Epitaxial growth and magnetic properties of half-metallic ${\mathrm{Fe}}_3{\mathrm{O}}_4$ on GaAs(100)

The growth and magnetic properties of epitaxial magnetite ${\mathrm{Fe}}_3{\mathrm{O}}_4$ on GaAs(100) have been studied by reflection high-energy electron diffraction, x-ray photoelectron spectroscopy, magneto-optical Kerr effect, and x-ray magnetic circular dichroism. The epitaxial ${\mathrm{Fe}}_3{\mathrm{O}}_4$ films were synthesized by in situ post growth annealing of ultrathin epitaxial Fe films at $500\mathrm{K}$ in an oxygen partial pressure of $5\times {10}^{-5}\mathrm{mbar}$. The XMCD measurements show characteristic contributions from different sites of the ferrimagnetic magnetite unit cell, namely, $\mathrm{Fe}_{\mathrm{td}}{}^{3+}$, $\mathrm{Fe}_{\mathrm{oh}}{}^{2+}$, and $\mathrm{Fe}_{\mathrm{oh}}{}^{3+}$. The epitaxial relationship was found to be ${\mathrm{Fe}}_3{\mathrm{O}}_4\left(100\right)⟨011⟩∕∕\mathrm{Ga}\mathrm{As}\left(100\right)⟨010⟩$ with the unit cell of ${\mathrm{Fe}}_3{\mathrm{O}}_4$ rotated by 45° to match that of GaAs(100) substrate. The films show a uniaxial magnetic anisotropy in a thickness range of about $2.0–6.0\mathrm{nm}$ with the easy axes along the $\left[0\overline{1}1\right]$ direction of the GaAs(100) substrate.

Figure 1

RHEED patterns of (a) GaAs(100) substrate after cleaning and in situ annealing, (b) $6.0\mathrm{nm}$ of epitaxial Fe on GaAs(100), and (c) ${\mathrm{Fe}}_3{\mathrm{O}}_4$ formed by annealing at $500\mathrm{K}$ in an oxygen partial pressure. The electron beam was along the $\left[0\overline{1}1\right]$ direction of the GaAs(100) substrate and the beam energy was set at $10.0\mathrm{kV}$ in the above diffraction.

Figure 2

Fe $2p$ core-level XPS spectroscopy from the $6.0\mathrm{nm}$ (nominal) sample excited by $\mathrm{Mg}\text{−}{K}_{\alpha }$. The x-ray is perpendicular to the surface of the sample.

Figure 3

Proposed epitaxial relation of (a) epitaxial ${\mathrm{Fe}}_3{\mathrm{O}}_4$ on (b) GaAs(100). The unit cells and the lattice distances have been indicated by solid lines. The ${\mathrm{Fe}}_3{\mathrm{O}}_4$ unit cell has been rotated by 45° in the plane of (100) relative to GaAs. The dash line in (b) indicates one possible unit accommodating one ${\mathrm{Fe}}_3{\mathrm{O}}_4$ unit cell. Note in (a) only Fe ions in the bottom layer of a unit cell have been shown. Insets show major axes of both materials.

Figure 4

MOKE loops of ${\mathrm{Fe}}_3{\mathrm{O}}_4∕\mathrm{Ga}\mathrm{As}\left(100\right)$ [up panel, (a)–(l)] and $\mathrm{Au}∕\mathrm{Fe}∕\mathrm{Ga}\mathrm{As}\left(100\right)$ [bottom panel, (m)–(p)]. The nominal thicknesses of the samples are indicated to the right of the panels. The directions of the external magnetic field are labeled on top of each column according to the GaAs(100) substrate. Note the magnetic field in the first row (a)–(d) was $-0.25–+0.25\mathrm{kOe}$, while in other rows (e)–(p), it was $-0.75–+0.75\mathrm{kOe}$.

Figure 5

(a) XAS and (b) XMCD of ${\mathrm{Fe}}_3{\mathrm{O}}_4$ $\left(6.0\mathrm{nm}\right)∕\mathrm{Ga}\mathrm{As}\left(100\right)$. Three peaks in the Fe $L_3$ edge related to the $\mathrm{Fe}_{\mathrm{td}}{}^{3+}$, $\mathrm{Fe}_{\mathrm{oh}}{}^{2+}$, and $\mathrm{Fe}_{\mathrm{oh}}{}^{3+}$ with different energy can be identified.