Temperature-dependent magnetic relaxation was studied in a La2CuO4+del
ta crystal in the low-temperature range 2-10 K. The experimental data
exhibited a non-vanishing magnetic relaxation in the low-temperature l
imit. These data could be well interpreted by a quantum-creep theory,
suggesting the non-vanishing relaxation at low temperatures due to qua
ntum tunneling of vortices. The effective Euclidean action determined
as HBAR/\d ln M/d ln t\ showed a quadratic temperature dependence. The
quantitative analysis of the effective Euclidean action yields a clas
sical activation energy, crossover temperature and zero-temperature qu
antum-tunneling rate.