Vortex pinning in a rotated YBa₂Cu₃O_7-δ crystal

Magnetization-vector measurements were made on an YBCO (YBa₂Cu₃O_7-δ) crystal rotated about its c-axis in various fixed fields at different temperatures. The pinning torque per vortex (τ_p) in the ab-plane is found to have a broad distribution in strength, similar to previous findings for polycrystalline samples of YBCO and BKBO (Ba_0.57K_0.43BiO₃). Simple modeling shows that the τ_p distribution function rises linearly with increasing rp and then drops rapidly to zero. With increasing field at each temperature, the average τ_p decreases gradually after rising to a broad peak. The peak value of τ_p/ μ ( μ being the vortex moment) defines H_P, a characteristic pinning field per vortex, and it is found to decrease with increasing temperature (T) in a distinctly two-component manner. One component of H_p falls off very rapidly as (1 - T/T_c)^β with β ≈ 10, while the other component descends linearly to zero as T approaches T_c. A similar two-component behavior of H_p versus T is found in polycrystalline YBCO, whereas in polycrystalline BKBO H_P decreases gradually with increasing T at all temperatures. The strongly temperature dependent H_p component in YBCO may derive from vortex pinning by oxygen vacancies, as suggested theoretically.