M. Sawicki, E. Guziewicz, M. I. Lukasiewicz, O. Proselkov, I. A. Kowalik, W. Lisowski, P. Dluzewski, A. Wittlin, M. Jaworski, A. Wolska, W. Paszkowicz, R. Jakiela, B. S. Witkowski, L. Wachnicki, M. T. Klepka, F. J. Luque, D. Arvanitis, J. W. Sobczak, M. Krawczyk, A. Jablonski, W. Stefanowicz, D. Sztenkiel, M. Godlewski, T. Dietl
A series of (ZnO)m(CoO)n digital alloys and superlattices grown by atomic
layer deposition has been investigated by a range of experimental methods. The
data provide evidences that the Co interdiffusion in the digital alloy
structures is sufficient to produce truly random Zn1-xCoxO mixed crystals with
x up to 40%. Conversely, in the superlattice structures the interdiffusion is
not strong enough to homogenize the Co content along the growth direction
results in the formation of (Zn,Co)O films with spatially modulated Co
concentrations. All structures deposited at 160\circC show magnetic properties
specific to dilute magnetic semiconductors with localized spins S = 3/2 coupled
by strong but short range antiferromagnetic interactions that lead to low
temperature spin-glass freezing.
It is demonstrated that ferromagnetic-like features, visible exclusively in
layers grown at 200\circC and above, are associated with an interfacial mesh of
metallic Co granules residing between the substrate and the (Zn,Co)O layer.
This explains why the magnitude of ferromagnetic signal is virtually
independent of the film thickness as well as elucidates the origin of magnetic
anisotropy. Our conclusions have been derived for layers in which the Co
concentration, distribution, and aggregation have been determined by:
secondary-ion mass spectroscopy, electron probe micro-analysis, high-resolution
transmission electron microscopy with capabilities allowing for chemical
analysis; x-ray absorption near-edge structure; extended x-ray absorption
fine-structure; x-ray photoemission spectroscopy, and x-ray circular magnetic
dichroism. Macroscopic properties of these layers have been investigated by
superconducting quantum interference device magnetometery and microwave
dielectric losses allowing to confirm the important role of metallic
inclusions.
View original:
http://arxiv.org/abs/1201.5268
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