Magnetism and superconductivity in La1.875Ba0.125-ySryCuO4+delta and La1.6-yNd0.4SryCuO4

mu(+)SR and neutron diffraction measurements show that at low temperatures in La-1.875-Ba0.125CuO4, there is phase separation into a long range ordered magnetic component of SDW-like nature, identified with the LTT phase and a superconducting component, identified with the LTO phase. Partial substitution of Pa by Sr at a fixed doping level of 1/8 leads to gradual decrease of the magnetic freezing temperature, accompanied by a decrease in the fraction of the magnetic component. The La1.875Ba0.025Sr0.100CuO4 composition exhibits freezing of the Cu electronic moments into a disordered arrangement. Excess interstitial oxygen leads to a rapid recovery of superconductivity and a marked depression of the magnetic volume fraction. Study of the LTT phase - after stabilisation with partial substitution of La by Nd - away from the 1/8 doping level, reveals no evidence of long range magnetic ordering with magnetic fluctuations surviving down to 3 K.