Columnar defects (CD’s) inclined at 45° from the c axis have been introduced in single crystals of different compounds: ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$, ${\mathrm{Tl}}_2$${\mathrm{Ba}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$, ${\mathrm{Tl}}_2$${\mathrm{Ba}}_2$${\mathrm{Ca}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{10}$, ${\mathrm{Tl}}_{2/3}$${\mathrm{Bi}}_{1/3}$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_7$, and ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$. In each case, hysteresis loops have been recorded with the magnetic field parallel to the columns (H${\to }_{\mathit{a}}$∥CD) and perpendicular to them (H${\to }_{\mathit{a}}$⊥CD). Two regimes are evidenced: an isotropic pinning enhancement [${\mathit{J}}_{\mathit{c}}$(H${\to }_{\mathit{a}}$⊥CD)≡${\mathit{J}}_{\mathit{c}}$(H${\to }_{\mathit{a}}$∥CD)] at low temperatures and a directional effect [${\mathit{J}}_{\mathit{c}}$(H${\to }_{\mathit{a}}$⊥CD)<${\mathit{J}}_{\mathit{c}}$(H${\to }_{\mathit{a}}$∥CD)] at higher temperatures. The isotropic regime is ascribed to the occurrence of a ‘‘totally adjusted’’ state where the vortices zigzag between the CD’s and the ab planes, keeping their mean direction along the applied field. An analysis is proposed for the crossover between these two regimes, which accounts for the observed influences of the temperature and electronic anisotropy. The vanishing of the difference between the two configurations at zero field is discussed in terms of a complete reorientation of the vortices along the CD’s in the case ${\mathit{H}}_{\mathit{a}}$⊥CD. © 1996 The American Physical Society.

• Received 4 January 1996

DOI:https://doi.org/10.1103/PhysRevB.54.656