Identifying local dopant structures and their impact on the magnetic properties of spintronic materials

The local structures of transition-metal dopant atoms in semiconductor host lattices of dilute magnetic semiconductors are determined using resonant inelastic x-ray scattering. Ligand field multiplet calculations are employed to analyze experimental data and show when dopant atoms substitute into the lattice and when they form metallic or oxide clusters. For ZnO doped with Mn, low-temperature ferromagnetism is shown to be intrinsic, but can be quenched by the formation of MnO clusters. For CeO${}_2$ doped with Co, room-temperature ferromagnetism is due to clusters of Co, while substitution into the lattice leads to paramagnetic behavior.

Figure 1

Experimental and calculated Mn $L$-edge spectra for ZnO:Mn. The top-left panel shows XAS spectra for one sample. The upper spectrum with dots is experimental and the lower smooth spectrum is calculated. The right panel and bottom-left panel show $L_2$ and $L_3$ RIXS, respectively. In each, the black spectrum with circular dots is experimental MnO RIXS, the blue (dark gray) spectrum with square dots is experimental Zn${}_{0.67}$Mn${}_{0.33}$O, the blue (dark gray) smooth spectrum is calculated with $10\mathit{Dq}=0.95$ eV, the red (gray) spectrum with dots is experimental Zn${}_{0.89}$Mn${}_{0.11}$O, and the red (gray) smooth spectrum is calculated with $10\mathit{Dq}=-0.50$ eV.

Figure 2

Contour plots showing the evolution of the RIXS spectra of Mn as the crystal field changes. The top panel shows $L_3$ RIXS intensity for varying crystal-field strengths, and the bottom panel shows the same for $L_2$ RIXS. The white lines show the best fits for the octahedral ($10\mathit{Dq}=0.95$ eV) and tetrahedral ($10\mathit{Dq}=-0.50$ eV) cases.

Figure 3

Experimental and calculated Co $L$-edge spectra for CeO${}_2$:Co. The top-left panel shows XAS spectra for one sample, where the blue (dark gray) line with dots is experimental and the red (light gray) smooth line is calculated. The top-right panel shows the calculated change in RIXS spectral intensity for a varying ligand field strength, $10\mathit{Dq}$. The two bottom panels contain experimental and calculated RIXS spectra for the different samples at excitation energies $A$ (upper) and $B$ (lower). The bottom-left panel shows the RIXS for the paramagnetic samples $3\%\hspace{0.5em}H$ and $5\%\mathit{LP}$ (described in text) plotted in blue (dark gray) with large and small dots, respectively. The red (light gray) smooth line shows the best-fit calculation. The bottom-right panel shows the experimental spectra for the ferromagnetic sample, $3\%\mathit{HP}$, plotted in blue (dark gray) with large dots, as well as best-fit spectra plotted in red (light gray) with open circles. The best-fit spectra are a summation of the $3\%\hspace{0.5em}H$ and metallic Co spectra shown below them.