Correlation Of Cu Research Articles

Structure—function correlation of Cu(II)-and Cu(I)-di-Schiff-base complexes during the catalysis of superoxide dismutation

Aquo-[ N,N′-bis(2-(6-methyl-pyridyl) methylene)-1,4-butanediamine] ( N,N′ ,N″,N‴)-copper(II) perchlorate, [Cu (II)-(Pu-6-MePy)-(H 2O)] (ClO 4) 2, a copper(II)-di-Schiff base was synthesized and reduced to the Cu(I) form. The successful preparation was controlled by IR and in the case of the Cu(II) complex additionally by EPR spectroscopy. [Cu(II)-(Pu-6-MePy)(H 2O)] ClO 4) 2 has an electronic absorption maximum at 717 nm of ε 717 = 108 M -1 cm −1. The EPR parameters are g ⊥ = 2.059 and g ⊥ = 2.235 which are very close to those of the respective values of Cu(II) in intact Cu 2Zn 2 superoxide dismutase (SOD). The electronic absorption maximum of the Cu(I) complex is at 474 nm ( ε 717 = 12 123 M −1 cm −1) and, as expected, no EPR is seen. The crystal structure of [Cu(II)-(Pu-6-MePy) (H 2O)](ClO 4) 2 shows a monomer while the reduced Cu(I) form reveals a dimer. Both complexes comprise the same SOD mimetic activity of 0.3% compared to that of the cytosolic enzyme. As the reduction of the Cu(II) complex proceeds within 10–40 min it is unlikely that the Cu(I) dimer is formed during the reduction oxidation cycle of superoxide dismutation which occurs at a timescale of milliseconds. The Cu(I) dimer is a promising superoxide dismutase mimicking compound in an aerobic reducing environment as, for example, in cytosol.

Distribution of heavy metal and PCB contaminants in the sediments of an urban estuary: The Hudson River

Surficial sediments obtained from sediment cores were collected over 100km along the axis of the lower Hudson River in June 1994, November 1994, May 1995 and April 1996 and showed the presence of anthropogenic Ag, Cd, Cu, Pb, Zn, PCBs in all samples. Contaminant distributions in the Hudson River estuary show two types of trends: Ag, Cu and Pb show an increasing trend down-estuary with maximum values in New York Harbor sediments; in contrast, Cd, Zn and total PCBs display a decreasing trend toward New York Harbor where urban sources are also apparent. Silver is a useful tracer of urban sources of contaminants in the Hudson River estuary and polychlorinated biphenyls (PCBs) are useful source indicators of upriver sources. Correlations of Cu and Pb with Ag suggest that Ag, Cu and Pb are dominated by down-estuary sources such as wastewater effluent. The history of their inputs suggests that they have been progressively transported downstream. Correlations of Cd and Zn with total PCBs indicate that these contaminants are dominated by upriver sources, where they are removed and diluted downstream along with the sediment transport.

Study of Cu–O and Cu–La Pair Correlations in Superconducting La2CuO4.1System by Cu K-Edge EXAFS

We report Cu K-edge extended X-ray absorption fine structure (EXAFS) spectra up to high momentum transfer measured on a superconducting La 2 CuO 4.1 single crystal using in-plane polarization of synchrotron light. The EXAFS data with a high signal to noise ratio has enabled the precise determination of the Debye-Waller factors and the pair correlations of Cu–O (planar) and Cu–La pairs of the structurally complex superconducting system. We have observed an anomalous behaviour of the pair correlations at ∼170 K and attributed it to an interplay between temperature-dependent structural modulation of the perovskite and rock-salt layers in the title compound.

Effects of heavy metals on nitrifying bacteria

Laboratory evaluations were conducted to study the toxic responses of heavy metals such as copper and nickel of an autotrophic culture of strict nitrifiers (Nitrosomonas sp. and Nitrobacter sp.) in continuous flow stirred tank reactors (CSTR). One of the CSTRs was operated as a suspended growth (SG) system while the other was operated as an attached & suspended growth (A&SG) system. Nitrification inhibition in a SG and A&SG systems was investigated with the emphasis on the effect of shock loading of copper and nickel. As a result of the copper and nickel test, Nitrosomonas sp. was found to be equally or more sensitive than Nitrobacter sp. However, a higher influent nickel concentration of 50 mg/L was needed to cause a similar percent inhibition of ammonium oxidation than the copper concentration of 5 mg/L. A geochemical equilibrium speciation model, MINTEQA2/PRODEFA2, was used to compute the concentrations of various chemical species present in the wastewater for both systems. The high correlations of Cu(NH3)4+2 and Ni(NH3)4+2 with percent inhibition were found and it was thought that they were probably the species responsible for the inhibition of ammonia oxidation.

