Magnetic hysteresis loops and fast relaxation of vortex lattice in YBaCuO single crystal

The magnetic moment values /spl Mscr/ at conventional magnetic moment relaxation at constant magnetic field and in magnetic hysteresis loop experiment are directly related and are the same if /spl musub 0/(d/spl Mscrdt)/sub CR/=/spl chisub 0(dB/sub edt)/sub MHL/. Dependencies of /spl Mscr/(dB/sub edt) measured by vibrating sample magnetometer on YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta single crystals at temperatures ranging from 2.5 K to 54 K are analyzed. Activation energy U/sub e/=-kT[ln(dB/sub edt)-C] vs. /spl Mscr/ is plotted with parameter C being adjusted to form a smooth curve. In this way the critical sweep rate (dB/sub edt)c=exp(C) corresponding to the critical state U/sub e/(/spl Mscrsub c/)=0 is determined. A detailed analysis shows that In(dB/sub edt)c increases approximately linearly with temperature. This dependence can be removed by scaling the activation energy by a temperature dependent factor. At temperatures below 5 K departure from the thermally activated relaxation occurs, the origin of which can be the quantum tunnelling effect.