Eur. Phys. J. B 63, 437-444 (2008)
https://doi.org/10.1140/epjb/e2008-00252-4

Nanosized superparamagnetic precipitates in cobalt-doped ZnO

¹ Walther-Meißner-Institut, Bayerische Akademie der Wissenschaften, 85748 Garching, Germany
² European Synchrotron Radiation Facility, 38043 Grenoble Cedex 9, France
³ Angewandte Physik II, Brandenburgische Technische Universität Cottbus, 03046 Cottbus, Germany
⁴ Institut für Anorganische Chemie, Rheinische Friedrich-Wilhelms-Universität Bonn, 53117 Bonn, Germany

Corresponding author: ^a opel@wmi.badw.de

Received: 26 March 2008
Revised: 16 May 2008
Published online: 3 July 2008

Abstract

The existence of semiconductors exhibiting long-range ferromagnetic ordering at room temperature still is controversial. One particularly important issue is the presence of secondary magnetic phases such as clusters, segregations, etc. These are often tedious to detect, leading to contradictory interpretations. We show that in our cobalt doped ZnO films grown homoepitaxially on single crystalline ZnO substrates the magnetism unambiguously stems from metallic cobalt nano-inclusions. The magnetic behavior was investigated by SQUID magnetometry, X-ray magnetic circular dichroism, and AC susceptibility measurements. The results were correlated to a detailed microstructural analysis based on high resolution X-ray diffraction, transmission electron microscopy, and electron-spectroscopic imaging. No evidence for carrier mediated ferromagnetic exchange between diluted cobalt moments was found. In contrast, the combined data provide clear evidence that the observed room temperature ferromagnetic-like behavior originates from nanometer sized superparamagnetic metallic cobalt precipitates.