Precious metal mineralogy in porphyry-, skarn-, and replacement-type ore deposits of the Ertsberg (Gunung Bijih) District, Irian Jaya, Indonesia

Petrographic and electron microprobe analyses of native Au and a variety of sulfide minerals indicate a generally high but widely varying native Au fineness and a Cu-Fe sulfide host for most Ag in the Gunung Bijih Timur complex of Cu-Au skarn deposits, the Big Gossan and Dom Cu skarn deposits, and the Grasberg porphyry Cu-Au deposit. Native Au rims bornite in veins and forms inclusions in bornite and chalcopyrite in samples from Grasberg and from the Gunung Bijih Timur complex, as well as filling open space in quartz veins in the complex. Skarn and quartz veins in the Big Gossan area host native Au in association with pyrite, pyrrhotite, galena, chalcopyrite, sphalerite, and arsenopyrite.Native Au from the Gunung Bijih Timur complex has a wide but bimodal range of fineness: 920 to 990 and 340 to 820 (avg = 860). Native Au from Grasberg samples is < or = 930 fine (avg = 960). Fineness of Big Gossan Au ranges from 540 to 960 (avg = 830), although compositional ranges are small within most individual samples. Native Au from Grasberg falls into two groups with respect to Pd: Pd-bearing (< or = 3,500 ppm; avg = 1,800 ppm) and with no detectable Pd. Fire assay of Grasberg ore reveals trace but detectable (10 ppb < [Pd] < 500 ppb, avg = < or = 50 ppb) concentrations of Pd and no detectable Pt. Native Au from the Gunung Bijih Timur complex averages twice as much Cu as that from Grasberg (1.7 vs. 0.8 wt % Cu). Native Au from Big Gossan has uniformly low Cu contents (< or = 1,200 ppm Cu; avg = 300 ppm).Mass-balance calculations and analytical data indicate that the bulk of the Ag in the Gunung Bijih Timur complex is contained by bornite, indicating a partitioning of Ag into bornite, relative to native Au or a discrete Ag mineral. This is supported by the consistent positive correlation of Cu and Ag throughout the Gunung Bijih Timur complex, as well as in Grasberg and Dom. Ag-bearing galena occurs in the Big Gossan area, but is rare in the Grasberg, the Gunung Bijih Timur complex, and the Dom orebodies.The high fineness of native Au from the Gunung Bijih district represents the upper end of the reported range for porphyry- and skarn-type Cu-Au deposits but is similar to values reported elsewhere in Indonesia. The fineness of native Au in the Gunung Bijih Timur complex does not appear to be controlled by depth or distance from the local intrusive contact, mineral assemblage, or paragenetic relationship. The fineness of native Au from the Big Gossan area shows a positive correlation with elevation, suggestive of a lower temperature, more distal character than in the other skarns of the district. Differences in Pd contents may be related to acidity and redox conditions during native Au deposition.High Cu, high Pd, and low Ag compositions of Au grains in pan concentrates suggest a porphyry or Cu skarn source relative to a more distal source. Compositional characteristics identified in this study may have applications to exploration within the Ertsberg district and elsewhere.

Effects of heavy metals on nitrifying bacteria

Laboratory evaluations were conducted to study the toxic responses of heavy metals such as copper and nickel of an autotrophic culture of strict nitrifiers (Nitrosomonas sp. and Nitrobacter sp.) in continuous flow stirred tank reactors (CSTR).One of the CSTRs was operated as a suspended growth (SG) system while the other was operated as an attached & suspended growth (A&SG) system. Nitrification inhibition in a SG and A&SG systems was investigated with the emphasis on the effect of shock loading of copper and nickel. As a result of the copper and nickel test, Nitrosomonas sp. was found to be equally or more sensitive than Nitrobacter sp. However, a higher influent nickel concentration of 50 mg/L was needed to cause a similar percent inhibition of ammonium oxidation than the copper concentration of 5 mg/L. A geochemical equilibrium speciation model, MINTEQA2/PRODEFA2, was used to compute the concentrations of various chemical species present in the wastewater for both systems. The high correlations of Cu(NH3)4+2 and Ni(NH3)4+2 with percent inhibition were found and it was thought that they were probably the species responsible for the inhibition of ammonia oxidation.

