  Doping of quantum antiferromagnets is an established approach to investigate
the robustness of their ground state against the competing phases. Predictions
of doping effects on the ground state of the Shastry-Sutherland dimer model are
here verified experimentally on Mg-doped SrCu2(BO3)2. A partial incorporation
of Mg2+ on the Cu2+-site in the SrCu2(BO3)2 structure leads to a subtle but
systematic lattice expansion with the increasing Mg-doping concentration, which
is accompanied by a concomitant decrease in the spin gap, the Curie-Weiss
temperature and the peak temperature of the susceptibility. These findings
indicate a doping-induced breaking of Cu2+ spin-1/2 dimers which is also
corroborated by X-band EPR spectroscopy that points to a systematic increase in
intensity of free Cu2+ sites with increasing Mg-doping concentration. Extending
the Mg-doping up to nominal x = 0.10 or SrCu1.9Mg0.1(BO3)2, in the
magnetisation measurements taken up to 35 T, a suppression of the pseudo-1/8
plateau is found along with a clear presence of an anomaly at an onset critical
field H'C0 ~ 9 T. The latter, absent in pure SrCu2(BO3)2, emerges due to the
coupling of liberated Cu2+ spin-1/2 entities in the vicinity of Mg-doping
induced impurities.
