  Low density flux dynamics in Nd based cuprate (NdBa2Cu3O7-x) HTSC single
crystals trapped in 1 Oe range DC magnetic fields were investigated for the
first time. Complex measurements of time, temperature and field dependence of
magnetization were performed. Using the results obtained and the Anderson-Kim
model the normalized magnetization relaxation rate S and the averaged effective
pinning potential U were calculated. We show a significant (one or even two
orders) increase in the effective pinning potential in these samples in
comparison with measurements performed in high magnetic fields for similar
cuprate single crystals formerly. In a wide enough range of temperatures close
to Tc (0.72 < T/Tc < 0.93) it has been shown that the doping of NdBa2Cu3O7-x
single crystals with calcium atoms leads to the increase of the averaged
effective pinning potential with an insignificant decreasing of the critical
temperature.