Relationship between heavy metals and magnetic properties in a large polluted catchment: The Etang de Berre (south of France)

The purpose of this study was to test possible extension of the relationship found by several authors between heavy metals enriched materials and magnetic properties to environmental system by the analysis of various materials of mixed anthropic and natural origins. The samples studied came from the Berre lake (pre-industrial sediments, recent surface sediments, saltmarsh materials, atmospheric dusts) and from other sources such as leaves of plane trees and Marseille urban sewage sludge. Clear correlations between metallic pollutants (mainly Zn, Cd & Cr with r2 > 0.75, and weaker correlation for Cu, Ni, Pb & Fe) concentrations and simple magnetic parameters (Saturation Isothermal Remanent Magnetisation, Susceptibility), are observed. The results show that correlation factors are frequently higher between magnetic parameters and metallic pollutants than between iron and the same metallic elements. For the same chemical element, SIRM values are frequently better correlated than x values. Nature of magnetic grains involved is discussed.

Trace metal geochemical association in sediments from the Cairns region of the Great Barrier Reef, Australia

Some physico-chemical properties and concentrations of the trace metals Cu and Zn have been measured in sediments from the Coral Sea between the coast at Cairns (146°E 17°S) and the Great Barrier Reef. Surface sediments along an 18 km transect and a mid-transect core were examined. The acid soluble material increased from 20 to 36% and Ca increased from 12 to 47 mg g −1 with increasing distance from shore. Fe passed through a maximum of 32 mg g −1 at about 10 km offshore. The results show the pattern of sedimentation, with Fe representing terrestrial input, and acid soluble material and Ca representing CaCO 3. The CaCO 3 was mainly calcite with only minimal amounts of aragonite (coral). Microscopic examination showed shell material but no coral debris. The Cu concentration in the surface sediments (2.0–17 μg g −1) decreased with increasing distance from Cairns harbour (correlation coefficient Cu: dist −0.793). The Zn concentration (40–53 μg g −1) followed Fe (correlation coefficient Zn:Fe 0.932) with a maximum about 10 km offshore. There was no correlation of Cu and Fe (Cu:Fe 0.041) or Cu and Zn (Cu:Zn 0.178) showing that Cu and Zn came from different sources. The principle source of Cu is in the Cairns harbour region and may be a result of previous use of Cu-based marine antifouling paints. The Zn is dominantly associated with terrestrial inputs. Within the sediment profile 11 km offshore, the pattern of particle size was consistent with the top 6 cm of sediment having been resuspended during a cyclone. 210Pb concentrations also showed evidence of sediment mixing. Deeper in the sediment the average sedimentation rate was estimated at about 3.5 mm yr −1. Most of the CaCO 3 down the profile was calcite with no evidence of substantial input of coral material. The different sources of Cu and Zn could not be recognized within the core at this distance offshore.

Occurrence of heavy metal tolerance and co-tolerance in Deschampsia cespitosa (L.) Beauv. from European and Canadian populations.

