A μSR study of spin dynamics in the n-type superconductor Nd1.85Ce0.15CuO4+δ

Because of the low number of chemical degrees of freedom and the unique band at the Fermi level, the copper oxide n-type superconductor Nd1.85Ce0.15CuO4+d can be considered as a model system for the study of the physical properties related to the charge carrier density. Resistivity , thermopower and magnetic susceptibility in samples of Nd1.85Ce0.15CuO4+d with well-known charge carrier density n , controlled by varying the oxygen content from d=0 to d=0.06 ( n=(4.075-d)*2-8 ) , suggested a possible interaction between carriers well above Tc [1]. Here we present a mu+SR investigation performed with the aim of adding important information on the mechanism at the basis of such an interaction. ZF-mu+SR measurements were made as a function of temperature in the range 10-100K in 5 samples prepared under a total pressure of 1 atmosphere and at different oxygen partial pressures: 1 (sample A), 10-2 (sample B), 10-3 (sample C), 10-4 (sample D), 10-6 (sample E) atmospheres, and extended in the range 0.08-10K for samples A and E. LF-mu+SR measurements were done on sample A at H=100, 150, 250 G and on sample E (d=0.014, n=0.12) at H=30, 60, 100G. The samples D and E are superconducting with Tc=18K and Tc=25K (onset); the other samples are non superconducting. We observed that in all the samples except the sample E, the asymmetry curve in ZF at low T (T<20K) has a form very similar to the one expected in systems where a spin freezing sets in (like for example in spin glasses, SG). In a recent paper [2] the SG phase in sample A was confirmed experimentally by a clear maximum in the ZFC susceptibility curve (blocking temperature Tg=5K), the associated irreversible ZFC-FC behaviour and the time behaviour of the relaxation. By current mu+SR measurements, the magnetic phase diagram was more clearly established. Moreover, we suggest that an antiferromagnetic disordered phase could co-exist with the superconducting phase, at least in sample D