Origin of Ferromagnetism in Co-Implanted ZnO

Abstract

AbstractThe potential of room temperature ferromagnetism in many diluted magnetic semiconductor oxide systems has opened up a new route for realizing spintronic devices. Based on theoretical calculations and experimental observations, TiO2, in both the anatase and rutile phases, and ZnO have been extensively studied as a host matrix for transition metal doping, in particular with cobalt. In this work, the structural, magnetic, and electronic properties of Co-implanted n-type ZnO films grown on sapphire substrates have been studied in detail. X-ray diffraction and transmission electron microscopy results show the presence of a (\(10\overline{1}0\)) oriented hexagonal Co phase in the sapphire substrate, but not in the ZnO film. The diameter of the Co clusters is about 5–6 nm, forming a Co-rich layer in the substrate close to the ZnO/Al2O3 interface. However, the multiplet structure of the X-ray absorption spectra around the Co L 3 edge indicates that the implanted cobalt ions are in the Co2+ state in the ZnO film. Magnetization measurements show that there are two magnetic phases in the implanted region. One is the intrinsic room temperature ferromagnetism due to Co substitution on Zn sites in the ZnO layer, and the second magnetic phase originates from Co clusters in the sapphire substrate. The magnetic moment per substituted cobalt is about 2.81 μ B, which is very close to the theoretical expected value of 3 μ B/Co for Co2+ in its high spin state. Ferromagnetic resonance data show that the easy and hard axes have a periodicity of 60∘ in the film plane, in agreement with the hexagonal structure of the ZnO films. This sixfold in-plane magnetic anisotropy is attributed to the substitution of cobalt on Zn sites in the ZnO structure, and is a clear indication of long-range ferromagnetic ordering between substitutional cobalt ions in the single-crystalline ZnO films. Magnetic dichroism at the O K edge and the anomalous Hall effect are also observed in the Co-implanted ZnO films, supporting the intrinsic nature of the observed ferromagnetism.KeywordsSapphire SubstrateImplantation DoseRoom Temperature FerromagnetismAnomalous Hall EffectAsymmetry RatioThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.