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.
