Electron paramagnetic resonance study ofZn1−xCoxO:A predict

Abstract

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...

Role of defects in ferromagnetism inZn1−xCoxO: A hybrid density-functional study

“…7-9͒ in which Co is substituted for Zn rather than forming clusters. These studies also show that nanocrystalline Zn 1−x Co x O is paramagnetic in this concentration range and at low temperature; 8,9 ferromagnetism was observed only at very low temperature. Another agent must therefore be responsible for promotion of high-T c ferromagnetism in Zn 1−x Co x O.…”

“…Recent magnetic resonance, [27][28][29][30][31][32] carrier mobility, 33 positron annihilation spectroscopy, 34 and first-principles calculations [35][36][37] have given a fuller picture of defect-induced electronic states in ZnO. Magnetic resonance techniques 8,9,38 have been applied to Zn 1−x Co x O and Zn 1−x Mn x O films and nanoclusters. As noted above, these studies 8,9,38,39 as well as x-ray magnetic circular dichroism, 12,39 which probe the local crystal structure about the transition-metal ion, have concluded that the transition-metal ions substitute for a Zn ion.…”

“…Magnetic resonance techniques 8,9,38 have been applied to Zn 1−x Co x O and Zn 1−x Mn x O films and nanoclusters. As noted above, these studies 8,9,38,39 as well as x-ray magnetic circular dichroism, 12,39 which probe the local crystal structure about the transition-metal ion, have concluded that the transition-metal ions substitute for a Zn ion. Epitaxial growth of thin films with ͑110͒ orientation has been achieved, 12 and there is considerable anisotropy of the magnetism in the thin film.…”

Probing the Local Coordination Environment for Transition Metal Dopants in Zinc Oxide Nanowires

“…[3][4][5] Even in systems that appear to be quite pure, reported magnetic behavior can vary widely, ranging from room-temperature ferromagnetism 6 to no magnetic ordering at all. 7,8 This variance in magnetism is attributed to the multitude of synthetic methods used for the production of DMS materials, which can result in differences in dopant environment, structural disorder, or carrier concentration. Therefore, to extract the most accurate information from any magnetic data on a DMS material, it is vital to perform a rigorous structural characterization.…”

Magnetic properties of single crystalline Zn1−xCoxO thin films

“…In particular, the theoretical predictions based on the local spin density approximation (LSDA), triggered extensive studies of ZnO :TM alloys with a special focus on ZnO :Co as the most promising candidate for a room-temperature ferromagnetic (FM) semiconductor [5]. Many experiments have been reported on this material fabricated by a variety of methods [6,7,8,9,10,11,12,13,14,15,16,17,18,19] ; however, the magnetic properties of ZnO :Co still remain a controversial issue since the observed magnetic behavior appears to be strongly dependent on the preparation methods and is poorly reproducible.…”