P 28 ISSRNS 2012: Abstracts / Synchrotron Radiation in Natural Science Vol. 11, No 1 – 2 (2012)
ELECTRONIC STRUCTURE AND MAGNETISM OF (Zn,Co)O FILMS:
A SOFT X-RAY SPECTROSCOPY STUDY
I.A. Kowalik1∗, M.I. Lukasiewicz1, E. Guziewicz1, M. Godlewski1, F.J. Luque2, M.A. Nino3, A. Zakharov4, and D. Arvanitis5
1Institute of Physics, Polish Academy of Sciences, al. Lotnik´ow 32/46, 02–668 Warsaw, Poland
2Depto. F´ısica Materia Condensada, University Aut´onoma de Madrid, E–28049, Madrid, Spain
3IMDEA; Faculdad de Ciencias Modulo C-IX, Madrid, Spain
4MAX-Lab; Lund University, P.O. Box 118, SE–22100 Lund, Sweden
5Deptartment of Physics and Astronomy, Uppsala University, P.O. Box 516, 751 20 Uppsala, Sweden Keywords: synchrotron radiation, soft x-rays, XAS, XMCD, PEEM
∗e-mail : ikowalik@ifpan.edu.pl
ZnCoO is an important model material for spin- tronics applications. Here we exploit the element specificity of soft x-ray based spectroscopy and mi- croscopy to characterize spontaneous nanocrystals formation at the interface of ZnCoO films grown on Si substrates. The samples are grown by means of Atomic Layer Deposition. Selected samples are subsequently sputtered, leading to a crater in the central region, to allow for a particular area of the ZnCoO close to the Si interface to be exposed.
Here we show in situ results for a sample at both the crater region as well as outside, under quasi- identical experimental conditions. X-ray Absorp- tion Spectroscopy (XAS) and X-ray Magnetic Cir- cular Dichroism (XMCD) measurements were per- formed at the EPU based I1011 beamline, at the MAX-lab synchrotron radiation facility in Lund, Sweden [1]. The measurements were performed in the total electron yield (TEY) mode. The presented spectra were taken in the non sputtered region as well as in the Si/ZnCoO interface region within the crater obtained by ion sputtering. These results are complemented by means of X-PEEM at the beam line I311 of MAX-lab.
By using the elemental specificity of XAS the composition of the ZnCoO sample is probed for these two sample regions [2]. The sample measured as grown in the first few nanometers contains about 6% of Co ions, however the amount of Co is inho- mogeneous. The fine structure of the Co white lines exhibits differences for the as grown sample, as a function of the depth from the sample outer surface.
The Co L-edges for the as grown film and around the crater show the typical multiplet shape observed for ZnCoO magnetically diluted samples [3]. However within the crater both the Co L-edge and O K-edges exhibit differences and indicate both a different stoi- chiometry as well as different electronic state for the O and Co ion cores probed by XAS. The shape of XAS spectra obtained for the Co L-edge indicates that in the interface region the metallic Co is the dominant phase but not the only one, we observe the superposition with some CoO [4]. The differ- ence in the electronic state of the Co atoms in the surface and the interface region manifests also in the large variation in the number of holes of Co atoms in these two regions of the sample, illustrated as a
difference in the area under the L3XAS white line.
The number of holes for Co atoms in the surface re- gion is about 7, in the crater is about 3.5, when for metallic Co this value is 2.8 [2]. This result confirms the difference in the hybridization of Co with neigh- boring atoms. Inside the crater we not only observe a small amount of Co but also a small amount of Zn. We observe also that the electronic state of the Zn atoms in the interface region is different than in the surface region.
A small XMCD dichroic difference is found in the interface region at room temperature, when a magnetic field of 350 Gauss is applied in the sur- face plane. The dichroic response is much more pronounced at the L3 white line indicating that the orbital moment carried by the Co atoms is much stronger than for Co bulk. By means of the X-PEEM measurements Co rich nanocrystals are identified. A discussion of the XAS, XMCD, and X-PEEM data both for the outer interface and the inner interface will be presented. The formation of nanocrystals in the inner interface evidenced here may be of general character and therefore allow to clarify discrepancies of the magnetic response of ZnCoO films in the literature.
Acknowledgments: We acknowledge the Swedish Re- search Council, the EC Seventh Framework Programme (FP7/2007 – 2013) under grant Nr 226716 (ELISA) for access to MAX-lab and the European Regional De- velopment Fund, through Grant Innovative Economy (POIG.01.01.02-00-008/08).
References
[1] I.A. Kowalik et al., “Description of the new I1011 beamline for magnetic measurements using synchrotron radiation at MAX-lab,” J. Phys.: Conf.
Ser. 211 (2010) 012030.
[2] M. Sawicki et al., “Homogenous and heterogeneous magnetism in (Zn,Co)O,” arXiv:1201.5268.
[3] M. Kobayashi et al., “Antiferromagnetic interaction between paramagnetic Co ions in the diluted mag- netic semiconductor Zn1−xCoxO,” Phys. Rev. B 81 (2010) 075204.
[4] A.M. Mulders et al., “On the interface magnetism of thin oxidized Co films: Orbital and spin moments,”
J. Phys.: Condens. Mat. 21 (2009) 124211.
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