Effect of Co and O defects on the magnetism in Co-doped ZnO: Experiment and theory

The electronic structure of ${\mathrm{Zn}}_{1-x}{\mathrm{Co}}_x\mathrm{O}$ ($x=0.02$, 0.06, and 0.10) diluted magnetic semiconductors is investigated using soft x-ray emission spectroscopy and first-principles calculations. X-ray absorption and emission measurements reveal that most Co dopants are incorporated at the Zn sites and that free charge carriers are absent over a wide range of Co concentrations. The excess Co interstitials appear in the samples with high Co concentration ($10\mathrm{at.}\%$) but are isolated without any direct exchange interaction with substitutional Co atoms. The lack of free charge carriers and the direct Co-Co interactions is responsible for the absence of ferromagnetism in the samples. First-principles calculations suggest that the exchange interaction between substitutional Co atoms induces only an antiferromagnetic coupling, and strong ferromagnetism in Co-doped ZnO requires not only free charge carriers but also the Co interstitials directly interacting with substitutional Co atoms.

Figure 1

Co $2p$ x-ray absorption spectra of ${\mathrm{Zn}}_{1-x}{\mathrm{Co}}_x\mathrm{O}$ ($x=0.02$, 0.06, and 0.10). The spectra are normalized to the same peak height of the Co $L_3$ peak.

Figure 2

Zn $2p$ x-ray absorption spectra of ${\mathrm{Zn}}_{1-x}{\mathrm{Co}}_x\mathrm{O}$ ($x=0.02$, 0.06, and 0.10).

Figure 3

O $1s$ x-ray absorption spectra of ${\mathrm{Zn}}_{1-x}{\mathrm{Co}}_x\mathrm{O}$ ($x=0.02$, 0.06, and 0.10). The corresponding absorption spectra of ${\mathrm{Zn}}_{0.95}{\mathrm{Co}}_{0.05}\mathrm{O}$ and ZnO reference samples reported in Ref. 13 are added for comparison.

Figure 4

Co $2p$ x-ray absorption spectrum (top) and resonantly excited Co $L_{2,3}$ x-ray emission spectra of the corresponding sample. Labels in the emission spectra indicate the corresponding excitation energies shown in the XAS spectrum.

Figure 5

Co $L_{2,3}$ RIXS spectra of ${\mathrm{Zn}}_{1-x}{\mathrm{Co}}_x\mathrm{O}$ ($x=0.02$, 0.06, and 0.10), Co metal, and CoO samples resonantly excited at the Co $L_2$ edge.

Figure 6

(Color online) Various defect configurations in the ZnO wurtzite structure: (a) one $\mathrm{Co}\left(S\right)$ and two different nearest ${\mathrm{O}}_v$ defects, (b) two $\mathrm{Co}\left(S\right)$ atoms, (c) one $\mathrm{Co}\left(S\right)$ and two $\mathrm{Co}\left(I\right)$ atoms, and (d) three $\mathrm{Co}\left(S\right)$ and one $\mathrm{Co}\left(I\right)$ atoms.

Figure 7

Calculated unoccupied O $2p$ DOS: (a) for the model clusters with $\mathrm{Co}\left(S\right)$ and ${\mathrm{O}}_v$ defects, and (b) for the clusters with $\mathrm{Co}\left(S\right)$ and $\mathrm{Co}\left(I\right)$ defects.

Figure 8

Superposed XES spectra of Zn, Co, and O in the ${\mathrm{Zn}}_{0.90}{\mathrm{Co}}_{0.10}\mathrm{O}$ sample. All spectra are plotted on the common binding energy scale.

Figure 9

Calculated DOS of occupied Co $3d$ and O $2p$ states with $S$, $2S$, and $3S+I$ model configurations. The vertical lines are given to guide the eye when comparing experimental spectra and roughly show the main peak of O $2p$ and the center of gravity of Co $3d$ states.

Figure 10

Defect-formation energies as a function of the Fermi energy.