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Electric Polarization-Driven Modulation of Fe Adatoms on Ferroelectric $α$-In$_2$Se$_3$

Monirul Shaikh, Aleksander L. Wysocki

TL;DR

This work addresses voltage control of magnetic adatoms by leveraging the out-of-plane polarization of a 2D ferroelectric substrate, α-In2Se3. Fe adatoms on In2Se3 were studied with first-principles DFT+U to map adsorption sites, electronic structure, and magnetism as a function of adatom concentration and polarization. Two adsorption sites, B and C, compete, with polarization reversing their relative stability and with hopping barriers $E_b \approx 0.36$–$0.45$ eV, yielding a blocking temperature of about $T_B \approx 100$ K. The Fe $3d$ occupancy, magnetic moment, and MAE depend strongly on site and polarization, including a colossal MAE $\approx 214$ meV per Fe at high concentration for C with P up, which is largely suppressed when polarization is reversed. These results demonstrate a viable route to voltage-controlled tuning of magnetic adatom properties and motivate extending the approach to rare-earth adatoms for nanoscale spintronic and quantum devices.

Abstract

The interplay among structural, electronic, and magnetic properties of Fe adatoms on the surface of two-dimensional ferroelectric α-In$_2$Se$_3$ is investigated using first-principles electronic structure calculations, with a focus on how these properties are modulated by the direction of the electric polarization of the substrate. We identify two competing adsorption sites for Fe adatoms, whose relative stability depends on the adatom concentration and can be reversed by switching the electric polarization of α-In$_2$Se$_3$. The calculated energy barrier for thermally activated hopping between these sites is approximately 0.4 eV, corresponding to a blocking temperature of around 100 K. The hybridization between Fe and In$_2$Se$_3$ orbitals strongly depends on the adsorption site and polarization direction, driven by variations in the local adatom geometry. As a result, the electronic configuration of adatom, magnetic moment, and magnetic anisotropy exhibit a pronounced site dependence and can be effectively modulated by switching the electric polarization of the In$_2$Se$_3$ layer. In particular, at higher adatom concentrations, an exceptionally large perpendicular magnetic anisotropy, exceeding 200 meV per Fe atom, emerges for one polarization direction, but is largely diminished when the polarization is reversed. These findings indicate that ferroelectric substrates offer a promising route for voltage-controlled tuning of magnetic adatom properties via reversible polarization switching.

Electric Polarization-Driven Modulation of Fe Adatoms on Ferroelectric $α$-In$_2$Se$_3$

TL;DR

This work addresses voltage control of magnetic adatoms by leveraging the out-of-plane polarization of a 2D ferroelectric substrate, α-In2Se3. Fe adatoms on In2Se3 were studied with first-principles DFT+U to map adsorption sites, electronic structure, and magnetism as a function of adatom concentration and polarization. Two adsorption sites, B and C, compete, with polarization reversing their relative stability and with hopping barriers – eV, yielding a blocking temperature of about K. The Fe occupancy, magnetic moment, and MAE depend strongly on site and polarization, including a colossal MAE meV per Fe at high concentration for C with P up, which is largely suppressed when polarization is reversed. These results demonstrate a viable route to voltage-controlled tuning of magnetic adatom properties and motivate extending the approach to rare-earth adatoms for nanoscale spintronic and quantum devices.

Abstract

The interplay among structural, electronic, and magnetic properties of Fe adatoms on the surface of two-dimensional ferroelectric α-InSe is investigated using first-principles electronic structure calculations, with a focus on how these properties are modulated by the direction of the electric polarization of the substrate. We identify two competing adsorption sites for Fe adatoms, whose relative stability depends on the adatom concentration and can be reversed by switching the electric polarization of α-InSe. The calculated energy barrier for thermally activated hopping between these sites is approximately 0.4 eV, corresponding to a blocking temperature of around 100 K. The hybridization between Fe and InSe orbitals strongly depends on the adsorption site and polarization direction, driven by variations in the local adatom geometry. As a result, the electronic configuration of adatom, magnetic moment, and magnetic anisotropy exhibit a pronounced site dependence and can be effectively modulated by switching the electric polarization of the InSe layer. In particular, at higher adatom concentrations, an exceptionally large perpendicular magnetic anisotropy, exceeding 200 meV per Fe atom, emerges for one polarization direction, but is largely diminished when the polarization is reversed. These findings indicate that ferroelectric substrates offer a promising route for voltage-controlled tuning of magnetic adatom properties via reversible polarization switching.

Paper Structure

This paper contains 3 sections, 1 equation, 6 figures, 1 table.

Table of Contents

  1. RESULTS AND DISCUSSION
  2. CONCLUSIONS
  3. COMPUTATIONAL METHODS

Figures (6)

  • Figure 1: Crystal structure of the ferroelectric $\alpha$-In$_2$Se$_3$ monolayer. Panels (a) and (b) show side views with the electric polarization pointing upward and downward, respectively. Panels (c) and (d) show the corresponding top views. Atomic sites A, B, and C are indicated.
  • Figure 2: Preferred Fe adatom adsorption sites on the $\alpha$-In$_2$Se$_3$ monolayer. Panels (a) and (e) show the side and top views, respectively, of the Fe adatom at the B adsorption site with the electric polarization pointing up. Panels (b) and (f) show the same views for the C site with polarization up. Panels (c) and (g) show the B site with polarization down, and panels (d) and (h) show the C site with polarization down. Brown, green, and pink spheres represent Fe, Se, and In atoms, respectively. The structures correspond to relaxed configurations in a $6\times6$ lateral supercell. The bottom panels display the relaxed bond lengths between the Fe adatom and its nearest Se neighbors. A Jahn–Teller distortion breaks the threefold symmetry of the adsorption site in all cases except for the B site with polarization down.
  • Figure 3: Adsorption energy of the adatom at the B and C sites as a function of lateral supercell size (i.e., adatom concentration), for both directions of the In$_2$Se$_3$ electric polarization.
  • Figure 4: Energy barriers for thermally activated hopping of the Fe adatom between B and C adsorption sites, calculated using the CI-NEB method for the $6 \times 6$ lateral supercell. Results are shown for In$_2$Se$_3$ with electric polarization (a) upward and (b) downward.
  • Figure 5: Partial density of states (DOS) of an Fe adatom on In$_2$Se$_3$, calculated using a $6 \times 6$ lateral supercell for B and C adsorption sites and both polarization directions. In the top row, the gray-shaded regions represent the total DOS (scaled down by a factor of 36). The bottom row shows Fe $3d$ single-particle levels split by the ideal (i.e., no Jahn–Teller distortion) threefold-symmetric trigonal crystal field, along with their electronic occupations. Orbital degeneracy responsible for the Jahn–Teller distortion is indicated. Note that the level ordering in the middle row differs from the single-particle diagram due to the presence of the Hubbard $U$ interaction (see text).
  • ...and 1 more figures