Tunneling magnetoresistance between Co clusters coated with CO molecules

Films made of in-beam prepared Co clusters (mean diameter ≈4.5nm) coated with CO molecules show an unusual large tunneling magnetoresistance (TMR) of about 50% at T=1.7K. Using a model for the T-dependence of the TMR which includes T-dependent spin disorder at the cluster surface and higher-order tunneling processes below 4 K we obtain a spin polarization P of the tunneling electrons of |P|=0.80(3). Such a high |P| value can be explained by an interaction between the Co cluster surface atoms and the CO molecules which leads to preferred d-electron tunneling. An increased spin polarization of the d-electrons due to a charge transfer process from the CO molecule to the Co atom may be an additional reason for the unusual large TMR.