Absence of pinning mechanism change in the second magnetization peak of YBa2Cu3O6.5 at low temperature

We have measured hysteresis curves M(H) and magnetic relaxation M(t) for a YBa2Cu3O6.5 single crystal (Tc = 62.5 K) with the magnetic field H // c axi s. Magnetic relaxation was measured for values of field below and above the second magnetization peak Hp in hysteresis curves, at 1.8, 4.4 and 20 K. T he activation pinning energy U is obtained from relaxation data, generating U(M) vs M(t) curves. All U(M) curves follow a power law behavior U similar to M-alpha. At 20 K and 4.4 K the curves U(M) obtained for fields below th e second peak Hp, presents a distinct behavior from the curves obtained for fields above Hp. At 1.8 K all U(H,M) curves present the same power law beh avior with alpha similar to 1.5. This fact highly suggests the existence of the second magnetization peak with only one pinning mechanism.