Local Control of Single Atom Magnetocrystalline Anisotropy

Individual Fe atoms on a ${\mathrm{Cu}}_2\mathrm{N}/\mathrm{Cu}(100)$ surface exhibit strong magnetic anisotropy due to the crystal field. We show that we can controllably enhance or reduce this anisotropy by adjusting the relative position of a second nearby Fe atom, with atomic precision, in a low-temperature scanning tunneling microscope. Local inelastic electron tunneling spectroscopy, combined with a qualitative first-principles model, reveal that the change in uniaxial anisotropy is driven by local strain due to the presence of the second Fe atom.

Figure 1

(a–c) Diagrams and STM topographic images for (a) $\{2,0\}$ dimer, (b) $\{3/2,1/2\}$ dimer, and (c) $\{1,1\}$ dimer. Magnetic easy axes for Fe atoms are indicated by red lines: the ${\mathrm{Cu}}_2\mathrm{N}$ unit cell is shown in (a). Topographic scale bars correspond to 2 nm. (d–f) Measured IETS spectra (dots) and corresponding simulated spectra (lines) on all three types of dimers, for zero magnetic field and magnetic fields applied in the $z$ and $y$ directions. At-field spectra offset for clarity; the STM temperature is 330 mK. In (e), atoms $A$ and $B$ are distinguished; in (d) and (f), the atoms of the dimer are essentially identical, but both sets of spectra are presented for comparative purposes. The discrepancy between measured and simulated spectra near zero bias in (f) for 8 $T(y)$ can be accounted for by assuming a $\sim 2°$ misalignment of the magnetic field to the $y$ axis.

Figure 2

(a) Splitting of $d$-orbital energies in a linear crystal field. (b) Splitting of $d$-orbital energies in a crystal field where the ligand-metal-ligand angle $\theta$ is slightly smaller than 180°. The electron configurations for the orbital states ${\psi }_n$ are shown. (c) Schematic of a single Fe adsorbed onto ${\mathrm{Cu}}_2\mathrm{N}$, after structure calculated using DFT [5]: a section in the $yz$ plane along the Cu–N direction is shown. (d) Equivalent schematic section of the $\{2,0\}$ Fe dimer: dashed outlines show atom positions for the single Fe case. Since the central Cu atom is lifted due to bonding to two Fe atoms, $\theta$ is larger (closer to 180°) for the dimer than for a single Fe atom. (e) Schematic of a $\{3/2,1/2\}$ Fe dimer in the $xz$ plane. For a single Fe atom adsorbed onto ${\mathrm{Cu}}_2\mathrm{N}$, the N–Fe–N angle in the $xz$ plane $\varphi =180°$: in the case of the $\{3/2,1/2\}$ dimer, the presence of Fe $A$ compresses bond lengths along the Cu–N direction [as in (d)], so that for Fe $B$, $\varphi <180°$. Similarly, for the $\{1,1\}$ dimer (f), for both Fe atoms, $\varphi <180°$. In (e) and (f), dashed outlines show the unperturbed ${\mathrm{Cu}}_2\mathrm{N}$ lattice. Atomic displacements are only indicative and are not to scale.

Figure 3

IETS measurements performed on 14 different instances of the type $\{3/2,1/2\}$ dimer at 330 mK and zero magnetic field, showing a spread of the step positions for both atoms in the dimer. Spectra were collected at either 0.9 or 2 nA; no current dependence on the step position was observed. In the inset, the anisotropies of the two atoms of each dimer $D^{(A)}$ and $D^{(B)}$, obtained by fitting the measured IETS spectra, are plotted against each other, indicating a strong anticorrelation between the two atoms (with Pearson correlation coefficient $R=-0.93$). The error bars represent the fitting uncertainty for $D$.