Adsorption-Induced Switching of Magnetic Anisotropy in a Single Iron(II) Phthalocyanine Molecule on an Oxidized Cu(110) Surface

We examined the zero-field splitting of an iron(II) phthalocyanine (FePc) attached to clean and oxidized Cu(110) surfaces and the dependence on an applied magnetic field by inelastic electron tunneling spectroscopy with STM. The symmetry of the ligand field surrounding the Fe atom is lowered on the oxidized surface, switching the magnetic anisotropy from the easy plane of the bulk to the easy axis. The zero-field splitting was not observed for FePc on a clean Cu(110) surface, and the spin state converts from triplet to singlet due to the strong coupling of Fe $d$ states with the Cu substrate, as is also confirmed by photoelectron spectroscopy. These findings demonstrate the importance of coupling at the molecule-substrate interface for manipulating the magnetic properties of adsorbates.

Figure 1

Schematic model of a (a) FePc molecule and (b) $\mathrm{Cu}(110)(2\times 1)\mathrm{\text{−}}\mathrm{O}$ added-row structure. Topographic STM images of (c) $\alpha$- and (d) $\beta$-FePc on $\mathrm{Cu}(110)(2\times 1)\mathrm{\text{−}}\mathrm{O}$ taken at 0.4 K ($4\times 4{\mathrm{nm}}^2$, $I_t=100\mathrm{pA}$, and $V_s=-0.5\mathrm{V}$). Small rectangles in (c) and (d) show a unit cell of the $(2\times 1)\mathrm{\text{−}}\mathrm{O}$ structure.

Figure 2

The $dI/dV$ spectra of (a) $\alpha$- and (b) $\beta$-FePc on $\mathrm{Cu}(110)(2\times 1)\mathrm{\text{−}}\mathrm{O}$ taken as a function of $B_z$ ($0\sim 11\mathrm{T}$) applied perpendicularly to the sample surface. Each spectrum was acquired by keeping an STM tip above the Fe atom with an open loop condition. The spectra except for $B_z=0$ are vertically offset for clarity. (c) The magnetic field dependence of step energies. The curves are the results of least squares fitting. (d) The energy diagram of ZFS for bulk FePc and a single FePc molecule on $(2\times 1)\mathrm{\text{−}}\mathrm{O}$.

Figure 3

Fe $2p$ spectra of a FePc monolayer on (a) Cu(110) and (b) $\mathrm{Cu}(110)(2\times 1)\mathrm{\text{−}}\mathrm{O}$ surfaces taken at room temperature. The incident photon energy used was 900 eV. The inset shows the $dI/dV$ spectra of FePc on Cu(110) and $\mathrm{Cu}(110)(2\times 1)\mathrm{\text{−}}\mathrm{O}$ at $B_z=0$. The spectrum for Cu(110) is vertically shifted for clarity. The conductances at 10 mV of both spectra are almost the same.