The authors report ferromagnetism at room temperature in Co doped CeO2−δ thin films grown by pulsed laser deposition on SrTiO3 and Si substrates. On SrTiO3 ceria is epitaxied and displays a high crystalline quality. On Si the films are textured with a dominant orientation. While the ferromagnetism is little affected by the amounts of structural defects, it depends sensitively on the oxygen pressure during growth and annealing. This indicates that oxygen vacancies could be involved in the magnetic coupling between Co ions. Furthermore, the epilayers grown on SrTiO3 display a large magnetic anisotropy with an out of plane easy axis.
The magnetic properties of Co 2ϩ ions in epitaxial ͑Zn,Co͒O layers with 10% Co concentration have been studied by electron paramagnetic resonance spectroscopy. The Co-related EPR spectrum is characterized by a 200-G broad anisotropic single line with g factors close to those of the isolated Co 2ϩ ion. The temperature dependence of the EPR signal follows a Curie-Weiss law with a critical temperature of ϩ12 K. We find no evidence for a high-temperature ferromagnetic state. Magnetocrystalline anisotropy is observed and a canted spin arrangement is suggested.Developments in spin-transport electronics depend in large part on the elaboration of ferromagnetic semiconductors with Curie temperatures above room temperature. The diluted magnetic semiconductors ͑DMS's͒ in which a fraction of nonmagnetic elements is substituted by magnetic transition-metal ͑TM͒ ions are ideal candidates. By interaction between the localized magnetic moments and band electrons or holes, it is possible to control magnetic order by means of carrier concentration. 1-4 The wide-band-gap zincoxide-based DMS's attract currently considerable attention. On the basis of a carrier Fermi sea interacting with localized spins, high Curie temperatures have been predicted for ZnO with Mn and hole doping. 5 Recent ab initio calculations predict that incorporation of V, Cr, Fe, Co, or Ni in ZnO in the 5-25% concentration range should give rise to metallic behavior and a ferromagnetic state without the need for additional doping. 6,7 The latter modeling, based on a partially occupied TM d band, 8 shows that the down spin 3d states of the TM ions enter the conduction band and that ferromagnetic double exchange interaction predominates. Electron doping should enhance this phenomenon.Several experiments have been carried out in order to verify these predictions and to determine the Curie temperature T C as a function of the nature of the TM ion, the TM concentration, and the carrier concentration. Ferromagnetic features, with T C between 280 and 350 K, have been reported for Co ͑Refs. 9 and 10͒ and V ͑Ref. 11͒ -doped ZnO films, grown on ␣-Al 2 O 3 substrates. The magnetic properties were shown to depend strongly on the carrier density ͑n-type͒, required to be in the range 10 18 -10 20 cm Ϫ3 . 9,11 The growth conditions (O 2 pressure and substrate temperature͒ as well as thermal annealing have also been found to influence the electron concentration, the magnetization, and the Curie temperature. 12,13 In particular, growth temperatures above 600°C can produce inhomogeneous Zn 0.75 Co 0.25 O films with ferromagnetic Co clusters and reduced electrical resistivity.Presently, there is a lack of information on the sites occupied by the TM ions in the ZnO host, as well as on the magnetic state of these ions, since all experiments were based on the macroscopic magnetization of the films. It is of fundamental importance to determine the electronic state of the TM ions in a high concentration range in order to understand the interplay between carrier concentration and magnet...
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