HYPERFINE FIELDS AT THE BA SITE IN THE ANTIFERROMAGNET YBA2CU3O6.05

We report a Ba nuclear quadrupole resonance (NQR) study of the antifer romagnetic state of YBa2Cu3O6.05 (Neel temperature T-N = 415 K) perfor med between 16 and 402 K. The Zeeman perturbed Ba-137 NQR spectrum yie lds information on two hyperfine fields present at the Ba site: the el ectric field gradient (EFG) and the internal magnetic field arising fr om the Cu(2) sublattice magnetization. The absolute value of the EFG i s in remarkable agreement with cluster and band structure calculations thus demonstrating again that both methods provide a satisfying elect ronic bond picture for the Y-Ba-Cu-O compounds [except for the planar Cu(2) site]. The temperature dependence of the EFG arises from thermal expansion only. The internal field, B(T), has been deduced from the m odulation of the Ba spin-echo intensity. A calculation of the dipolar field at the Ba site produced by Cu(2) d electrons yields a value that is about three times larger than the experimental result. The discrep ancy could be explained by assuming that part of the magnetic moment i s located at oxygen ions. The temperature variation of B(T) follows, u p to 402 K, a power law [B(0)-B(T)]/B(0)=AT(alpha) with alpha=1.82(22) which agrees quite well with the result of a Cu(2) in-plane determina tion of the sublattice magnetization. Furthermore, this result is in a ccord with a spin-wave model for a quasi-two-dimensional (2D) antiferr omagnet. The ''critical exponent'' beta is estimated to be less than o r equal to 0.18 which is in accord with values proposed by models for 2D ordered magnetic systems. Thus YBa2Cu3O6.05 behaves, in terms of it s spin dynamics, as a quasi-2D antiferromagnet and this character can be studied either at out-of-plane Ba or at in-plane Cu(2) sites.