(a) SQUID curves at 300 K of a 1% Co doped ZnO thin film compared to a nominally undoped ZnO film. Both samples have been annealed in Zn at for 1 h. The inset magnifies the low-field behavior of both samples at 300 K. (b) Depiction of the remanent magnetization (, filled squares) and the temperature below which the field-cooled and zero-field-cooled curves are separated (asterisks). The fused silica substrate itself as well as undoped, Co or Zn-interstitial doped samples show similar effects.
(a) Absorption and (b) MCD spectra of in the near infrared (left) and visible (right) spectral range below the band gap.
(a) Level scheme of the crystal field transitions of in ZnO in the visible and near infrared spectral range. The sign of the MCD transitions () or (−) is obtained by use of the Kastler diagrams in (b).
Normalized intensity of the magnetic field and temperature dependent measurements of the MCD detected on the crystal field transitions of at 0.9 and 2.10 eV and the near band edge MCD in 5% Co doped ZnO. Data are normalized to unity at maximum field to show their similarity.
Change in the midinfrared absorption due to annealing of the films in Zn vapor. The inset shows the related increase in conductivity.
Low temperature EPR spectrum of ZnO with 10% Co and a calculated spectrum (dashed line) using and . The inset shows the temperature variation of the EPR intensity. The drawn line was calculated assuming a thermally excited state higher in energy.
Magnetoresistance of samples at different temperatures; (a) , (b) , (c) , and (c) , respectively.
(a) Band structure scheme for the impurity band in ZnCoO without an external magnetic field. [(b)–(d)] Influence of an external magnetic field on the band structure of ZnCoO. At small magnetic fields and low doping concentrations (a) the magnetic field reduces the DOS which leads to a positive MR. (b) At higher fields the DOS increases again and a negative MR is observed. (c) High Co doping leads to a broadening of the impurity band. The band splitting results in an increasing DOS and therefore in a negative MR.
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