54 research outputs found
Fe-implanted ZnO: Magnetic precipitates versus dilution
Nowadays ferromagnetism is often found in potential diluted magnetic
semiconductor systems. However, many authors argue that the observed
ferromagnetism stems from ferromagnetic precipitates or spinodal decomposition
rather than from carrier mediated magnetic impurities, as required for a
diluted magnetic semiconductor. In the present paper we answer this question
for Fe-implanted ZnO single crystals comprehensively. Different implantation
fluences and temperatures and post-implantation annealing temperatures have
been chosen in order to evaluate the structural and magnetic properties over a
wide range of parameters. Three different regimes with respect to the Fe
concentration and the process temperature are found: 1) Disperse Fe and
Fe at low Fe concentrations and low processing temperatures, 2)
FeZnO at very high processing temperatures and 3) an intermediate
regime with a co-existence of metallic Fe (Fe) and ionic Fe (Fe and
Fe). Ferromagnetism is only observed in the latter two cases, where
inverted ZnFeO and -Fe nanocrystals are the origin of the
observed ferromagnetic behavior, respectively. The ionic Fe in the last case
could contribute to a carrier mediated coupling. However, their separation is
too large to couple ferromagnetically due to the lack of p-type carrier. For
comparison investigations of Fe-implanted epitaxial ZnO thin films are
presented.Comment: 14 pages, 17 figure
G. bulloides dissolution index of surface sediments
The Atlantic is regarded as a huge carbonate depocenter due to an on average deep calcite lysocline. However, calculations and models that attribute the calcite lysocline to the critical undersaturation depth (hydrographic or chemical lysocline) and not to the depth at which significant calcium carbonate dissolution is observed (sedimentary calcite lysocline) strongly overestimate the preservation potential of calcareous deep-sea sediments. Significant calcium carbonate dissolution is expected to begin firstly below 5000 m in the deep Guinea and Angola Basin and below 4400 m in the Cape Basin. Our study that is based on different calcium carbonate dissolution stages of the planktic foraminifera Globigerina bulloides clearly shows that it starts between 400 and 1600 m shallower depending on the different hydrographic settings of the South Atlantic Ocean. In particular, coastal areas are severely affected by increased supply of organic matter and the resultant production of metabolic CO2 which seems to create microenvironments favorable for dissolution of calcite well above the hydrographic lysocline
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