Spin-dependent transport in multiwalled carbon nanotubes

We report on the observation of spin-polarized tunneling magnetoresistance (TMR) in Co-contacted multiwalled carbon nanotubes (MWCNT). At 4.2 K, we find an increase of the TMR with decreasing junction bias. When the junction bias current is 0.5 nA, the highest value of −36% for the TMR ratio is found. The enhanced TMR ratio is explained by the onset of co-tunneling effects. We also find an asymmetric switching behavior which is tentatively attributed to an additional exchange biasing in the electrode material. For different samples, the TMR can assume both negative and positive values, indicating that the sign of the spin polarization of the tunneling electrons can be reversed. Apparently, the spin polarization sign depends sensitively on the detailed electronic configuration of the contact between Co and MWCNT.