By sampling soils and plants from a variety of metal-contaminated sites in Canada and Europe, heavy metal tolerance and co-tolerance have been investigated in the grass Deschampsia cespitosa L. (Beauv.). Soils were analyzed for water extractable Ca, as well as for their heavy metal content (Cu, Ni, Zn, Pb, Cd, Co), electrical conductivity, pH and organic matter. Canadian plant material was collected from non-contaminated areas, as well as sites contaminated by copper and nickel (Sudbury) and nickel and cobalt (Cobalt). European plants were sampled from reference sites and from zinc and lead contaminated locations (Bleikuhle, Pochsand/Germany). Using cloned material the 'in parallel' root elongation technique was employed to determine tolerance indices to individual metals (Cu, Ni, Zn, Pb). Among the Canadian plants, patterns of tolerances matched patterns of soil contamination. Thus, at the Cobalt sites, Cu was not present in elevated amounts, and the plants did not exhibit Cu tolerance. Also, evidence suggests that the close correlation of Cu and Ni tolerance among the Sudbury plants was not the result of Cu-Ni co-tolerance but occurred because ions of both metals were present in elevated amounts at these sites. In addition to the tolerances expected on soil chemistry, plants taken from metal contaminated sites exhibited enhanced tolerance also to metal ions not elevated in the soils of their origin. This is interpreted as general, non-specific, low level co-tolerance. Thus, most Cobalt and Sudbury plants exhibited incidental tolerance at a higher level to Zn and Pb. Correlation analysis revealed very close relations both between Zn and Pb, and especially between Ni and Zn, supporting findings from previous studies. It was concluded that among the Canadian populations tested in this study, Ni tolerance conferred Zn tolerance. Among the European plants examined, the expected tolerances to Zn and Pb were found. However, whereas some control plants from non-contaminated sites exhibited relatively high tolerances to these metal ions, the tolerance of plants from mines was lower than expected. The Zn and Pb tolerant mine populations had no tolerance to Cu and only very low tolerance to Ni, suggesting that the enhanced Zn tolerance associated with Ni tolerance in the Canadian plants is not necessarily reversible, i.e. Zn tolerance does not necessarily confer Ni tolerance.

Analysis of the Electric Field Gradients and the Knight Shifts at All Cu and O Nuclei in YBa2Cu3O7

Electronic configurations and local susceptibilities at all copper and oxygen sites in YBa 2 Cu 3 O 7 are determined from the analysis of the electric field gradients (EFG's) and the Knight shifts (shifts). Besides main contributions from 3 d holes for Cu sites and 2 p σ holes for O sites, it is necessary to consider 4 p electrons of Cu and 2 p π holes of O for accurate analysis of the EFG's, and 4 s electrons of Cu and 2 s holes of O for the shifts. Difference in the observed EFG's between Cu(1) and Cu(2) is due to the 4 p -electron contributions, and the 3 d -electron states are determined to be quite similar. Each site of Cu and O shares almost same amount of holes. The local spin susceptibilities, however, are fairly different in order of magnitude as Cu(2), Cu(1), O(2, 3), O(4), implying a strong correlation in Cu(2).

63Cu Nuclear Spin-Lattice Relaxation Study for Low-Temperature Structural Transition in (La1-xBax)2CuO4aroundx=0.06

The nuclear spin-lattice relaxation rate 1/ T 1 of 63 Cu was measured in the range from 1.5 to 300 K for x =0.0625 and x =0.08 in (La 1- x Ba x ) 2 CuO 4 . In the high-temperature tetragonal (HTT) and the low-temperature orthorhombic (LTO) phases in the normal state, 1/ T 1 T for both samples follows the Curie-Weiss law, C /( T +θ), suggesting that the two-dimensional (2D) antiferromagnetic (AF) fluctuation among Cu spins is dominant, similarly to the (La 1- x Sr x ) 2 CuO 4 system. In the HTT and LTO phases, no significant differences except for a slight difference in θ were observed between the two samples. In the low-temperature tetragonal (LTT) phase for x =0.0625, 1/ T 1 diverges at about 35 K, confirming the existence of magnetic ordering. The magnetic ordering at the finite temprature suggedsts that the dominant 2D-AF correlation of Cu spins in the HTT or LTO phase becomes the 3D-AF correlation derived from the some interactions along the c axis in the LTT phase.

Platinum-group element and gold contents in the Skouries porphyry copper deposit, Chalkidiki Peninsula, northern Greece

