Cation- and lattice-site-selective magnetic depth profiles of ultrathin ${\mathrm{Fe}}_3{\mathrm{O}}_4$(001) films

A detailed understanding of ultrathin film surface properties is crucial for the proper interpretation of spectroscopic, catalytic, and spin-transport data. We present x-ray magnetic circular dichroism (XMCD) and x-ray resonant magnetic reflectivity (XRMR) measurements on ultrathin ${\mathrm{Fe}}_3{\mathrm{O}}_4$ films to obtain magnetic depth profiles for the three resonant energies corresponding to the different cation species ${\mathrm{Fe}}_{\mathrm{oct}}^{2+}$, ${\mathrm{Fe}}_{\mathrm{tet}}^{3+}$, and ${\mathrm{Fe}}_{\mathrm{oct}}^{3+}$ located on octahedral and tetrahedral sites of the inverse spinel structure of ${\mathrm{Fe}}_3{\mathrm{O}}_4$. By analyzing the XMCD spectrum of ${\mathrm{Fe}}_3{\mathrm{O}}_4$ using multiplet calculations, the resonance energy of each cation species can be isolated. Performing XRMR on these three resonant energies yields magnetic depth profiles that each correspond to one specific cation species. The depth profiles of both kinds of ${\mathrm{Fe}}^{3+}$ cations reveal a $\left(3.9\pm 1.0\right)\text{−}\AA$-thick surface layer of enhanced magnetization, which is likely due to an excess of these ions at the expense of the ${\mathrm{Fe}}_{\mathrm{oct}}^{2+}$ species in the surface region. The magnetically enhanced ${\mathrm{Fe}}_{\mathrm{tet}}^{3+}$ layer is additionally shifted about $2.9\pm 0.4\AA$ farther from the surface than the ${\mathrm{Fe}}_{\mathrm{oct}}^{3+}$ layer.

Figure 1

(a) XAS and (b) XMCD spectrum at the Fe $L_{2,3}$ edge for the ${\mathrm{Fe}}_3{\mathrm{O}}_4$ film, taken at $4\mathrm{T}$ external magnetic field, at room temperature, and in TEY mode. A step function was subtracted from the XAS spectrum. Black dots represent data; green, red, and blue spectra are multiplet calculations for the three cation species of ${\mathrm{Fe}}_3{\mathrm{O}}_4$, and the violet line is their sum. The cation spectra are offset for better visibility. (c) Cation stoichiometry used to obtain the fit in (a) and (b).

Figure 2

XRMR data (open circles) and corresponding fits (solid lines) from the ${\mathrm{Fe}}_3{\mathrm{O}}_4$ film, recorded at the three resonant energies of the XMCD $L_3$ edge, using the modeled magneto-optical depth profiles of Fig. 3. Data were recorded with a magnetic field of 200 $\mathrm{mT}$ along the ${\mathrm{Fe}}_3{\mathrm{O}}_4$[001] direction at room temperature.

Figure 3

(a) Close-up of the surface magneto-optical depth profile of the ${\mathrm{Fe}}_3{\mathrm{O}}_4$ film, together with the optical density obtained from off-resonant XRR fits (black line). (b) (${\mathrm{Fe}}_{\mathrm{oct}}$-O-terminated) model of the magnetite unit cell, in scale with Fig. 3. Comparison with the model in (b) illustrates the sizes of the enhanced regions being roughly half a unit cell of magnetite (four cation layers).

Figure 4

Multiplet simulations of XMCD spectra collected in TEY mode expected for (a) a ${\mathrm{Fe}}_{\mathrm{oct}}$-O-terminated ${\mathrm{Fe}}_3{\mathrm{O}}_4\left(001\right)$ surface, (b) the surface found in this work, and (c) an ideal SCV surface. (d)–(f) Cation profiles for each model, with a simulated roughness of $2.5\AA$. The dashed line indicates the sensitivity of the TEY signal on the sample depth $d_{{\mathrm{Fe}}_3{\mathrm{O}}_4}-z$, which exponentially decays with the electron escape depth ${\lambda }_{\mathrm{e}}=30\AA$.