Nanosized superparamagnetic precipitates in cobalt-doped ZnO

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.Comment: 20 pages, 6 figures; details about background subtraction added to section III. (XMCD

Synthesis, Characterization and ESR Studies of Zn1−x Co x O Nanoparticles

Zn1-x Co (x) O polycrystalline nanoparticles with different (x=0.03, 0.05, 0.1, 0.15, 0.2, 0.25, and 0.3) compositions were synthesized using the sol-gel technique. The effects of doping ratio and annealing temperature on structure and magnetic properties were investigated systematically. The phase, crystal structure, and microstructure of the Zn1-x Co (x) O nanoparticles were characterized using X-Ray diffraction and scanning electron microscope. Electron spin resonance spectra of Zn1-x Co (x) O nanoparticles were collected at room temperature on a Bruker EMX model X-band spectrometer operating at a frequency of 9.50 GHz