The Skouries porphyry copper deposit is located in the eastern Chalkidiki peninsula, northern Greece. Along with several other porphyry copper occurrences, this deposit establishes the northeastern part of the Servomacedonian zone as a porphyry copper region. Amphibolite, mica schist, and two-mica gneiss are the predominant rock types exposed in the Skouries area. These rocks have been intruded by the Skouries porphyry stock, which is of subalkaline composition and is Tertiary in age (18 Ma). The Skouries porphyry intrusion and the surrounding country rocks are strongly fractured and intensely altered by hydrothermal fluids. In addition to intense silicification, a prominent alteration type is potassic, whereas phyllic and propylitic types are less common.Mineralization mainly includes chalcopyrite, pyrite, bornite, chalcocite, and magnetite in the form of veins, stockworks, and disseminations. Native gold and electrum commonly are present as small inclusions in chalcopyrite. Although lateral zoning is not defined clearly, a vertical zoning in terms of both metal mineralogy and overall metal content is apparent. A zone of oxidation (azurite-malachite) covers the upper part of the ore and is underlain by a zone of reduced rock rich in covellite and chalcocite, which in turn is underlain by the most important ore type, including a chalcopyrite-pyrite-bornite-magnetite assemblage.Both weakly and intensively altered porphyry samples from the deposit were analyzed for major oxides, trace elements, and all platinum-group elements (PGE). The stock exhibits high levels of Cu, Au, Ag, B, Bi, Co, Se, and Te. Barium and Sr contents are also high while Rb is low, resulting thereby in high K/Rb and low Rb/Sr ratios. Palladium concentrations range from less than 2 to 480 ppb. Ruthenium content ranges from 8 to 41 ppb and Pt from 2 to 10 ppb. Osmium, Ir, and Rh content is below detection limit. The average Au/Ag ratio is 0.87, which is consistent with the range of several worldwide porphyry copper deposits.On the basis of factor analysis of selected geochemical data, a strong positive interelement correlation (r > or = 0.83) between Au, Ag, Pd, nd Te coupled with their association with silica appears to support a correlation of these precious metals with silicification rather than potassic-type alteration. Such a relationship may suggest similar behavior of these metals and a common origin. A positive correlation of Cu with iron (r = 0.86), which is consistent with the association of chalcopyrite with magnetite, and the low correlation of Te and Pd with Cu may suggest that these metals and, to a lesser extent, copper have been redistributed during a late stage of evolution of the Skouries porphyry system. Furthermore, the close association of Te with precious metals, combined with the relatively low temperature for telluride minerals stability, may indicate that at least partly precious metal tellurides have been formed in a relatively shallow and less hot environment.

Variation of Cu-O charge-transfer energies in YBa2Cu3O7-x thin films studied by photoemission spectroscopy.

Cu 2p core-level spectra have been obtained on ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathit{x}}$ thin films with different transition temperatures. The intensity ratios of the Cu 2p satellite to main peaks show a systematic decrease as the superconductivity transition temperatures drop. The experimental trend not only indicates the reduction of some Cu atoms from ${\mathrm{Cu}}^{2+}$ to ${\mathrm{Cu}}^{1+}$ state, but also reveals the correlation of Cu d--O p charge-transfer energy on ${\mathit{T}}_{\mathit{c}}$. This analysis is based on a simple configuration-interaction model simplified for impurity clusters. The observed trend suggests, as the removal of oxygen decreases ${\mathit{T}}_{\mathit{c}}$ in the thin films, that the charge-transfer energy between Cu d and O p levels becomes smaller. This, in turn, increases the superexchange energy J. The relations between the oxygen content, the electronic structure, and the magnetic interaction energies are consistent with existing experimental data for ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathit{x}}$ compounds.

Superconductivity in doped planar CuO2 systems

The authors present a model to describe high-Tc superconductivity in doped planar CuO2 systems. Each doped hole on the oxygen site interacts through strong antiferromagnetic (AF) coupling with two neighbouring copper holes to form Cu-O spin singlet pairs and interacts with a Cu-RVB state to lead to the condensation of O holes in momentum space, which is responsible for the superconductivity. The ground state of this system and the spin correlation of Cu and O holes are also studied. The results are in good agreement with previous works.

Nuclear Quadrupole Resonance in (La1-xSrx)2CuO4System

Nuclear quadrupole resonance (NQR) of Cu in (La 1- x Sr x ) 2 CuO 4 system (0.075≦ x ≦0.15) has been performed where the system is transformed from the high- T c superconducting to the normal metallic state. The NQR frequency increases and the spectrum is broadened with increasing x . The nuclear spin-lattice relaxation rate 1/ T 1 shows a remarkable distribution for more than x =0.125. For the nonsuperconducting compound of x =0.15, each component of 1/ T 1 is suppressed as compared to that in the superconducting compound of x =0.075 which is strongly enhanced by antiferromagnetic spin correlation of Cu– d hole, and is found to obey the Korringa relation, T 1 T =const. from 1.4 to 90 K. This dramatic change of 1/ T 1 is accompanied by the disappearance of T c . It is concluded that the superconductivity is realized with the background of the A.F. spin correlation.

