Vortex phase diagrams of YBa2Cu4O8 in H parallel c and H parallel b

Vortex phase diagram of YBa2Cu4O8 has been investigated by means of the AC susceptibility and the DC magnetization. Phase diagram in H parallel c is very different from Bi2212 and Y123 because the onset field of the magnetization peak increases rapidly as T decreases at temperatures below 40 K. The AC susceptibility and the DC magnetization measurements in H parallel b reveal that the irreversibility line is almost independent of the applied field (at ∼40 K). We argue that there must be an additional pinning mechanism in YBa2Cu4O8 at temperatures below 40 K compared to Bi2212 and Y123. A possible candidate for pinning centers is a spin density wave which was recently predicted by our group