  We have performed extensive zero field muSR experiments on pure YBa2Cu3O6+y
and diluted Y-rare-earth substituted Y0.92Eu0.08Ba2Cu3O6+y and
Y0.925Nd0.075Ba2Cu3O6+y at light hole-doping. A common magnetic behavior is
detected for all the three families, demonstrating negligible effects of the
isovalent Y-substituent disorder. Two distinct regimes are identified,
separated by a crossover, whose origin is attributed to the concurrent thermal
activation of spin and charge degrees of freedom: a thermally activated and a
re-entrant quenched-hole antiferromagnetic regime. The peculiar temperature and
hole density dependence of the magnetic moment m(h,T) fit a model with a (spin)
activation energy for the crossover between the two regimes throughout the
entire investigated range. The magnetic moment is suppressed by a simple
dilution mechanism both in the reentrant regime (0<h<0.056) and in the
so-called Cluster Spin Glass state coexisting with superconductivity
(0.056<h<0.08). We argue a common magnetic ground state for these two doping
regions and dub it quenched-hole antiferromagnet. Conversely frustration
prevails in the thermally activated regime, that vanishes at the same
concentration where superconductivity emerges, suggesting the presence of a
quantum critical point at h_c=0.056(2).
