Ferromagnetic Behavior above Room Temperature of Fe-Ion-Implanted 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 × 10 ions/cm 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 ZnO2. 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 (∆M ) 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.