Dynamic Atomic Reconstruction: How ${\mathrm{Fe}}_3{\mathrm{O}}_4$ Thin Films Evade Polar Catastrophe for Epitaxy

Polar catastrophe at the interface of oxide materials with strongly correlated electrons has triggered a flurry of new research activities. The expectations are that the design of such advanced interfaces will become a powerful route to engineer devices with novel functionalities. Here, we investigate the initial stages of growth and the electronic structure of the spintronic ${\mathrm{Fe}}_3{\mathrm{O}}_4/\mathrm{MgO}(001)$ interface. Using soft x-ray absorption spectroscopy, we have discovered that the so-called A-sites are completely missing in the first ${\mathrm{Fe}}_3{\mathrm{O}}_4$ monolayer. This discovery allows us to develop an unexpected but elegant growth principle in which, during deposition, the Fe atoms are constantly on the move to solve the divergent electrostatic potential problem, thereby ensuring epitaxy and stoichiometry at the same time. This growth principle provides a new perspective for the design of interfaces.

Popular Summary

The physical properties of the surfaces and interfaces of a solid can markedly differ from those in the solid’s bulk, particularly in cases when the surface or interface involves non-neutral crystal planes. For insulators, the stacking of such polar planes causes the electrostatic potential to diverge, and this so-called “polar catastrophe” destabilizes the system in a dramatic manner. This situation results in the surface or interface behaving very differently from that of the bulk. The polar catastrophe presents an opportunity to design new interfaces with potential for new emergent phenomena, provided that we understand the atomic structure and growth modes of polar interfaces. Here, we investigate the polar interface between ${\mathrm{Fe}}_3{\mathrm{O}}_4$ and the MgO (001) substrate, one of the most commonly used interfaces in spintronics.

We use molecular beam epitaxy to ensure layer-by-layer growth of ${\mathrm{Fe}}_3{\mathrm{O}}_4$ films only a few angstroms thick in ultraclean conditions. Using soft x-ray absorption spectroscopy, we discover that certain types of Fe ions are missing in the first ${\mathrm{Fe}}_3{\mathrm{O}}_4$ monolayer. Together with data from thicker films, we propose an unexpected but elegant growth principle in which not only the surface but also the subsurface Fe atoms are constantly on the move during deposition to solve the divergent electrostatic potential problem. Having identified this “dynamic atomic reconstruction” growth principle, we conclude that we truly have to think differently and openly about how polar interfaces can grow.

We expect that our findings will be useful in the research field of thin-film devices and nanoscience to create novel functionalities.

Figure 1

${\mathrm{Fe}}_3{\mathrm{O}}_4$ on MgO (001). (a) Structure of ${\mathrm{Fe}}_3{\mathrm{O}}_4$. (b) Buildup of the polar catastrophe of ${\mathrm{Fe}}_3{\mathrm{O}}_4$ on MgO (001): charged planes, and the corresponding charge, electric field, and electric potential. (c,d) RHEED and LEED patterns, respectively, of a 200-nm epitaxial ${\mathrm{Fe}}_3{\mathrm{O}}_4$ (001) film showing the characteristic $(\sqrt{2}\times \sqrt{2})\mathrm{R}45°$ superstructure. (e) RHEED intensity oscillations of the specularly reflected beam. The electron beam was incident along the [100] direction, with a primary energy of 20 kV. (f) Resistivity as a function of temperature of a 10-nm and a 200-nm thin film, showing the presence of the Verwey transition.

Figure 2

Fe $L_3$ XAS spectra of ${\mathrm{Fe}}_3{\mathrm{O}}_4$ films. The film thickness varies from 0.67 to 8 ML. The reference spectra of bulk ${\mathrm{Fe}}_3{\mathrm{O}}_4$, bulk ${\mathrm{YBaCo}}_3{\mathrm{FeO}}_7$ (${\mathrm{Fe}}^{3+}$ in tetrahedral coordination) [28], bulk FeO (${\mathrm{Fe}}^{2+}$ in octahedral coordination) [32], and bulk ${\mathrm{Fe}}_2{\mathrm{O}}_3$ (${\mathrm{Fe}}^{3+}$ in octahedral coordination) are also included. All spectra were measured at 300 K. The gray line indicates the energy position of the main peak in the spectrum of bulk ${\mathrm{Fe}}_2{\mathrm{O}}_3$. The full Fe $L_{2,3}$ spectral range is presented in Ref. [31].

Figure 3

The extracted spectral weight of each Fe site versus the film thickness. The spectral weight of the B-site ${\mathrm{Fe}}^{3+}$, the B-site ${\mathrm{Fe}}^{2+}$, and the A-site ${\mathrm{Fe}}^{3+}$ are given by the green, blue, and red squares, respectively. Green, blue, and red curves depict the concentrations of the B-site ${\mathrm{Fe}}^{3+}$, the B-site ${\mathrm{Fe}}^{2+}$, and the A-site ${\mathrm{Fe}}^{3+}$, respectively, in our model (see the text). The Fe content ($y$) of the ${\mathrm{Fe}}_y\mathrm{O}$ films derived from the average valence is shown in the bottom panel. The error bars reflect the deviations of the fits to the experimental data.

Figure 4

Model for the growth process of polar ${\mathrm{Fe}}_3{\mathrm{O}}_4$ (001) thin films. (a) One monolayer, (b) two monolayers, and (c) three monolayers.