We present a magneto-optical study of the coupling between Manganese spin and band carriers in wide bandgap diluted magnetic semiconductors (DMS). We use reflectivity, transmission and photoluminescence. We analyze similarities and differences between the giant Zeeman effect in (Ga,Mn)N and in (Zn,Mn)O.
ZnO and GaN are wide bandgap semiconductors with the wurtzite structure, with a weak spin-orbit coupling and a strong electron-hole exchange interaction within the excitons. This combination was an origin of controversies in the determination of basic material parameters. It results also in a complex behavior of the giant Zeeman effect of A and B excitons in the presence of magnetic impurities. It was shown for (Zn,Co)O [1], that not only the transition energies, but also the oscillator strengths are strongly affected by spin-carrier coupling.
We studied (Ga,Mn)N epilayers (grown by MBE), in which most of the Mn ions are in the d⁴ electronic configuration (Mn³⁺) with spin S=2 [2]. In the wurtzite structure, spin-orbit coupling in Mn³⁺ leads to a significant spin anisotropy, with the c-axis as a hard axis. We analyzed the circular polarization causes by the giant Zeeman effect, and we find a ferromagnetic p-d exchange. It is the same sign as in the case of Cr²⁺ in II-VI DMS's - another example of the d⁴ electronic configuration.
In ZnO, as in other II-VI's, Mn ions are in Mn²⁺ state with spin 5/2 and zero orbital momentum. Then the anisotropy is negligible. Thus, the temperature and magnetic-field dependence of the giant Zeeman effect in (Zn,Mn)O follows the usual Brillouin function. Assuming the "normal ordering" for the valence band in (Zn,Mn)O samples grown by MOCVD, the p-d exchange is found to be antiferromagnetic for Mn²⁺, as for Co²⁺ [1].
[1] W.Pacuski et al., Phys. Rev. B 73, 035214 (2006)
[2] S.Marcet et al., cond-mat/0604025 (2006)