Direct Measurement of spin polarization in ferromagnetic-C₆₀ interfaces using point-contact andreev reflection

An important step has been developed combining the potential of spintronics with organic electronics to reveal the promising field of molecular spintronics, which can offer more flexibility, higher recyclability, and low-production costs compared with inorganic devices. Room temperature magnetoresistance (MR) of 5% has been obtained from C₆₀ spin valves with the structure Co(20 nm)/Al₂ O₃(1.2 nm)/C₆₀ (5-60 nm)/Py(20 nm)/Al(1.5 nm). We observe an asymmetric dependence at low temperatures of the MR with voltage, and surprisingly with the magnetic field as well. This behavior has been attributed to the organic interface formed at the junction, which results in a change of the ferromagnet's spin polarization. The spin polarization at the organic-ferromagnetic interface is extracted by measuring the bias dependence of the conductance of a metallic-superconducting point contact and analyzed the spectra with the modified Blonder-Tinkham-Klapwijk theory. Point-contact Andreev reflection measurements reveal that the Co-C₆₀ possess spin polarization of 30% ± 1%, compared with 40% ± 1% for our Co films without C₆₀. This could account for the asymmetry of the MR in spin valves incorporating C₆₀ and the need for an alumina spacer to maximize the MR.