Magnetic-field-dependent microwave absorption in the superconducting state of single crystals of Bi₂Sr₂CaCu₂O _8+x

The derivative of the microwave absorption versus DC magnetic field strength is observed near zero field in the superconducting state of single crystals of Bi₂Sr₂CaCu₂O_8+x. The absorption intensity decreases as the DC magnetic field is rotated away from the crystallographic c axis, which indicates preferential orientation of weak link current loops parallel to the largest crystal facet. The hysteresis of the low field absorption is also anisotropic because of the anisotropy of flux pinning barriers. Direct microwave absorption measurements at higher fields show a continued increase in absorption above 0.1 T which is associated with flux motion. The temperature, field and orientation dependence of this absorption are reported and interpreted in terms of flux motion. Barriers to flux motion in the surface regions of the crystal are estimated.