Surface barriers and bulk pinning in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_4$${\mathrm{O}}_8$

Strong surface barrier effects have been observed for the motion of vortices in single-crystal ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_4$ ${\mathrm{O}}_8$. 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 ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_4$ ${\mathrm{O}}_8$ single crystals, surface barriers dominate at low field with little bulk pinning. At 35 K, bulk pinning turns on at about 1 T.