Electronic structure of the high-$T_{\mathrm{C}}$ ferromagnetic semiconductor (Ga,Fe)Sb: X-ray magnetic circular dichroism and resonance photoemission spectroscopy studies

The electronic structure and the magnetism of the ferromagnetic semiconductor (Ga,Fe)Sb, whose Curie temperature $T_{\mathrm{C}}$ can exceed room temperature, were investigated by means of x-ray absorption spectroscopy (XAS), x-ray magnetic circular dichroism (XMCD), and resonance photoemission spectroscopy (RPES). The line-shape analyses of the XAS and XMCD spectra suggest that the ferromagnetism is of intrinsic origin. The orbital magnetic moments deduced using XMCD sum rules were found to be large, indicating that there is a considerable $3d^6$ contribution to the ground state of Fe. From RPES, we observed a strong dispersive Auger peak and nondispersive resonantly enhanced peaks in the valence-band spectra. The latter is a fingerprint of the correlated nature of Fe $3d$ electrons, whereas the former indicates their itinerant nature. It was also found that the Fe $3d$ states have a finite contribution to the density of states at the Fermi energy. These states, presumably consisting of majority-spin $p\text{−}d$ hybridized states or minority-spin $e$ states, would be responsible for the ferromagnetic order in this material.

Figure 1

(a) XAS and (b) XMCD spectra of (Ga,Fe)Sb compared with those of (In,Fe)As:Be [30], bcc Fe [37, 39], ${\mathrm{FeCr}}_2{\mathrm{O}}_4$ [40], and $\gamma \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$ [41]. The XMCD spectra of bcc Fe, ${\mathrm{FeCr}}_2{\mathrm{O}}_4$, and $\gamma \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$ are multiplied by 0.5 for ease of comparison.

Figure 2

(a) XAS and XMCD spectra of the 13.7% Fe-doped (Ga,Fe)Sb before and after HCl etching. Intrinsic and extrinsic components are also separately shown by red and orange dashed curves, respectively, together with the spectrum of $\alpha \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$. The inset shows the intrinsic XAS and XMCD spectra expanded at the Fe $L_3$ edge. The second derivative of the XAS spectrum is also shown to emphasize weak features, labeled $a–d$. Here, the signs of the XMCD and the second derivative spectrum have been reversed. (b) Magnetic-field dependence of XMCD and visible-light MCD intensities, shown by circles and solid curves, respectively.

Figure 3

(a) and (d) Resonance photoemission spectroscopy (RPES) spectra of (Ga,Fe)Sb with 6% Fe and 13.7% Fe taken with photon energies across the Fe $L_3$ absorption edge. The color and the baseline positions of the spectra represent the photon energies as depicted by triangles on the XAS spectra in (c) and (f). Here, the off-resonance spectra shown in (b) and (e) have been subtracted from all the spectra. (g) and (h) False-color plots of the second derivatives of the RPES spectra in (a) and (d). The dispersive normal Auger peak and the nondispersive resonance features are indicated by white dashed lines and red dashed lines, respectively.

Figure 4

(a) RPES spectrum of the 13.7% Fe-doped sample taken with $h\nu =707$ eV (thick gray curve) and fitting results (black curve). Fitting components of two Gaussian functions for features $\alpha$ and $\beta$ and one asymmetric Gaussian function for the Auger peak are also shown separately. (b) and (c) RPES spectra of the 6% and 13.7% Fe-doped samples near $E_{\mathrm{F}}$. Here, the colors are the same as those used in Figs. 3 and 3. Note that the RPES spectra of the 6% Fe-doped sample have been smoothed and multiplied by 3.5 for easy comparison. (d) Constant-initial-state spectra for features $\alpha$ and $\beta$ and the intensity at $E_{\mathrm{F}}$ and the Auger peak intensity as functions of photon energy. For comparison, XAS spectra are also plotted.

Figure 5

Schematic energy diagram of Fe-doped III-V semiconductors. The $d$ levels of Fe are shown on the sides, and the valence band and the conduction band of the host GaSb are shown by green boxes in the middle. Due to $p\text{−}d\left(t_2\right)$ hybridization, the $t_2$ orbitals are split into the bonding $\left(t_b\right)$ and antibonding $\left(t_a\right)$ states having both $p$ and $t_2$ characters. States with predominant $p$ and $t_2$ characters are indicated by green and gray boxes, respectively. The electron occupancy of the ${\mathrm{Fe}}^{3+}$ state is illustrated by red and black arrows, while that of ${\mathrm{Fe}}^{2+}$ is illustrated by blue and black arrows.