Origin of the Anomalous Magnetic Circular Dichroism Spectral Shape in Ferromagnetic ${\mathrm{Ga}}_{1-x}{\mathrm{Mn}}_x\mathrm{As}$: Impurity Bands inside the Band Gap

The electronic structure of a prototype dilute magnetic semiconductor (DMS), ${\mathrm{Ga}}_{1-x}{\mathrm{Mn}}_x\mathrm{As}$, is studied by magnetic circular dichroism (MCD) spectroscopy. We prove that the optical transitions originated from impurity bands cause the strong positive MCD background. The MCD signal due to the $E_0$ transition from the valence band to the conduction band is negative indicating that the $p\mathrm{\text{−}}d$ exchange interactions between the $p$ carriers and $d$ spin is antiferromagnetic. The negative $E_0$ MCD signal also indicates that the hole doping of the valence band is not so large as previously assumed. The impurity bands seem to play important roles for the ferromagnetism of ${\mathrm{Ga}}_{1-x}{\mathrm{Mn}}_x\mathrm{As}$.

Figure 1

MCD spectrum of paramagnetic ${\mathrm{Ga}}_{1-x}{\mathrm{Mn}}_x\mathrm{As}$ ($x=0.004$) at 6 K and 1 T. Reported energies [21] of the critical points of GaAs are shown by broken vertical lines. The inset shows the MCD spectrum of paramagnetic ${\mathrm{Cd}}_{1-x}{\mathrm{Mn}}_x\mathrm{Te}$ ($x=0.08$) [8] at 15 K and 1 T. The horizontal scales are adjusted so that the $E_0$ and $E_0+{\Delta }_0$ positions of the two materials coincide.

Figure 2

MCD spectra, at 6 K and 1 T, of ${\mathrm{Ga}}_{1-x}{\mathrm{Mn}}_x\mathrm{As}$ samples with various Mn contents. The spectra of paramagnetic samples ($x=0.004$ and 0.011) are magnified fourfold for clarity. A MCD spectrum reported by Ref. 12 is shown by a dotted curve in an arbitrary unit vertical scale.

Figure 3

Schematic decomposition of the anomalous positive MCD signal around the absorption edge of ${\mathrm{Ga}}_{1-x}{\mathrm{Mn}}_x\mathrm{As}$. The negative $E_0$ peak, positive $E_0+{\Delta }_0$ peak, and negative $L$ signal are due to transitions between the valence band and conduction band. The broad and positive MCD signal is due to transitions to impurity bands (IBs).

Figure 4

Mn-content dependence of the photon energies corresponding to the minimum and maximum of the MCD intensity at 6 K. The data from Ref. 12 are also shown by open circle and box. Reported energies [21] of the critical points of GaAs are shown by horizontal lines.