Soft-x-ray spectroscopic investigation of ferromagnetic Co-doped ZnO

Abstract

The electronic properties of cobalt-doped ZnO were investigated through site-selective and element-sensitive x-ray-absorption spectroscopy in the vicinity of the Co L 2,3 edge, the oxygen K edge, and at the Zn L 3 edge. The spectroscopic measurements of the ferromagnetic cobalt-doped ZnO films appear to have additional components in the O K edge x-ray-absorption spectrum not observed in the undoped films. The observed features may derive from both hybridization with unoccupied Co 3d states and also from lattice defects such as oxygen vacancies. Only minor changes in the Zn L 3 edge spectra were observed. These observations are consistent with a polaron percolation model in which the ferromagnetic coupling is mediated by shallow donor electrons trapped in oxygen vacancies and couples the Co atoms substituted on Zn sites in the hexagonal wurtzite ZnO structure. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2165916͔ Dilute magnetic semiconductors ͑DMS͒ are an especially interesting area of research because of their potential applications in spin electronics and magneto-optics. DMS based on the wide-band-gap semiconductor ZnO doped with a transition metal have been predicted theoretically to be good candidates for room-temperature ferromagnetism. 1 Subsequently Ueda et al. 2 reported ferromagnetism in Codoped ZnO. Following their work many conflicting reports have attributed the origin of ferromagnetic behavior as being due to substitution by Co atoms in the ZnO 2-6 or due to clustering of Co atoms in secondary phases that are ferromagnetic; 7 while others report no ferromagnetic behavior even though Co occupies the substitutional sites. 8 Three different mechanisms have been proposed to explain the origin of ferromagnetism in transition-metal-doped ZnO: ͑I͒ A model of ferromagnetism where there is an exchange interaction mediated by carriers in the valence or conduction band and the localized moment of the ion, 1 ͑II͒ a double exchange mechanism in which hopping of 3d electrons from one ion to the next results in ferromagnetic behavior. 9 , and ͑III͒ an impurity band model 10 where localized ionic moments create magnetic polarons in a defect-related donor impurity band. Clearly, it is desirable to find a way to distinguish between these models and to decide whether any of them is the correct model for doped ZnO systems. Hence, it is important to verify that the transition-metal dopants occupy the Zn site and that ferromagnetism can result. In light of earlier reports such as that of Wi et al. 8 substitutional cobalt may be a necessary but not sufficient condition for ferromagnetism. This realization may reconcile some of the apparently conflicting experimental reports.In this report, element specific soft-x-ray-absorption techniques were used to help address the origin of roomtemperature ferromagnetism in Co-doped ZnO grown by pulsed laser deposition ͑PLD͒. The power of x-rayabsorption spectroscopy ͑XAS͒ at the Co L 2,3 edge allows the identification of the oxidation state and site symmetry of the cobalt i...

Defect induced low temperature ferromagnetism in Zn1−xCoxO films

“…Figure 7 shows the comparison of O K edge XAS spectra from the undoped reference ZnO sample, the Zn 0.9 Co 0.1 O sample S1, and the Zn 0.9 Co 0.1 O sample S2. The spectral line shape of the O K edge agrees well with the XAS spectra obtained from ZnO powder, 41 PLD grown Co-doped ZnO, 42 and chemical vapor deposition prepared samples. 43 The region between 530 and 538 eV can be attributed to the hybridization between O 2p and Zn 4s states followed by the region between 539 and 550 eV which is due to the hybridization between O 2p and Zn 4p states.…”

Defect induced ferromagnetism in Co-doped ZnO thin films

“…Figure 2 (d) shows the comparison of O K edge XAS spectra from the undoped reference ZnO sample, the Zn 0.9 Co 0.1 O sample S1, and the Zn 0.9 Co 0.1 O sample S2. The spectral line shape of the O K edge agrees well with the XAS spectra obtained from ZnO powder [18], PLD grown Co-doped ZnO [19], and CVD prepared samples [20]. The region between 530 and 538 eV can be attributed to the hybridization between O 2p and Zn 4s states followed by the region between 539 and 550 eV which is due to the hybridization between O 2p and Zn 4p states.…”

“…Above 550 eV the spectrum can be assigned to the hybridization between O 2p and Zn 4d states. The broadening of the spectral features observed in the S1 sample in comparison with the sample S2 as well as to the undoped reference ZnO sample at 537, 540 and 543 eV were recently assigned to the presence of the oxygen vacancies [19] showing that the oxygen-related defect concentration in the sample S1 is higher than in sample S2. Zn L 2,3 XAS spectra shown in the inset of Fig.…”

Spectroscopic investigation of an intrinsic room temperature ferromagnetism in Co doped ZnO nanoparticles

“…The intensity of peak at 536.7 eV decreases with increase in 'Co' concentration (see top-inset figure) and the broadening of other peaks with energy greater than around 538 eV. Similar features and broadening were reported in [28,31]. These broadenings are attributed to the presence of 'O' vacancies and 'Co' doping.…”

“…Based on the literature [26,27], the spectral features are assigned as follows. In the case of both pure and Co doped ZnO: (i) the features in the energy region between 532 and 539 eV are assigned to the hybridization of O 2p with highly dispersive Zn 3d4s/Co 3d states, forming the bottom of the conduction band with prominent peak at 536.7 eV due to the transition of O 1s electrons to more localized (non-dispersive) O 2p states (2p z and 2p x+y ) [28], (ii) the features between 539 and 547 eV are assigned for the transition to the states formed due to the hybridization between O 2p states with Zn 4p and Co 4p states and (iii) the spectral features above 547 eV is emerged due to the hybridization of O 2p states with the extended higher orbitals of 'Zn' and 'Co' [29]. As 'Co' concentration increases in ZnO, there is an increase in the intensity of the pre-edge peak at 531.6 eV (see bottom-inset figure).…”