6 research outputs found
Na atomic order, Co charge disproportionation and magnetism in NaCoO for large Na contents
We have synthesized and characterized four different stable phases of Na
ordered NaCoO, for . Above 100 K they display similar
Curie-Weiss susceptibilities as well as ferromagnetic spin fluctuations
in the CoO planes revealed by Na NMR data. In all phases from
Co NMR data we display evidences that the Co disproportionate already
above 300 K into non magnetic Co and magnetic Co
sites on which holes delocalize. This allows us to understand that metallic
magnetism is favored for these large Na contents. Below 100 K the phases
differentiate, and a magnetic order sets in only for at
22 K. We suggest that the charge order also governs the low energy
scales and transverse couplings
59Co NMR study of the Co states in superconducting and anhydrous cobaltates
Co NMR spectra in oriented powders of NaCoO and in its
hydrated superconducting phase (HSC) NaCoO,1.3HO reveal a
single electronic Co state with identical independent NMR shift tensor.
These phases differ markedly from NaCoO, in which we resolve 3
types of Co sites. The large T variation of their spin susceptibilities and the anisotropy of the orbital susceptibility allow us
to conclude that charge disproportionation occurs, in a non magnetic Co
and two magnetic sites with about 0.3 and 0.7 holes in the multiplet.
The data are consistent with those for the single Co site in the anhydrous and
HSC phase assuming the expected Co charge.Comment: 5 pages, 3 figures, accepted for publication in Phys. Rev. Let
Antiferromagnetic properties of a water-vapor-inserted YBa<sub>2</sub>Cu<sub>3</sub>O<sub>6.5</sub> compound studied by NMR, NQR, and μSR
We present a detailed NQR, NMR and SR study of a magnetic phase
obtained during a topotactic chemical reaction of YBaCuO
high- temperature superconductor with low-pressure water vapor. Our studies
give straightforward evidence that the ''empty'' Cu(1) chains play the role of
an easy water insertion channel. It is shown that the NQR spectrum of the
starting material transforms progressively under insertion of water, and
completely disappears when one HO molecule is inserted per unit cell.
Similarly, a Cu ZFNMR signal characteristic of this water inserted material
appears and grows with increasing water content, which indicates that the
products of the reaction are non-superconducting antiferromagnetic phases in
which the bilayers are ordered. These antiferromagnetic phases are felt by
proton NMR which reveals two sites with static internal fields of 150 and about
15 Gauss respectively. Two muon sites are also evidenced with similar local
fields which vanish at K. This indicates that the magnetic
phases have similar N\'{e}el temperatures as the other bilayer undoped
compounds. An analysis of the internal fields on different sites of the
structure suggests that they can be all assigned to a single magnetic phase at
large water content in which the Cu(1) electron spins order with those of the
Cu(2). It appears that even samples packed in Stycast epoxy resin heated
moderately at a temperature (200C) undergo a reaction with epoxy
decomposition products which yield the formation of the same final compound. It
is then quite clear that such effects should be considered quite seriously and
avoided in experiments attempting to resolve tiny effects in such materials, as
those performed in some recent neutron scattering experiments.Comment: 12 pages of revised text including 12 Figure