We report on magnetic measurements carried out in a ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.65}$ deoxygenated single crystal with superconducting transition temperature $T_c=62.5\mathrm{}\hspace{0.5em}\mathrm{K}.$ The so-called fishtail or second magnetization peak ${(H}_p)$ has been observed in magnetization curves from 60 K down to 1.8 K. In the region $6\hspace{0.5em}\mathrm{K}<T<\mathrm{}60\mathrm{}\hspace{0.5em}\mathrm{K},$ $H_p$ increases with a positive curvature as the temperature is lowered. For $T<\mathrm{}6\mathrm{}\hspace{0.5em}\mathrm{K},$ $H_p$ increases linearly with temperature and an extrapolation to $T=0\mathrm{}$ gives $H_p\mathrm{}\left(0\right)=2.9\mathrm{}\hspace{0.5em}\mathrm{T}.$ This value occurs just above a decoupling field $H_d,$ above which there is a change in the temperature dependence of the irreversibility line, a departure from the relation $(1\mathrm{}-{T/T}_c{)}^m.$ Magnetic relaxation measurements for fields in the region of the second magnetization peak indicate a change in the mechanism originating the second peak below 5 K. At higher temperatures the peak position changes with time. For temperatures lower than 5 K there is no differences in relaxation below and above $H_p,$ and the position of the peak does not change with time.

• Received 15 January 1999

DOI:https://doi.org/10.1103/PhysRevB.60.102