Abstract
The magnetism of pure and Pb-doped ${\text{Bi}}_{2}{\text{Sr}}_{2}{\text{Co}}_{2}{\text{O}}_{y}$ (BSCO) crystals has been investigated with positive muon-spin rotation and relaxation $({\ensuremath{\mu}}^{+}\text{SR})$ spectroscopy. The entire volume of both materials enters into a magnetic state at low temperatures, occurring below 4.7 K for Pb-doped BSCO and below 1.0 K for pristine BSCO. By combining ${\ensuremath{\mu}}^{+}\text{SR}$ and susceptibility measurements, it is clarified that Pb-doped BSCO is a ferromagnet with a Curie temperature $({T}_{C})$ of 4.7 K and its ordered internal magnetic field $({H}_{\text{int}})$ is almost parallel to the $c$ axis. Since the relationship between the reduced $T$ and reduced ${H}_{\text{int}}$ for Pb-doped BSCO is very similar to that for BSCO, the origin of the magnetic transition in both crystals is thought to be explained by common physics. Interestingly, we also detect the existence of a magnetic anomaly far above ${T}_{\text{C}}$. This occurs at $\ensuremath{\approx}60\text{ }\text{K}$, coinciding with the metal-to-insulator transition that was observed for both materials by resistivity measurements.
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