Role of Defect Sites and Ga Polarization in the Magnetism of Mn-Doped GaN

We report a study of the Mn local structure, magnetism, and Ga moments in molecular beam epitaxy grown Mn-doped GaN films. Using x-ray absorption spectroscopy and magnetic circular dichroism, we find two distinct Mn sites and a Ga moment antiparallel to Mn. First-principles calculations reproduce this phenomenology and indicate that Mn preferentially populates Ga sites neighboring N split interstitial defects. These results show that defects may strongly affect the Mn ordering and magnetism, and that the GaN valence band is polarized, providing a long-range ferromagnetic ordering mechanism for ${\mathrm{Ga}}_{1-x}{\mathrm{Mn}}_x\mathrm{N}$.

Figure 1

X-ray absorption spectroscopy (blue curves) and magnetic circular dichroism (red curves) at the Mn $L_{3,2}$ edges for each Mn concentration, taken at 4.2 K in a $+2\mathrm{T}$ field. Feature $A$ on the primary $L_3$ peak is consistent with the ${\mathrm{Mn}}^{2+}$ calculation. The intensity of feature $B$, $\sim 0.7\mathrm{eV}$ above $A$, scales with Mn doping. The dotted curve superimposed on the 4.5% XAS is a two-site Mn $3d^5$ high spin atomic calculation with no crystal field splitting. Inset: doping dependence of the ratio of the intensities of the two features.

Figure 2

Temperature dependence of the XMCD intensity for the two features in the 1.5 and 3.6% Mn samples. Inset: doping dependence of the Curie temperature associated with feature $B$, derived from the XMCD data. Arrows above the two highest Mn concentration points indicate that the Curie temperature is above 300 K, the highest temperature for which XMCD was measured.

Figure 3

XAS (blue) and XMCD (red) at the Ga $L_3$ edge in a $+5\mathrm{T}$ field at 4.2 K. The sign of the initial dichroism suggests a Ga moment that is antiparallel to the Mn $3d$ moments. The dashed line is an estimate of the area under the XMCD curve based on a fit (dotted line) to two Gaussians of opposite sign.

Figure 4

Upper left: calculated local structure of a Mn (green) atom next to an N (red) split interstitial defect. (Ga atoms are in blue.) (a) Ground state unoccupied Mn DOS for different defects. (b) Experimental XAS spectra (colored) for different Mn concentrations, compared to calculations in the presence of core holes for substitutional ${\mathrm{Mn}}_{\mathrm{Ga}}$, for one and two Mn atoms next to a N split interstitial (bottom three spectra), and for different superpositions of these spectra (top two calculated). The energy scale is set using total energy differences, and the experimental spectra are shifted to match the most intense $L_3$ peaks with the simple ${\mathrm{Mn}}_{\mathrm{Ga}}$ calculation. The spectra for the substitutional and N split interstitial defects were calculated using 96 atom cells; all others used 32 atom cells.