Ising Supercriticality and Universal Magnetocalorics in Spiral Antiferromagnet Nd$_3$BWO$_9$

The celebrated analogy between the pressure-temperature phase diagram of a liquid-gas system and the field-temperature phase diagram of an Ising ferromagnet has long been a cornerstone for understanding universality in critical phenomena. Here we extend this analogy to a highly frustrated antiferromagnet, the kagome-layered spiral Ising compound Nd$_3$BWO$_9$. In its field-temperature phase diagram, we identify a critical endpoint (CEP) and an associated Ising supercritical regime (ISR). The CEP of the metamagnetic transition is located at ${μ_0H_c} \simeq 1.04$ T and $T_c \simeq 0.3$ K. Above this point, the ISR emerges with supercritical crossover lines that adhere to a universal scaling law, as evidenced by the specific heat and magnetic susceptibility measurements. Remarkably, we observe a universally divergent Grueneisen ratio near the emergent CEP, $Γ_H \propto 1/t^{β+γ-1}$, with $β + γ \simeq 1.563$ the critical exponents of the 3D Ising universality class and $t \equiv (T - T_c)/T_c$ the reduced temperature. Our adiabatic demagnetization measurements on Nd$_3$BWO$_9$ reveal a lowest temperature of 195 mK achieved from 2 K and 4 T. Our work opens new avenues for studying supercritical physics and efficient cooling in layered-kagome, rare-earth RE$_3$BWO$_9$ family and, more broadly, in Ising-anisotropic magnets like spin ices.