Origin of Perpendicular Magnetic Anisotropy and Large Orbital Moment in Fe Atoms on MgO

We report on the magnetic properties of individual Fe atoms deposited on MgO(100) thin films probed by x-ray magnetic circular dichroism and scanning tunneling spectroscopy. We show that the Fe atoms have strong perpendicular magnetic anisotropy with a zero-field splitting of $14.0\pm 0.3\mathrm{meV}/\text{atom}$. This is a factor of 10 larger than the interface anisotropy of epitaxial Fe layers on MgO and the largest value reported for Fe atoms adsorbed on surfaces. The interplay between the ligand field at the O adsorption sites and spin-orbit coupling is analyzed by density functional theory and multiplet calculations, providing a comprehensive model of the magnetic properties of Fe atoms in a low-symmetry bonding environment.

Figure 1

(a) STM image of two Fe atoms on a ML MgO(100) grown on Ag(100) ($4\mathrm{nm}\times 4\mathrm{nm}$, tunnel current $I_t=5\mathrm{pA}$, tunnel voltage $V_t=100\mathrm{mV}$). (b) Side view of DFT-calculated binding geometry and charge density [color scale, $1e/(\mathrm{au}{)}^3$; Fe, green; O, red; Mg, blue). (Middle sketch) Top view ball model of the binding geometry. (c) Oblique view of DFT-calculated valence electron spin density contours (positive spin polarization, red; negative, blue).

Figure 2

(a) Measured and simulated XAS over the Fe $L_3$ and $L_2$ edges for 0.03 ML Fe on a MgO film on Ag(100) with an average thickness of 3 ML ($T=2.5\mathrm{K}$, $B=6.8\mathrm{T}$, incident angle $\theta$, total electron yield mode). (b) XMCD spectra for both geometries. (c) Out-of-plane magnetization curve measured by first saturating the sample at 6.8 T (red) and $-6.8\mathrm{T}$ (green) and then moving to the respective field value ($T=2.5\mathrm{K}$). The values of the magnetization are obtained from the maximum of the XMCD signal at 704 eV. The solid line represents $⟨2S_z(B)⟩+⟨L_z(B)⟩$ determined by the multiplet fit with a saturation moment of $5.2{\mu }_B$. (d) Sketch of the measurement geometry. The magnetic field is aligned to the incident beam.

Figure 3

Energy level diagram resulting from the multiplet simulation of the XAS and XMCD spectra (for the full diagram, see Ref. [34]). The $S_z$ and $L_z$ values in the labels are the respective expectation values; the $⟨⟩$ signs have been omitted for brevity. The cubic crystal field quenches the orbital moment and creates two spin quintuplets with $B_1$ and $B_2$ symmetry, which are both further separated by the SOC and the Zeeman energy. Labels $|0⟩$–$|4⟩$ and $|5⟩$–$|9⟩$ denote the states deriving from the $B_1$ and $B_2$ quintuplets, respectively.

Figure 4

(a) STM differential conductance ($dI/dV$) spectrum on $\mathrm{Fe}/1\mathrm{ML}$ $\mathrm{MgO}(100)/\mathrm{Ag}(100)$; $dI/dV$ on bare MgO is shown for comparison ($T=0.6\mathrm{K}$, $B=0\mathrm{T}$, modulation with $V_{\text{mod}}=150\mu \mathrm{V}$ rms at $f=806\mathrm{Hz}$, set point before opening the feedback loop $I_t=1\mathrm{nA}$, $V_t=30\mathrm{mV}$). (b) Positive conductance step at out-of-plane fields of 0 T (green) and 4 T (blue). (Inset) Field splitting of step energies. (c) Sketch of the magnetic states and the allowed IETS excitations. (d) IETS feature corresponding to the superposition of $V_{35}$, $V_{25}$, $V_{47}$, and $V_{46}$, measured with a spin-polarized tip ($T=1.2\mathrm{K}$, $B=2\mathrm{T}$, $V_{\text{mod}}=1.5\mathrm{mV}$, $I_t=1\mathrm{nA}$, $V_t=100\mathrm{mV}$). (Inset) Spin-polarized spectrum in the same energy window as (a). Jagged edges at the conductance steps originate from the superposition of inelastic spin excitations and spin pumping, and they reveal that the tip is magnetic.