The celebrated analogy between the pressure-temperature phase diagram of a liquid-gas system and the field-temperature phase diagram of a ferromagnet has long been a cornerstone for understanding universality of phase transitions and critical phenomena. Here we extend this analogy to a highly frustrated antiferromagnet, the spiral Ising compound ${\mathrm{Nd}}_3{\mathrm{BWO}}_9$ with kagome layers. In its phase diagram, we identify a metamagnetic transition line with a critical endpoint (CEP) located at ${\mu }_0{H}_{\mathrm{c}}\simeq 1.04\mathrm{T}$ and $T_{\mathrm{c}}\simeq 0.3\mathrm{K}$. Above the CEP, an Ising supercritical regime emerges with crossover lines that follow a universal scaling law, as evidenced by the specific heat, magnetic susceptibility, and magnetocaloric measurements. Remarkably, we observe critically enhanced magnetic cooling near the emergent CEP, characterized by a divergent magnetic Grüneisen ratio ${\mathrm{\Gamma }}_H\propto 1/t^{\beta +\gamma -1}$, with $\beta +\gamma \simeq 1.563$ the sum of critical exponents of the 3D Ising universality class and $t\equiv (T-T_{\mathrm{c}})/T_{\mathrm{c}}$ the reduced temperature. Adiabatic demagnetization from 2 K and 4 T reaches a minimum temperature of 195 mK, via a self-cascading process that combines supercritical and topological cooling. Our findings open a new avenue for studying supercritical phenomena and magnetic refrigeration with the frustrated rare-earth compounds ${\mathrm{RE}}_3{\mathrm{BWO}}_9$ and, more broadly, in Ising-anisotropic antiferromagnets such as spin ices.

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• Critical phenomena
• Frustrated magnetism
• Magnetic phase transitions
• Magnetocaloric effect
• Antiferromagnets
• Magnetic insulators
• Magnetic systems
• Quantum many-body systems
• Adiabatic demagnetization
• Magnetization measurements
• Scaling methods
• Specific heat measurements