Long range magnetic ordering in a spin-chain compound, Ca3_3CuMnO6_6, with multiple bond distances

The results of ac and dc magnetization and heat capacity measurements as a function of temperature (T = 1.8 to 300 K) are reported for a quasi-one-dimensional compound, Ca$_3$CuMnO$_6$, crystallizing in a triclinically distorted K$_4$CdCl$_6$-type structure. The results reveal that this compound undergoes antiferromagnetic ordering close to 5.5 K. In addition, there is another magnetic transition below 3.6 K. Existence of two long-range magnetic transitions is uncommon among quasi-one-dimensional systems. It is interesting to note that both the magnetic transitions are of long-range type, instead of spin-glass type, in spite of the fact that the Cu-O and Mn-O bond distances are multiplied due to this crystallographic distortion. In view of this, this compound could serve as a nice example for studying "order-in-disorder" phenomena.Comment: Physical Review (in press

Magnetic anomalies in the spin chain system, Sr3_3Cu1−x_{1-x}Znx_xIrO6_6

We report the results of ac and dc magnetization (M) and heat-capacity (C) measurements on the solid solution, Sr$_3$Cu$_{1-x}$Zn$_x$IrO$_6$. While the Zn end member is known to form in a rhombohedral pseudo one-dimensional K$_4$CdCl$_6$ structure with an antiferromagnetic ordering temperature of (T$_N$ =) 19 K, the Cu end member has been reported to form in a monoclinically distorted form with a Curie temperature of (T$_C$ =) 19 K. The magnetism of the Zn compound is found to be robust to synthetic conditions and is broadly consistent with the behavior known in the literature. However, we find a lower magnetic ordering temperature (T$_o$) for our Cu compound (~ 13 K), thereby suggesting that T$_o$ is sensitive to synthetic conditions. The Cu sample appears to be in a spin-glass-like state at low temperatures, judged by a frequency dependence of ac magnetic susceptibility and a broadening of the C anomaly at the onset of magnetic ordering, in sharp contrast to earlier proposals. Small applications of magnetic field, however, drive this system to ferromagnetism as inferred from the M data. Small substitutions for Cu/Zn (x = 0.75 or 0.25) significantly depress magnetic ordering; in other words, T$_o$ varies non-monotonically with x (T$_o$ ~ 6, 3 and 4 K for x = 0.25, 0.5, and 0.67 respectively). The plot of inverse susceptibility versus temperature is non-linear in the paramagnetic state as if correlations within (or among) the magnetic chains continuously vary with temperature. The results establishComment: 7 pages, 7 figures, Revte

Effects of Macrobrachium hainanense predation on benthic community functioning in tropical Asian streams

1. Macrobrachium hainanense is a large predatory palaemonid shrimp, present at high densities in pools of low-order forested streams in Hong Kong. The present study investigated the impacts of M. hainanense on benthic community structure and functions in pools of two streams: Tai Po Kau Forest Stream and Tai Shing Stream. 2. Repeated whole-pool experiments involving shrimp density manipulations (removal, control and addition) were conducted in both streams between October 2000 and April 2002, and included a wet (May to September) and two dry (October to April) seasons. The three objectives of the study were to determine the effects of M. hainanense predation on benthic macroinvertebrate abundance and species richness, rates of leaf litter breakdown because of effects on detritivores, and periphyton standing stocks by way of an effect on herbivores. 3. Wet season results showed consistent reductions in benthos densities and species richness following heavy rainfall, irrespective of shrimp manipulation. These results suggested that spate-induced disturbances might override biotic effects and play a dominant role in structuring benthic communities in stream pools in Hong Kong. 4. No significant, reproducible effects on any of the response variables measured in either stream were found during the dry season. Litter breakdown was reduced in the absence of shrimps during one experiment only, suggesting it might be a type I error. These results signified no effect of shrimp removal on benthic communities, or on the functional processes of litter breakdown, or on periphyton accumulation. The large scale of the experimental units (8-40 m2), refuge availability, and the presence of benthic predatory fishes that cropped excess prey made available by removal of M. hainanense, may have contributed to the lack of any effect, despite the abundance of the predatory shrimps.link_to_subscribed_fulltex