Probing Superexchange Interaction in Molecular Magnets by Spin-Flip Spectroscopy and Microscopy

The superexchange mechanism in cobalt phthalocyanine (CoPc) thin films was studied by a low temperature scanning tunneling microscope. The CoPc molecules were found to form one-dimensional antiferromagnetic chains in the film. Collective spin excitations in individual molecular chains were measured with spin-flip associated inelastic electron tunneling spectroscopy. By spatially mapping the spin-flipping channels with submolecular precision, we are able to explicitly identify the specific molecular orbitals that mediate the superexchange interaction between molecules.

Figure 1

CoPc multilayers on Pb. (a) STM image ($V=0.9\mathrm{V}$, $I=0.03\mathrm{nA}$) of the self-assembled multilayers of CoPc molecules on Pb(111) film (26 ML thick). (b) Molecular structure of CoPc. (c) STM image ($V=0.6\mathrm{V}$, $I=0.1\mathrm{A}$) showing the relative stacking of the 1st and 2nd CoPc layers. (d) STM image showing the relative stacking of the 2nd and 3rd CoPc layers. The white and black dots indicate the centers of the molecules on the 2nd and 3rd layers, respectively. (e) Stacking geometry of CoPc molecules. The molecular layers are spaced $3.5\pm 0.1\AA$ apart. The inserts show the orientation and displacement between molecules in adjacent layers.

Figure 2

STS for molecules in the first and second layers. For all spectra, $T=0.4\mathrm{K}$ and the tunneling junction is set at $V=6\mathrm{mV}$, $I=0.2\mathrm{nA}$. The bias modulation was 0.05 mV at 1991 Hz. (a) Superconducting gap observed on bare Pb surface and an isolated CoPc molecule adsorbed on Pb. The insert shows an image of the molecule (8 mV, 0.1 nA). (b) Kondo resonance acquired on a molecule in the second layer. The Kondo peak is split in magnetic field. (c) Zeeman splitting for a molecule away from the top site of the first layer. (d) Magnetic field dependence of the Zeeman energy in (c).

Figure 3

Spin-flip spectra of individual CoPc chains. (a) Spectrum acquired with the STM tip positioned on the third molecular layers. The tunneling gap was set at $V=50\mathrm{mV}$, $I=0.2\mathrm{nA}$. The bias modulation was 1 mV (rms) at 1991 Hz. The arrows indicate the inelastic tunneling threshold voltages. (b) Spectra on the third layer in different magnetic fields. The STS were acquired at a set point of $V=10\mathrm{mV}$ and $I=0.15\mathrm{nA}$ at $T=0.4\mathrm{K}$. The arrows indicate the three steps induced by 11 T magnetic field. The lock-in modulation was 0.05 mV at 1991 Hz. (c) Schematic of the spin singlet to triplet transition. The triplet states are labeled by the magnetic quantum number $S_z$. $S_c$ is the total spin of the chain. (d) Spectra acquired on the fourth and fifth layers. The curves are offset vertically for clarity. (e) Eigenstates of the Heisenberg Hamiltonian ($J>0$) with $S=1/2$ and $N=3$, 4.

Figure 4

Superexchange mechanism. (a) Illustration of the superexchange interaction in a molecular chain. (b) Spin configuration in superexchange. The bridging N atom is marked by a dot. The nodal plane (dashed line) of the $E_g$ orbital passes through two pyrrole N atoms and the central ${\mathrm{Co}}^{2+}$ ion. (c) The spectra at different locations of the molecule. (d) Spin-flip image of a CoPc molecule on the third layer at $T=0.4\mathrm{K}$. The bias voltage was fixed at $-17.2\mathrm{mV}$ [indicated by arrows in (c)] during mapping. The mapping with positive bias has similar shape, but less contrast. The modulation was 2 mV at 799 Hz. The molecular structure is superimposed on the image. The dashed circle indicates the bridging N atom. (e) Charge density of $E_g$ orbital yielded by the first-principles calculation.