Electron tunneling studies of Mn12-Acetate

We used self-assembling tunnel junctions (SATJs) to study the electron transport through films of the molecular magnets, Mn12-Acetate. Pulse laser deposition (PLD) was used to deposit two monolayers of Mn12-Acetate on thin Pt wires (diameter 0.001 in). The electron tunneling current was measured with typical bias voltages from -1 to 1 V at liquid helium temperature, 4.2 K. I, dI/dV , and d2I/dV 2 signals were directly acquired with the aid of a current amplifier and two lock-in-amplifiers. Results show that the differential conductance is approximately 10? 6 S for bias voltages 0.04 V < or =| V |< or = V and exhibits a strong voltage dependence. In the region | V |< or = 0.04 V, we find a zero-bias feature (ZBF) in which the differential conductance is suppressed. In some samples, we observe I -V staircases which we attribute to electrons "hopping" between the electrodes and the molecules. The observed hysteresis was attributed to the slow relaxation of molecules re-orienting within the junction. Abrupt conductance jumps at a bias voltage of -0.12 V were also observed and may indicate state transitions in the Mn12-Acetate molecules. Furthermore, we observed that the zero bias feature (ZBF) can switch from an enhancement to a suppression of the differential conductance. A dip and dry (DAD) method was also used to form films of Mn12-Acetate on Al and Pt wires. Although the conductances were similar to those obtained using the PLD method, there were some subtle differences. In particular, we did not observe the I -V staircases and the state jumps were more ambiguous. The differential conductance for the Mn12-Acetate films on Al wires were typically 10- 7 S, which we attributed to the oxide layer on Al surfaces. We have also found substantial changes in the I - V characteristics when the Pt wires coated with the Mn12-Acetate films were stored in 10-2 Torr for 6 months. In particular, we observed many new features such as peaks in the conductance as a function as the bias voltage. We believe that these effects may be caused by the slow oxidation of the Mn12-Acetate molecules.