Electron paramagnetic resonance in transition metal-doped ZnO nanowires

The wide-band-gap zinc oxide-based diluted magnetic semiconductors currently attract considerable attention due to their possible use in spintronic devices. In this work, we studied ZnO nanowire samples synthesized on $10×10mm2$ $a$-plane sapphire substrates by high-pressure pulsed laser deposition. The samples were characterized by scanning electron microscopy (SEM) and electron paramagnetic resonance (EPR) in the $X$-band (⁠$≃9.3$ GHz) from $T=4$ to 300 K. According to the SEM pictures, the nanowires exhibit a length of about $1μm$ and are aligned perpendicular to the substrate surface. The structures have a hexagonal cross section and their diameter ranges from 60 nm up to 150 nm. For the lowest nominal concentrations of $xMn=3$ at. % and $xCo=5$ at. %, we detect the anisotropic EPR spectra of isolated $Mn2+$ (⁠$3d5$⁠, $S6$⁠) and $Co2+$ (⁠$3d7$⁠, $F4$⁠), respectively, on Zn sites. The detection of the well-resolved anisotropic spectra proves a coherent crystallographic orientation of the nanowires. The linewidth was larger than the best values reported in the literature. Nevertheless, it was possible to identify two different components, $A$ and $B$⁠, of the reported spectra. From the temperature dependence of the EPR intensity, we found that both components exhibit paramagnetic behavior and are present in a concentration ratio of $NB∕NA=1.4$⁠. In the case of the Mn-doped ZnO wires, the linewidth increases with increasing Mn concentration due to the dipole-dipole interaction of the paramagnetic ions. At the highest used nominal concentration, $xMn=10$ at. %, an additional broad single line is observed.