Magnetic nanoparticle formation in Fe implanted ZnO

Low temperature doping with transition metals is currently accepted as the “silver bullet” for the creation of diluted magnetic semiconductors (DMS). Among them, ion implantation offers the possibility for doping at almost any temperature. However, tiny, hardly detectable ferromagnetic nanoparticles created due to phase separation already during implantation can wrongly give the impression of a DMS by means of a pronounced integral magnetization. In this work we would like to present systematic investigations on the formation of ferromagnetic nanoparticles due to 57Fe implantation into ZnO single crystals applying several sensitive analysis techniques. At an implantation temperature of 623 K and implantation energy of 180 keV, a distinct dependence of the nanoparticle formation on the Fe ion fluence was observed. Below a fluence of 2x1016 cm-2, all Fe is found in nonmagnetic charged states indicating no -Fe-nanoparticle formation as was confirmed using X-ray diffraction. From a fluence of 2x1016 cm-2, superparamagnetic -Fe-nanoparticles are present. They were evidenced by zero-field cooled (ZFC) and field cooled (FC) magnetization curves, x-ray diffraction and transmission electron microscopy. The superparamagnetic blocking temperatures increase with the implanted fluence. An annealing at 823 K for 15 min enhanced the Fe particle formation, but Fe nanoparticles were oxidized after an annealing at 1073 K for 15 min. A further annealing at 1073 K for 3.5 h induced the formed of ZnFe2O4. In a conclusion, we definitely rule out the formation of a diluted magnetic semiconductor in Fe-implanted ZnO at process temperature above 623 K.