Development of magnetism in Fe-doped magnetic semiconductors: Resonant photoemission and x-ray magnetic circular dichroism studies of (Ga,Fe)As

Fe-doped III-V ferromagnetic semiconductors (FMSs) such as (In,Fe)As, (Ga,Fe)Sb, (In,Fe)Sb, and (Al,Fe)Sb are promising materials for spintronic device applications because of the availability of both $\mathrm{n}$- and $\mathrm{p}$-type materials and the high Curie temperatures. On the other hand, (Ga,Fe)As, which has the same zinc-blende crystal structure as the Fe-doped III-V FMSs, shows paramagnetism. The origin of the different magnetic properties in the Fe-doped III-V semiconductors remains to be elucidated. To address this issue, we use resonant photoemission spectroscopy (RPES) and x-ray magnetic circular dichroism (XMCD) to investigate the electronic and magnetic properties of the Fe ions in a paramagnetic $\left({\mathrm{Ga}}_{0.95},{\mathrm{Fe}}_{0.05}\right)\mathrm{As}$ thin film. The observed Fe $2p\text{−}3d$ RPES spectra show that the Fe $3d$ states are similar to those of ferromagnetic (Ga,Fe)Sb. The estimated Fermi level is located in the middle of the band gap in (Ga,Fe)As. The Fe $L_{2,3}$ XMCD spectra of $\left({\mathrm{Ga}}_{0.95},{\mathrm{Fe}}_{0.05}\right)\mathrm{As}$ show preedge structures, which are not observed in the Fe-doped FMSs, indicating that the minority-spin (↓) $e_{\downarrow }$ states are vacant in $\left({\mathrm{Ga}}_{0.95},{\mathrm{Fe}}_{0.05}\right)\mathrm{As}$. The XMCD results suggest that the carrier-induced ferromagnetic interaction in $\left({\mathrm{Ga}}_{0.95},{\mathrm{Fe}}_{0.05}\right)\mathrm{As}$ is short ranged and weaker than that in the Fe-doped FMSs. The experimental findings suggest that the electron occupancy of the $e_{\downarrow }$ states contributes to the appearance of ferromagnetism in the Fe-doped III-V semiconductors, for $\mathrm{p}$-type as well as $\mathrm{n}$-type compounds.

Figure 1

Characterization of the $\left({\mathrm{Ga}}_{0.95},{\mathrm{Fe}}_{0.05}\right)\mathrm{As}$ thin film. (a) RHEED pattern of the $\left({\mathrm{Ga}}_{0.95},{\mathrm{Fe}}_{0.05}\right)\mathrm{As}$ film taken along the [−110] azimuth after the MBE growth. (b) Normalized reflection visible-light MCD spectra measured at $T=10\mathrm{K}$ under magnetic fields ${\mu }_{0}H=0.2$, 0.5, and 1 T applied perpendicular to the film. (c) Visible-light MCD– $H$ curves measured at $E_1 (h\nu =2.73$ eV).

Figure 2

Fe $L_{2,3}$-edge XAS and XMCD spectra of $\left({\mathrm{Ga}}_{0.95},{\mathrm{Fe}}_{0.05}\right)\mathrm{As}$ at $T=10\mathrm{K}$. (a) XAS spectrum of after-etching (purple curve) and before-etching (green curve) samples, where dashed black curves represent the background. (b), (c) Magnetic field dependence of XAS and XMCD spectra, respectively. Here, the spectra have been normalized to the height at $h\nu =708\mathrm{eV}$. In the inset, the XMCD spectra at the $L_3$ edge are enlarged and the preedge structures are clearly observed.

Figure 3

Spin and orbital magnetic moments estimated using the XMCD sum rules. (a) Magnetic field dependence of orbital- (left axis, round markers) and spin- (right axis, triangle markers) magnetic moments. Here, the error originates from the shape of the background and the magnetic field dependence of the XMCD intensity. (b) Magnetic field dependence of the orbital magnetic moment-to-spin magnetic moment ratio.

Figure 4

Magnetic field dependence of the XMCD of $\left({\mathrm{Ga}}_{0.95},{\mathrm{Fe}}_{0.05}\right)\mathrm{As}$. (a) XAS and XMCD spectra, where the photon energies used for the $\mathrm{XMCD}–H$ measurements are indicated by dashed bars. (b) $\mathrm{XMCD}–H$ curves taken at various $h\nu$’s, where the $\mathrm{XMCD}–H$ curves have been normalized such that the slopes of the curves between 0 and 0.4 T become the same. In the inset, the normalized $\mathrm{XMCD}–H$ curves between 0.1 and 1 T are enlarged, where the black line represents the XMCD intensity proportional to ${\mu }_{0}H$. (c) Fitting of the magnetization curve taken at $h\nu =707.7\mathrm{eV}$. The fitting result is shown by the solid black curve. Here, the PM and SPM components of the fitting result are represented by black dotted and dashed-dotted curves, respectively.

Figure 5

RPES spectra of an unetched $\left({\mathrm{Ga}}_{0.95},{\mathrm{Fe}}_{0.05}\right)\mathrm{As}$ thin film. (a) RPES spectra taken at various $h\nu$’s across the Fe $L_3$ absorption edge. The colors of the spectra correspond to those of the markers on the XAS spectrum in the right panel. The RPES spectrum represented by the thick red curve in the left panel is taken at $h\nu =707.9\mathrm{eV}$. (b) Second-derivative image of the RPES spectra, which are obtained by subtracting the off-resonant spectrum [black spectrum in (a) from the spectra in (a)]. (c) CIS spectra $E_B=1.0$, 3.6, and 6.6 eV. For comparison, XAS spectra before and after etching are also plotted. (d) Fitting of the PDOS spectrum taken at $h\nu =707.9\mathrm{eV}$. The solid gray, solid black, solid red, and solid green curves represent the PDOS spectrum, sum of fittings, an asymmetric Gaussian fitting, and a Gaussian fitting, respectively. Here, the PDOS spectrum is obtained by subtracting the off-resonant spectrum from the RPES spectrum taken at $h\nu =707.9\mathrm{eV}$.

Figure 6

Schematic energy diagrams of (a) (${\mathrm{Ga}}_{0.95},{\mathrm{Fe}}_{0.05})\mathrm{Sb}$ [33] and (b) (${\mathrm{Ga}}_{0.95},{\mathrm{Fe}}_{0.05}$)As.

Figure 7

Mg-Kα XPS spectra of $\left({\mathrm{Ga}}_{0.95},{\mathrm{Fe}}_{0.05}\right)\mathrm{As}$ and (Ga,Fe)Sb near the VBM at room temperature. Here, orange and blue markers represent the XPS spectra of $\left({\mathrm{Ga}}_{0.95},{\mathrm{Fe}}_{0.05}\right)\mathrm{As}$ and (Ga,Fe)Sb, respectively. The solid lines are the results of the linear fitting. The estimated positions of the VBM of $\left({\mathrm{Ga}}_{0.95},{\mathrm{Fe}}_{0.05}\right)\mathrm{As}$ and (Ga,Fe)Sb are $E_B=0.65\pm 0.16\mathrm{eV}$ and 0.29 ± 0.13 eV, respectively.

Figure 8

Orbital hybridization. (a) Schematic image of the orbital hybridization. (b) Isolate orbital-energy ratio $\left[\left({\alpha }_1-{\alpha }_2\right)/\beta \right]$ dependence of the energy splitting by the hybridization $\left(\mathrm{\Delta }E\right)$. Here, $\mathrm{\Delta }E$ is defined as the change of the energy difference between antibonding and bonding states caused by the hybridization.