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Walther-Meißner-Institut (WMI), Bayerische Akademie der Wissenschaften
Chair for Technical Physics (E23), Technische Universität München

The nature of ferromagnetism in cobalt-doped ZnO


July 05, 2008


SPP1285The rapidly emerging field of spintronics requires material systems combining ferromagnetism (FM) with the versatile electronic properties of semiconductors. Diluted magnetic semiconductors (DMS) such as Mn-doped InAs or GaAs, where FM ordering of the magnetic dopants is mediated by mobile charge carriers, are very attractive in this regard. Unfortunately, these well established DMS have Curie temperatures TC < 175 K, preventing room temperature (RT) applications. In contrast, TC > 300 K has been predicted for cobalt-doped ZnO and ferromagnetic behavior has been reported. However, there is an ongoing debate on whether this material is really a DMS or the observed behavior is caused by magnetic nanometer-sized precipitates of the Co dopant atoms embedded in a nonmagnetic ZnO matrix.

To unambiguously clarify the nature of ferromagnetism in ZnO:Co thin films a careful and systematic study has been performed by a team of researchers in the group of Prof. Gross (Walther-Meißner-Institut, Bayerische Akademie der Wissenschaften and Physik-Department, Technische Universität München) in collaboration with scientists of the University of Bonn and the ESRF at Grenoble. The study has been supported by the German Science Foundation (DFG) within the Priority Program 1285 on "Semiconductor Spin Electronics" and the Excellence Initiative via the "Nanosystems Initiative Munich (NIM)". In this study SQUID magnetometry, x-ray magnetic circular dichroism (XMCD), and AC susceptibility measurements have been combined with careful x‑ray and high resolution TEM experiments. In particular, XMCD spectra in both the total electron (TEY) and the fluorescence yield (FY) modes were simultaneously recorded, allowing for an element-specific distinction between surface and bulk magnetic properties. The obtained data provide clear evidence that the ZnO:Co thin films under study are not homogeneous DMS. Rather the observed RT magnetic behavior is caused by nanometer-sized superparamagnetic Co precipitates, which are directly evidenced by XMCD and energy-filtering transmission electron microscopy (EFTEM).

We note that our data do not prove that the realization of a DMS is impossible for ZnO:Co. However, much more effort is required to unambiguously determine the nature of ferromagnetism, and conclusions based on superficial studies should be considered with care.

Journal reference

Nanosized superparamagnetic precipitates in cobalt-doped ZnO
M. Opel, K.-W. Nielsen, S. Bauer, S.T.B. Goennenwein, J.C. Cezar, D. Schmeisser, J. Simon, W. Mader and R. Gross
Eur. Phys. J. B 63, 437- 444 (2008) | DOI: 10.1140/epjb/e2008-00252-4


Prof. Dr. Rudolf Gross
Walther-Meißner-Institut, Bayerische Akademie der Wissenschaften
und Physik-Department, TU München
Tel.: +49 (0)89 / 289 – 14201
E-Mail: Rudolf.Gross@wmi.badw.de
Web: http://www.wmi.badw-muenchen.de/

image description

Effective spin magnetic moment ms,eff, of Co in ZnO:Co derived from TEY (blue) and FY (red) XMCD measurements. The FY data follow the sample magnetization M (green line). The TEY data fit well to a Brillouin function (blue line) for paramagnetic Co2+ ions. The inset shows an elemental map of Co obtained by EFTEM, displaying Co enriched regions of about 4 nm diameter.