Higher-Magnesium-Doping Effects on the Singlet Ground State of the Shastry-Sutherland SrCu₂(BO₃)₂

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 slight 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 that is also corroborated by X-band EPR spectroscopy that points to a systematic increase in the intensity of free Cu²⁺ sites with increasing Mg-doping concentration. Extending the Mg-doping up to nominal x = 0.10 yielding SrCu_1.9Mg_0.1(BO₃)₂, in the magnetization 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 μ₀H'_C0 ≈ 9 T. The latter, absent in pure SrCu₂(BO₃)₂, emerges due to the pairwise coupling of liberated Cu²⁺ spin-1/2 entities in the vicinity of Mg-doping induced impurities.