Antiferromagnetic properties of a water-vapor-insertedYBa2Cu3O6.5compound studied by NMR, NQR, and

Abstract

We present a detailed NQR, nuclear magnetic resonance (NMR), and SR study of the magnetic phase obtained during a topotactic chemical reaction of YBa 2 Cu 3 O 6.5 high-temperature superconductor with lowpressure water vapor. 65Cu-enriched samples have been used for NQR/NMR studies which allows to get a good resolution in the Cu͑1͒ NQR and Cu͑2͒ zero field NMR (ZFNMR) spectra. It is shown that the NQR spectrum of the starting material transforms progressively under insertion of water, and almost completely disappears when about one H 2 O molecule is inserted per unit cell. Similarly, a 65Cu ZFNMR signal characteristic of this water inserted material appears and grows with increasing water content, which indicates that the products of the reaction are nonsuperconducting antiferromagnetic phases in which the copper electronic magnetic moments in the CuO 2 bilayers are ordered. The use of 65 Cu-enriched samples allowed us to reliably resolve three different copper resonances which correspond to different internal magnetic fields. The antiferromagnetic phases are also felt by proton NMR which reveals two sites with static internal fields of 150 and about 15 Gauss, respectively. SR studies performed on a series of samples prepared in the same way as the 65 Cu-enriched ones reveal two muon sites with the same local fields as the proton sites, which vanish at T Ϸ 400 K. This indicates that muons preferentially occupy proton vacancy sites, and that the magnetic phases have similar Néel temperatures as the other bilayer undoped cuprate compounds. An analysis of the internal fields on the different spin probes suggests that they can be all assigned to a single magnetic phase at large water content in which the Cu͑1͒ electron spins order with those of the Cu͑2͒. The detailed evolution of the spectra with the progressive increase of water content is shown to be compatible with a coexistence of phases during the early stages ot the reaction. It appears that even samples packed in paraffin underwent a transformation of a substantial part of the sample after 6 years storage in atmosphere. Samples packed in Stycast epoxy resin heated moderately to a temperature ͑200°C͒ undergo a reaction with epoxy decomposition products which yield the formation of the same final compound. It is clear that such effects should be considered quite seriously and avoided in experiments attempting to resolve tiny effects in these materials, such as those performed in some recent neutron scattering experiments.

Distance to Massive Metal Body—a Paradoxical Parameter that Regulates the Intensity of the Hydration of YBa2Cu3O6.75

“…The lattice parameters of exposed YBCO correspond to the maximally oxidized state of YBCO [9]. On the other hand, our study has not revealed radical changes in the structure that would be evidence of the transformation of YBCO under the action of absorbed water into the pseudo-YBa 2 Cu 4 O 8 phase, as had been observed in [10], or into another phase. There is ob-served only some change in the phase composition caused by the deposition of the impurity phases.…”

Formation of magnetic moments in the cuprate superconductor Hg0.8Cu0.2Ba2Ca2Cu3O8+δ below Tc seen by NQR

“…Now we would like to discuss the possible origin of these internal magnetic fields. We can exclude the presence of the water as a source of the weak antiferromagnetism in the CuO 2 -plane as was suggested recently [25], since our measurements were performed on the fresh samples. Furthermore, the values of the deduced magnetic fields in our case is much larger than those measured in the water-inserted YBaCuO compounds.…”

Polarized neutron measurement of magnetic order inYBa2Cu3O6.45

“…This makes it unlikely there is magnetism stemming from water. 15 Also water appears to enter the sample more easily when the chains are highly ordered leaving large free spaces in the chains. The moments cannot stem from a c-axis component of the sloping background found in the experiment as this does not peak at the magnetic lattice point.…”