We present a comprehensive study of the low-temperature magnetism and field-tuned thermodynamics of the layered Shastry-Sutherland compound ${\mathrm{Eu}}_2{\mathrm{MgSi}}_2{\mathrm{O}}_7$. ${\mathrm{Eu}}^{2+}$ ions ($4f^7, S=7/2$) form a quasi-two-dimensional orthogonal-dimer network, realizing a large-spin Heisenberg system on a Shastry-Sutherland geometry that is prone to geometric frustration. Magnetic susceptibility and magnetization measurements reveal nearly isotropic ${\mathrm{Eu}}^{2+}$ moments with weak antiferromagnetic interactions. Specific-heat data uncover a primary long-range ordered state at $T_{\mathrm{N}1}=0.8\mathrm{K}$ and a second anomaly near $T_{\mathrm{N}2}=1.0\mathrm{K}$ that is rapidly suppressed by magnetic field, suggesting that it is more field sensitive and may not correspond to an independent thermodynamic phase transition. By separating the magnetic heat capacity, we determine the full magnetic-entropy surface $S_{\mathrm{m}}(T,B)$, which captures the smooth field-driven evolution from the ordered state to a correlated and eventually polarized paramagnetic regime. The resulting isentropic trajectories demonstrate efficient adiabatic demagnetization, allowing cooling from 1.5–3.1 K at 5 T to minimum temperatures of 0.21–0.50 K. Comparison with the benchmark refrigerant ${\mathrm{Gd}}_3{\mathrm{Ga}}_5{\mathrm{O}}_{12}$ shows that ${\mathrm{Eu}}_2{\mathrm{MgSi}}_2{\mathrm{O}}_7$ exhibits enhanced sub-Kelvin magnetic heat-storage capacity, reflecting its large accessible spin entropy and weak interactions. These results establish ${\mathrm{Eu}}_2{\mathrm{MgSi}}_2{\mathrm{O}}_7$ as a rare example of a Eu-based frustrated magnet that combines weak interactions, a well-resolved low-temperature ordered state, and tunable thermodynamics suitable for sub-Kelvin cryogenic applications.

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• Magnetism
• Phase transitions
• Magnetic refrigeration