Ferromagnetic behavior above room temperature of Fe-ion-lmplanted ZnO

Zinc-oxide (0001) single crystals with a 0.5 mm thickness were prepared, and then 80 keV Fe ions with a dose of 3 × 10¹⁶ ions/cm ² were implanted into the ZnO single crystals at 350°C The implanted samples were post-annealed at 700, 800 and 900°C by rapid thermal annealing in an N₂ atmosphere for 5 min to remove the ion-implantation damage. The structure and the magnetic properties of Fe-ion-implanted ZnO were investigated by using X-ray diffraction and a superconducting quantum interference device magnetometer. The carrier transport properties were measured in the range from 5 K to 300 K. The X-ray diffraction results showed an island peak near 37°C, which was identified as ZnO ₂. The magnetization curve showed hysteresis loops at 5 K in the samples annealed at 700 and 800°C, showing ferromagnetism. Hysteresis loops were observed at 300 K in the samples annealed at 700°C The temperature dependence of the magnetization was taken in field cooling (FC) and zero field cooling (ZFC) in the samples annealed at 700°C The difference magnetization (ΔM) between the FC and the ZFC magnetizations did not converge to a zero value even at 340 K, showing ferromagnetism above room temperature. The magnetoresistance (MR) curve showed negative curvature up to 150 K which could be explained with the delocalized d state of ZnO implanted with Fe ions.