Static magnetic order and anisotropy of the layered cobalt dioxides Bi1.6Pb0.4Sr2Co2Oy and Bi2Sr2Co2Oy

The magnetism of a Pb-doped Bi2Sr2Co2Oy (BSCO) crystal has been investigated by positive muon-spin rotation and relaxation (μ+SR) spectroscopy. Weak transverse field (wTF)-μ+SR measurements show that the whole sample enters into a magnetic state below ∼4.5K. Combining the results of zero-field (ZF)-μ+SR experiment with susceptibility measurements, it is clarified that the sample is in a ferromagnetic ordered phase with a Curie temperature (TC) of 4.7 K and with the ordered internal magnetic field parallel to the c-axis within ∼±15∘. On the other hand, a pure BSCO crystal is also found to exhibit a bulk magnetic transition at 1.0 K by μ+SR. Since the relationship between the reduced T and reduced internal magnetic field for BSCO is almost equivalent to that for Pb-doped BSCO, the origin of the magnetic transition for both crystals is thought to be explained by common physics.