Absence of Intrinsic Ferromagnetic Interactions of Isolated and Paired Co Dopant Atoms in ${\mathrm{Zn}}_{1-x}{\mathrm{Co}}_x\mathrm{O}$ with High Structural Perfection

We report element specific structural and magnetic investigations on ${\mathrm{Zn}}_{1-x}{\mathrm{Co}}_x\mathrm{O}$ epitaxial films using synchrotron radiation. Co dopants exclusively occupy Zn sites as revealed by x-ray linear dichroism having an unprecedented degree of structural perfection. Comparative magnetic field dependent measurements by x-ray magnetic circular dichroism and conventional magnetometry consistently show purely paramagnetic behavior for isolated Co dopant atoms with a magnetic moment of $4.8{\mu }_B$. However, the total magnetization is reduced by $\sim 30\%$, demonstrating that Co-O-Co pairs are antiferromagnetically coupled. We find no sign of intrinsic ferromagnetic interactions for isolated or paired Co dopant atoms in $\mathrm{Co}:\mathrm{ZnO}$ films.

Figure 1

(a) Measured and calculated XANES spectra at the Zn $K$ edge of ZnO for two perpendicular orientations of the linear polarization of the light. The measurements were taken at 300 K under 10° grazing incidence. (b) Respective calculated and measured XLD signatures.

Figure 2

(a) Measured and calculated XANES spectra at the Co $K$ edge of $\mathrm{Co}:\mathrm{ZnO}$ at 300 K and 10° grazing incidence. (b) Measured (full squares) and calculated XLD signatures of substitutional Co (open circles) and Co-O-Co pairs (open triangles).

Figure 3

(a) XANES and XMCD spectra recorded at the Co $K$ edge at 6.7 K. (b) Two $M(H)$ curves recorded at the photon energy of the Co preedge XMCD feature at 6.7 K (squares) and at 40.5 K (circles) together with fitted Brillouin functions (see text).

Figure 4

SQUID $M(H)$ curves of $\mathrm{Co}:\mathrm{ZnO}$ at 5 K and 300 K including fits using Brillouin functions for $L=3$ (dashed line) and $L=1.07$ (full line). Inset: Field cooled versus zero field cooled curves at 10 mT corroborating paramagnetic behavior.