Hyperfine Interactions 105 (1997) 71–76 71 Phase separation in antiferromagnetic YBa 2 Cu 3 O 6+x C. Bucci a , R. De Renzi a , G. Guidi a , G. Allodi a,* and F. Licci b a Dipartimento di Fisica and Istituto Nazionale di Fisica della Materia, Universit` a di Parma, I-43100 Parma, Italy b Istituto MASPEC-CNR, Parma, Italy We present a detailed study of the ZF-μSR precession signals in AF YBa2Cu3O6+x for a series of 0.06  x  0.39 ceramic samples. Careful analysis of the temperature and oxygen content dependence shows a complex behaviour of the magnetic field at the muon site. The two main novelties are that a) the muon does not diffuse freely above 250 K, as previously stated, and that b) mobile holes phase separate, inducing a finite size reduction of the AF order parameter. 1. Introduction Low temperature ZF-μSR measurements of the order parameter in antiferromagnetic (AF) copper perovskites – obtained via the local field B μ – deviate significantly from a simple regular behaviour. Lightly doped samples [1,2] show a sharp increase of B μ at low temperature, which was interpreted [2] as a freezing temperature for the spin disturbance associated with holes transferred to the CuO 2 layers. However the order parameter, i.e. the reduced sublattice spontaneous magnetiza- tion, m(T )= M s (T )/M s (0), is measured in YBa 2 Cu 3 O 6+x also by elastic neutron scattering (ENS) [3] and zero-field NMR [4]. Different probes detect slightly differ- ent features, although all of them observe low temperature anomalies in m(T ). In order to reach a unified and coherent description of YBa 2 Cu 3 O 6+x intrinsic magnetic properties, one must gain a better understanding of the muon version of facts! A careful determination of the AF YBa 2 Cu 3 O 6+x intrinsic properties must consider in particular the multiple site occupancy, which takes place in YBa 2 Cu 3 O 6+x , and the information conveyed by the muons in the various sites as a function of temperature and oxygen doping still has to be fully disclosed. In this connection, La 2-x Sr x CuO 4 is a much simpler case for two reasons: muons occupy a single interstitial lattice site and the Cu magnetic moments order in the [110] * Partially supported by EEC grant CHRX CT930113.  J.C. Baltzer AG, Science Publishers