic devices.