Quantum Mechanical Tunneling in A Molecular Magnet Vertical Heterojunction*

We have fabricated EGaIn/CoPc/ITO heterojunctions where sublimated cobalt phthalocyanine (CoPc) films as thin as 5 nm are sandwiched between transparent bottom-layer ITO and top-layer soft-landing eutectic GaIn (EGaIn) electrodes. The roughness of the CoPc films was determined by AFM to be 1.6 nm, and a crystalline ordering with molecules lying parallel to the ITO surface was confirmed by XRD. The differential conductance measurements of the 5 nm thick sample reveal the onset of a superconducting gap below Tc at 6 K (the transition temperature of metastable β-Ga in the EGaIn contact) thereby providing incontrovertible evidence for direct quantum mechanical tunneling processes through the magnetic molecules in our heterojunctions. In addition, Simmons' model fits of our current-voltage characteristics show a weak temperature dependence of tunneling barrier width and height. External magnetic field dependence of the differential conductance suggests coupling of the tunnel currents to both electronic and magnetic states of the CoPc molecules.