Correlation of Cu(PTSM) localization with regional blood flow in the heart and kidney

In a dog model of myocardial ischemia, i.v. injection of [ 67Cu]Cu(PTSM) leads to an excellent linear correlation for both the heart and kidney between regional tissue radiocopper concentration and regional blood flow measured by radioactive micropheres. The 62Cu-labeled complex may serve as a generator-produced positron-emitting perfusion tracer for these organs.

Uptake of heavy metals by crops in relation to their concentration in the soil solution

Of 20 Dutch soils, concentrations in the soil solution and total contents of Zn, Cd, Pb and Cu were determined. In addition, distribution constants were measured for Cd and Pb. Soil solution concentrations of the heavy metals as well as their total contents in the soil and exchangeable fraction, calculated with the distribution coefficients, were compared with uptake in vegetables grown in pots. Log-log plots of concentrations in the plant and the soil solution concentrations showed the best correlation for Cu, Cd and Zn. Increasing Zn appeared to increase Cd uptake at high solution concentrations of Cd and to decrease uptake at low solution concentrations. For Pb generally no significant relationship between plant uptake and soil solution concentration or any other soil parameter for Pb could be found.

Urine and plasma free phenytoin concentration correlation.

In an attempt to find a simple, noninvasive method for estimating the plasma free concentration (CPf) of phenytoin (PHT), the relationship between urine PHT concentration (Cu) and CPf was studied in 40 epileptic patients who were 6 to 50 yr old. Cu was determined by gas-liquid chromatography and CPf by equilibrium dialysis at 37 degrees. Cu was generally greater than CPf, but correlation between the two was strong (r2 = 0.876) and in the same range as previously published results for saliva and CPf. Correlation of Cu with CPf was not influenced by patient age, urine pH, or urine flow rate. In 24 patients timed urine collections were obtained and calculated renal clearances of PHT were shown to increase in proportion to urine flow rates. Although it is not known if renal impairment and albuminuria will alter the relationship between Cu and CPf it appears that Cu may be useful in estimating the concentration of pharmacologically active PHT in plasma.

Spin Dynamics in Heusler Alloy Pd2MnSn

The dynamical spin susceptibility in the ferromagnetic Heusler alloy X 2 MnY (Pd 2 MnSn) is evaluated on the basis of the realistic band structure. The theoretical results of spin wave spectra of Pd 2 MnSn are in good agreement with the experimental ones, as have been for Cu 2 MnAl. The correlation between the spin densities of d-electrons of Mn is important to the spin waves with small and large q in both Cu 2 MnAl and Pd 2 MnSn. In Pd 2 MnSn the correlation between the spin density of d-electrons of Mn and that of conduction electrons is important for the spin waves with small and large q , but in Cu 2 MnAl this correlation decreases significantly for those with large q . These different q dependences of the spin-density correlation for Cu 2 MnAl and Pd 2 MnSn are consistent with the strong dependence on X atom of the magnetic interactions pointed out by Ishikawa et al.

Heat Capacity of a Strongly One-Dimensional Antiferromagnet Cu(C6H5COO)2·3H2O –Field-Induced Crossover of Spin-Dimensionality–

Strong one-dimensionality of the spin correlation in Cu(C 6 H 5 COO) 2 ·3H 2 O has been revealed by the heat capacity measurements in the temperature region from 20 K down to 0.07 K. The ratio of the estimated Neel temperature T N (0) (<0.1 K) under the zero-external magnetic field to an interaction energy in the chain, k B T N (0)/2| J | S ( S +1), is less than 0.7×10 -2 , where the exchange constant J has been determined as J / k B =-9.5±0.5 K from the heat capacity measurements between 0.5 K and 2.5 K. This one-dimensionality is comparable with or better than that of TMMC. An anomalous increase of the heat capacity has been found below 1.5 K under the external field up to 12 kOe applied along the c -axis which is perpendicular to the spin preferential direction. This anomaly can be explained qualitatively by considering the crossover effects of spin-dimensionality from the isotropic Heisenberg to the X Y symmetry which is induced by the external field.