  An anomalous distortion is often observed in the transfer characteristics of
graphene field-effect transistors. We fabricate graphene transistors with
ferromagnetic metal electrodes, which reproducibly display distorted transfer
characteristics, and show that the distortion is caused by metal-graphene
contacts with no charge-density pinning effect. The pinning effect, where the
gate voltage cannot tune the charge density of graphene at the metal
electrodes, has been experimentally observed; however, a pinning-free interface
is achieved with easily-oxidizable metals. The distortion should be a serious
problem for flexible electronic devices with graphene.
