Irreversibility temperatures in superconducting oxides: The flux-line-lattice melting, the glass-liquid transition, or the depinning temperatures

The magnetic-field dependence of the irreversibility temperatures follows an H=a[1-Tᵣ(H)/Tc(0)]ⁿ relationship with n{}1.5, for pure and alloyed YBa₂(Cu_1-x{M}ₓ)₃{O}_{7+{{δ}}}$ with x=0 and 0.02, where M=Al, Fe, Ni, and Zn, measured for an applied field parallel to the c axis. However, for M=Ni and x=0.04 and 0.06, n{}2.0. This relationship is not applicable for either Bi₂{Sr}₂CaCu₂{O}₈$ or (Bi,Pb${)}₂Sr₂{Ca}₂Cu₃{O}₁₀powders. It is also shown that the irreversibility temperature is a strong function of the magnetic hysteresis width ΔM for pure and alloyed{YBa}₂Cu₃{O}₇. These results and the measurements of the flux creep ΔM(t) for these specimens suggest that{T}ᵣ$(H) is a depinning line rather than a lattice melting or glass-to-liquid phase-transition temperature. However, the conventional flux-creep model cannot account for all of the observed temporal dependences of {Δ}M(t).