Tunneling magnetoresistance of C2H2 molecules sandwiched between Co clusters

The tunneling magnetoresistance (TMR) of samples containing well-defined Co clusters (≈4.5 nm mean diameter) embedded in C₂H₂ matrices essentially is independent of Co-cluster volume fraction v_Co and reveals a value of about 26% at T=2 K. This result is in contrast to that obtained for Co clusters embedded in C₂H₄ matrices (Phys. Rev. B 67 (2003) 094433). In the latter system the TMR strongly decreased with increasing v_Co indicating different possible orientation of the C₂H₄ molecule sandwiched between the Co clusters. We, therefore, conclude that the C₂H₂ molecules are sandwiched in a rather well-defined orientation between the Co clusters. They probably form double layers of C₂H₂ molecules with the C–C bond axis parallel to the Co cluster surface.