P 76 ISSRNS 2012: Abstracts / Synchrotron Radiation in Natural Science Vol. 11, No 1 – 2 (2012)
Co 3d STATES IN FERROMAGNETIC AND PARAMAGNETIC (Zn, Co)O FILMS – RESONANT PHOTOEMISSION STUDIES
E. Guziewicz∗, M.I. Lukasiewicz, K. Kopalko, L. Wachnicki, M. Godlewski Institute of Physics, Polish Academy of Sciences, Al. Lotnikow 32/46 , 02–668 Warsaw, Poland Keywords: synchrotron radiation, photoemission spectroscopy, zinc oxide, cobalt, semimagnetic semiconductors
∗e-mail : guzel@ifpan.edu.pl
Zinc oxide with cobalt has been recently in- tensively studied for prospective spintronic applica- tions. This is because high cobalt concentrations in ZnO matrix is expected due to the nearly identical Co2+ and Zn2+ ionic radii. Ferromagnetic ordering in ZnCoO films has been found and reported by sev- eral groups, but the origin of the ferromagnetism is still not fully clear. It has been related to formation of foreign phases, defects in the ZnO lattice, and Co metal precipitations inside the ZnCoO layer.
In this paper we present resonant photoemission results for a series of ferromagnetic and paramag- netic ZnCoO samples grown by the Atomic Layer Deposition (ALD) technique. The extensive struc- ture characterization show that the films are poly- crystalline and do not have any precipitations of cobalt oxides. In case of paramagnetic samples XANES and EXAFS studies indicate that most of Co2+ ions replace Zn2+ ions in the ZnO lattice, whereas in ferromagnetic layer about 20% of Co atoms are outside the substitutional positions and
form kind of metallic Co clusters [1]. It has been found that a ferromagnetic behavior is mostly re- lated to the Co-rich layer at the ZnCoO/Si interface [1, 2]. In addition to the ferromagnetic signal, par- ticularly well visible in thin FM samples, in all layers grown at high temperature (above 160◦C) a super- paramagnetic response has been observed. Based on the XPS profiling experiment this magnetization component has been assigned to metallic Co-rich nanoparticles that disperse over the film volume [1].
However, similar signal from Co-rich metallic inclu- sions has been observed for paramagnetic ZnCoO layers.
In the present study we investigated the Co3d contribution to the valence band electronic struc- ture of a series of ZnCoO samples with various mag- netic response. The Resonant Photoemission (RE- SPES) studies were directed to evaluate the Co3d contribution to the valence band of a series of Zn- CoO films. The experiment was performed at the I3 beamline at the MAX III storage ring at MAXlab,
Figure 1 : EDC spectra of the paramagnetic ZnCoO film with 10% of Co. The spectra were taken across the Co3p- Co3d resonance.
Figure 2 : EDC spectra of the ferromagnetic ZnCoO film with 6% of Co. The spectra were taken across the Co3p- Co3d resonance.
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ISSRNS 2012: Abstracts / Synchrotron Radiation in Natural Science Vol. 11, No 1 – 2 (2012) P 76 Sweden. Before the RESPES experiment each
ZnCoO/Si film was cleaned in the same way under a ultra-high vacuum condition (p ∼ 4 ∗ 10−10 Torr) using argon ions bombardment (U = 600 V, p(Ar)
= 5 ∗ 10−6 Tr, 40 minutes) and annealing to 200◦C (2h).
Photoemission spectra were recorded in the binding energy (BE) range between the Fermi level and 15 eV below. This BE region covers the ZnCoO valence band and the Zn3d core level. The overall resolution was evaluated as 200 meV at 50 eV pho- ton energy.
RESPES experiment is an effective tool for iden- tification of the Co3d contribution to the valence band electronic structure of the host ZnO material.
The resonant photoemission process is a result of interference between two photoemission paths that can be expressed as follows:
Co3p63d7+ h → [Co3p53d8]∗→ Co3p63d6+ e Co3p63d7+ h → Co3p63d6+ e
As a result we observe an increase of photoemis- sion intensity when photon energy is tuned to the Co3p → Co3d photo-ionization threshold.
We investigated a series of paramagnetic and fer- romagnetic ZnCoO samples. In Fig. 1 and Fig. 2 we present representative examples of photoemis- sion spectra taken across the Co3p →Co3d pho- toionization threshold. It was observed that En- ergy Distribution Curves (EDCs) of paramagnetic and ferromagnetic ZnCoO films differ considerably,
which indicate different Co3d states in ZnCoO lay- ers showing different magnetic behavior. For para- magnetic sample we found clear contributions of the Co3d states at BE 2 eV and 7 eV below the Femi level. Paramagnetic films show the RESPES en- hancement in the region between 5 eV and 7 eV be- low the Fermi level. We observed that resonant en- hancement of photoemission intensity does not scale with the cobalt content.
Acknowledgments: The research was partially sup- ported by the European Union within the European Regional Development Fund, through grant Innova- tive Economy (POIG.01.01.02-00-008/08). Synchrotron studies was supported by the European Community’s Seventh Framework Programme (FP7/2007-2013) un- der grant agreement n◦226716.
References
[1] M. Sawicki, E. Guziewicz, M.I. Lukasiewicz, et al., Phys. Rev B (under revision), arXiv: 1201.5268v1 [cond-mat.mtr1-sci] 25 January 2012
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