We have characterized Zn1−xCoxO (x=0.25) films grown on sapphire (0001) substrates by pulsed laser deposition using various growth conditions to investigate the growth condition dependence of properties of Co-doped ZnO films. The substrate temperature (TS) was varied from 300 to 700 °C and the O2 pressure (PO2) from 10−6 to 10−1 Torr. When TS is relatively low (≲600 °C), homogeneous alloy films with a wurtzite ZnO structure are grown and predominantly paramagnetic, whereas inhomogeneous films of wurtzite ZnO phase mixed with rock-salt CoO and hexagonal Co phases form when TS is relatively high and PO2 is fairly low (≲10−5 Torr). The presence of Co clusters leads to room temperature ferromagnetism in inhomogeneous films. The temperature dependence of the magnetization for the homogeneous Zn1−xCoxO (x=0.25) films shows spin-glass behavior at low temperature and high temperature Curie–Weiss behavior with a large negative value of the Curie–Weiss temperature, indicating strong antiferromagnetic exchange coupling between Co ions in Zn1−xCoxO. We have found that Co can be dissolved in ZnO over 40% under an optimum growth condition of TS=600 °C and PO2=10−5 Torr, where epitaxial homogeneous Zn1−xCoxO (x=0.25) films of the best crystalline quality are obtained.
We have investigated the effects of rapid thermal annealing under vacuum on the CoFe-doped ZnO [Zn1−x(Co0.5Fe0.5)xO] films grown by reactive magnetron co-sputtering. At least up to x=0.15, the films have the single phase of the same wurtzite structure as pure ZnO. Ferromagnetism was observed for the CoFe-doped ZnO films. We found that rapid thermal annealing leads to a remarkable increase in the spontaneous magnetization of the CoFe-doped ZnO as well as the electron concentration. The annealing also leads to a significant increase in the Curie temperature (TC), resulting in room temperature ferromagnetism with TC>300 K for the CoFe-doped ZnO films.
The crystal structure~EantiferroelectricPb(Yb,,~Nb,,JO, has been characterized. Both x-ray diffraction and transmission electron diffraction show two sets of superlattice reflections origina!ing respectively from the B i t e atom ordering and the antiparallel leadatom displacements which suffer within the orthorhombica,-boplane along the a,direction.The room-temperature crystal symmetry is considered to be orthorhombic with space group Pbnm ( D : : ) and lattice parameters a. = 5.918 A, 6 , = 23.453 k. and c, = 8.221 A. Highresolution electron microscopy confirms the diffraction results. A structural phase transition from the lower-symmetry phase to the cubic paraelectric phase occurs at about 302 "Con heating.
We report on the growth of spinel ZnCo2O4 films using reactive magnetron sputtering and their electrical and magnetic properties, with particular emphasis on the relation of Curie–Weiss temperature (TCW) and conduction type. The conduction type and carrier concentration in these films were found to be dependent on the oxygen partial pressure ratio in the sputtering gas mixture. The highest electron and hole concentration at 300 K were 1.37×1020 and 2.81×1020 cm−3, respectively. A ferromagnetic coupling (TCW>0) was observable in p-type ZnCo2O4, whereas an antiferromagnetic interaction (TCW<0) was found for n-type and insulating ZnCo2O4, revealing hole-induced ferromagnetic transition in ZnCo2O4.
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