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Surface barriers and bulk pinning in YBa2Cu4O8.

Strong surface barrier effects have been observed for the motion of vortices in single-crystal YBa[sub 2]Cu[sub 4]O[sub 8]. They are most easily visible between 25 and 50 K where a typical hysteresis loop for fields less than 1 tesla shows a classic Bean-Livingston shape with magnetization close to zero for the leg of the hysteresis loop where vortices are exiting the sample. At higher fields, bulk pinning grows dramatically with a magnetic hysteresis loop that rises and falls with a classic fishtail shape. For these YBa[sub 2]Cu[sub 4]O[sub 8] single crystals, surface barriers dominate at low field with little bulk pinning. At 35 K, bulk pinning turns on at about 1 T.