We performed zero and transverse field µSR experiments on a large number of YBa2Cu3O6+x samples. We detect coexistence of antiferro-type (AF) short range magnetism with superconductivity below T f 10 K in compositions 0.37 x 0.39. Most muons experience local AF fields, even when SQUID detects a full superconducting volume fraction, which points to a local minimal interference organization of short AF stripes embedded in the superconductor. A detailed phase diagram is produced and the consequences of the minimal interference are discussed. Neutron scattering also detects magnetic correlations, notably [10] in YBa 2 Cu 3 O 6.35 , exclusively dynamic in nature, although their static counterpart could be elusive due to a very short correlation length. The doubled magnetic unit cell indicates an antiferromagnetic (AF) structure, with a suggested stripe-like character.In all the samples explored so far by µSR [5,6,7,8,9] it has been hinted that the cluster spin glass nature of magnetism (low spin freezing temperature, T f , large distributions of B i and absence of long range order as from neutron diffraction [11,12]) might be favored by the disorder inherent in cation substituted perovskites, which directly influences the CuO 2 layers. Conversely in YBa 2 Cu 3 O 6+x (123, hereafter) the source of disorder, namely the basal CuO x layers, are farther removed from the CuO 2 layers, but systematic µSR data were lacking, prior to the present work. Our aim is to clarify whether the appearance of coexisting superconducting and magnetic properties is indeed intrinsic to the unperturbed underdoped CuO 2 layers and whether the two properties cooperate or interfere.We performed µSR measurements on twenty-four polycrystalline 123 samples (Y 1−24 ) prepared by the topotactic technique, which consists of oxygen equilibration of stoichiometric quantities of the two end member specimens, tightly packed in sealed vessels [13]. Low temperature annealing yields high quality homogeneous samples with an absolute error of δx=±0.02 in oxygen content per formula unit and a much smaller relative error between samples of the same batch. The width of the interval were the resistance drops from 90% to 10% of the onset value is 0.5 K at optimal doping and 6-7K at x 0.4 (vs. e.g. 10K in Ref.10). The hole content h was determined from the resistive T c for the superconducting samples [14], and from the Seebeck coefficient S at 290 K for the non-superconducting ones, using the exponential dependence [14] of S on h, with fit parameters determined from our series of samples [15]. Samples Y 1−8 , with oxygen content 0.20≤x≤0.32 and hole content per planar Cu atom 0.033≤h≤0.055, never superconduct, and their AF properties were reported previously [16]. Samples Y 9−24 , with 0.32≤x≤0.42 and 0.055≤h≤0.08, are superconductors and are the subject of the present work.The µSR experiment were performed on the MUSR spectrometer of the ISIS pulsed muon facility, where the external magnetic field H may be applied either parallel to the initial muon spin S µ , in...

We have performed extensive zero field µSR experiments on pure YBa2Cu3O6+y and diluted Yrare-earth substituted Y0.92Eu0.08Ba2Cu3O6+y and Y0.925Nd0.075Ba2Cu3O6+y at light hole-doping. A common magnetic behavior is detected for all the three families, demonstrating negligible effects of the isovalent Y-substituent disorder. Two distinct regimes are identified, separated by a crossover, whose origin is attributed to the concurrent thermal activation of spin and charge degrees of freedom: a thermally activated and a re-entrant antiferromagnetic regime. The peculiar temperature and hole density dependence of the magnetic moment m(h, T ) fit a model with a (spin) activation energy for the crossover between the two regimes throughout the entire investigated range. The magnetic moment is suppressed by a simple dilution mechanism both in the re-entrant regime (0 ≤ h ≤ 0.056) and in the so-called Cluster Spin Glass state coexisting with superconductivity (0.056 < h 0.08). We argue a common magnetic ground state for these two doping regions and dub it frozen antiferromagnet. Conversely either frustration or finite-size effects prevail in the thermally activated antiferromagnetic state, that vanishes at the same concentration where superconductivity emerges, suggesting the presence of a quantum critical point at hc = 0.056(2).

We present Muon spectroscopy investigations on SmFeAsO 1−x F x showing coexistence of magnetic order and superconductivity only in a very narrow F-doping range. The sharp crossover between the two types of order is similar to that observed in LaFeAsO 1−x F x , suggesting a common behavior for the 1111 pnictides. The analysis of the muon asymmetry demonstrates that the coexistence must be nanoscopic, i.e., the two phases must be finely interspersed over a typical length scale of few nm. In this regime both the magnetic and the superconducting transition temperatures collapse to very low values. Our data suggest a competition between the two order parameters.