Correlated substitution in paramagnetic ${\text{Mn}}^{2+}$-doped ZnO epitaxial films

Epitaxial films of ${\text{Mn}}^{2+}$-doped ZnO were deposited by pulsed laser deposition on $\alpha {\text{-Al}}_2{\text{O}}_3\left(0001\right)$ using targets created from ${\text{Mn}}^{2+}$-doped ZnO nanoparticles. Using x-ray absorption spectroscopy and x-ray magnetic circular dichroism, Mn(II) was found to substitute for Zn(II) in the würtzite ZnO lattice with only a paramagnetic dichroic component from the Mn and no magnetic component from either the O or Zn. The dichroism reveals that, while substitutional, the ${\text{Mn}}^{2+}$ distribution in the ZnO lattice is not stochastic. Rather, ${\text{Mn}}^{2+}$ has a tendency to substitute with higher effective local concentrations than anticipated from a stochastic doping model.

Figure 1

TEM of ${\text{Mn}}_{0.05}{\text{Zn}}_{0.95}\text{O}/{\text{Al}}_2{\text{O}}_3\left(0001\right)$ deposited from colloidal ${\text{Mn}}^{2+}:\text{ZnO}$ nanoparticle target. (a) Low magnification micrograph showing a smooth film comprised of (b) columnar grains labeled nano1, nano2, and nano3. (c) High-resolution micrograph of ${\text{Mn}}_{0.05}{\text{Zn}}_{0.95}\text{O}/{\text{Al}}_2{\text{O}}_3$ interface.

Figure 2

(Color online) Mn $K$-shell XANES showing similarity between representative PLD grown Mn:ZnO film (thick red) and Mn:ZnO nanoparticles (thick blue) along with Mn standards. Near-edge spectra from bottom to top are todorokite $\left[{\left(\text{Mn},\text{Mg},\text{Ca},\text{Ba},\text{K},\text{Na}\right)}_2{\text{Mn}}_3{\text{O}}_{12}\cdot 3{\text{H}}_2\text{O}\right]$, Mn ferrite $\left({\text{MnFe}}_2{\text{O}}_4\right)$, Mn:ZnO nanoparticles, Mn:ZnO film, rhodochrosite $\left({\text{MnCO}}_3\right)$, and Mn metal. Curves have been offset for clarity.

Figure 3

(Color online) Fourier transform of the $\text{Mn}K$-shell EXAFS of a PLD grown ${\text{Mn}}_{0.05}{\text{Zn}}_{0.95}\text{O}$ film (red dotted), ${\text{Mn}}_{0.05}{\text{Zn}}_{0.95}\text{O}$ nanoparticles (blue dashed-dot), and the $\text{Zn}K$-shell EXAFS of ZnO powder showing substantial Mn substitution within the ZnO lattice.

Figure 4

(Color online) (a) Low-temperature in-field x-ray absorption (solid) and dichroism (dashed) at the $\text{Mn}{L}_3$ edge for ${\text{Mn}}_{0.05}{\text{Zn}}_{0.95}\text{O}$ (blue), ${\text{Mn}}_{0.025}{\text{Zn}}_{0.975}\text{O}$ (red), and ${\text{Mn}}_{0.002}{\text{Zn}}_{0.998}\text{O}$ (green) at 5 Tesla. (b) Low-temperature x-ray absorption (solid) and dichroism (dashed) for 5% Mn-doped ZnO film. The in-plane magnetic field values range from 0 T (bottom) to 5 T (top) in steps of 1 T. The dichroism spectra have been scaled by a factor of 10 with respect to the x-ray absorption spectra.

Figure 5

(Color online) Temperature dependence of the maximum of the $\text{Mn}{L}_3$ dichroism on a per-Mn basis for the 0.2% Mn:ZnO (triangles), 2.5% Mn:ZnO (squares), and 5% Mn:ZnO (circles) thin films. Lines are drawn to guide the eyes.

Figure 6

(Color online) Brillouin fits (solid) and expected normalized Mn magnetization (dashed) assuming stochastic distribution of Mn ions for ${\text{Mn}}_{0.002}{\text{Zn}}_{0.998}\text{O}$ (green triangles), ${\text{Mn}}_{0.025}{\text{Zn}}_{0.975}\text{O}$ (red squares), and ${\text{Mn}}_{0.05}{\text{Zn}}_{0.95}\text{O}$ (blue circles).