Electron Transport through Single ${\mathrm{Mn}}_{12}$ Molecular Magnets

We report transport measurements through a single-molecule magnet, the ${\mathrm{Mn}}_{12}$ derivative $[{\mathrm{Mn}}_{12}{\mathrm{O}}_{12}({\mathrm{O}}_2\mathrm{C}\mathrm{\text{−}}{\mathrm{C}}_6{\mathrm{H}}_4\mathrm{\text{−}}\mathrm{SAc}{)}_{16}({\mathrm{H}}_2\mathrm{O}{)}_4]$, in a single-molecule transistor geometry. Thiol groups connect the molecule to gold electrodes that are fabricated by electromigration. Striking observations are regions of complete current suppression and excitations of negative differential conductance on the energy scale of the anisotropy barrier of the molecule. Transport calculations, taking into account the high-spin ground state and magnetic excitations of the molecule, reveal a blocking mechanism of the current involving nondegenerate spin multiplets.

Figure 1

(a) Side view of a ${\mathrm{Mn}}_{12}$ molecule with tailor-made ligands containing acetyl-protected thiol end groups ($R={\mathrm{C}}_6{\mathrm{H}}_4$). Atoms are color labeled: manganese (orange), oxygen (dark red), carbon (gray), sulfur (yellow). The molecule diameter is about 3 nm. (b) Schematic drawing of the ${\mathrm{Mn}}_{12}$ molecule (red circle) trapped between electrodes. A gate changes the electrostatic potential on the molecule enabling energy spectroscopy. (c) Scanning electron microscopy image of the electrodes. The gap is not resolvable. Scale bar corresponds to 200 nm.

Figure 2

(a) Differential conductance (gray scale) as a function of gate voltage ($V_g$) and bias voltage ($V_b$) ($T=3\mathrm{K}$, $R={\mathrm{C}}_6{\mathrm{H}}_4$). A region of complete current suppression (left degeneracy point, arrow) and low-energy excitations with negative differential conductance (right degeneracy point) are observed. The dashed line near the left degeneracy point indicates the suppressed diamond edge (gray scale from $-0.8$ [black] to 1.4 nS [white]). (b) $I-V_b$ at the gate voltage indicated in (a) with a line. NDC is clearly visible as a decrease in $|I|$ upon increasing $|V_b|$. Upon applying a magnetic field, current is increased for negative bias. (c)–(f) Same as (a) for 4 transport regions in which the 14 meV excitation is indicated with arrows. Bias voltage ranges are $V_b=\pm 30\mathrm{mV}$, $V_b=\pm 20\mathrm{mV}$, $V_b=\pm 25\mathrm{mV}$, and $V_b=\pm 15\mathrm{mV}$, respectively. (c) and (d) are the right and center charge transport regions of the sample in (a), respectively. (e) and (f) are measured in devices with the $R={\mathrm{C}}_{15}{\mathrm{H}}_{30}$ ligands.

Figure 3

(a) Differential conductance (gray scale) as a function of gate voltage ($V_g$) and bias voltage ($V_b$) for another device ($T=1.5\mathrm{K}$, $R={\mathrm{C}}_6{\mathrm{H}}_4$) in which the 2 and 3 meV excitations are also observed (negative bias). The 14 meV excitation is indicated by a white arrow. The origin of the step in the diamond edge at positive bias is unclear but most likely not related to the magnetic properties of the molecule (gray scale from $-3$ [black] to 10 nS [white]). (b) $I-V_b$ along the line in (a).

Figure 4

Model calculations. (a) Energy diagram of the $S_z$ states for the electronic ground state ($\alpha =0$, blue) and first excited state ($\alpha =1$, red) of the neutral molecule ($N=n$). (b),(c) Differential conductance (gray scale from $-0.8$ [black] to 1 a.u. [white]) modeling CCS (b) and NDC (c). Parameters are listed in Ref. 30.