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

TL;DR

This study uncovers supercritical behavior and universal magnetocaloric effects in the frustrated antiferromagnet Nd$_3$BWO$_9$, revealing critical phenomena analogous to liquid-gas transitions and potential for magnetic cooling applications.

Contribution

It extends the analogy between liquid-gas and magnetic phase diagrams to a frustrated antiferromagnet, identifying a critical endpoint and supercritical regime with universal scaling laws.

Findings

Identification of a critical endpoint at 1.04 T and 0.3 K.

Observation of a supercritical crossover adhering to universal scaling.

Demonstration of enhanced magnetic cooling near the critical endpoint.

Abstract

The celebrated analogy between the pressure-temperature phase diagram of a liquid-gas system and the field-temperature phase diagram of 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 Nd$_3$BWO$_9$ with kagome layers. In its phase diagram, we identify a metamagnetic transition line with a critical endpoint (CEP) located at $\mu_0H_{\mathrm{c}} \simeq 1.04$ T and $T_{\mathrm{c}} \simeq 0.3$ K. Above the CEP, an Ising supercritical regime} emerges with supercritical crossover lines that adhere to a universal scaling law, as evidenced by the specific heat, magnetic susceptibility, and magnetocaloric measurements. Remarkably, we observe a highly sensitive field dependence in the magnetic cooling near the emergent CEP, characterized by a divergent magnetic Gr\"uneisen ratio $\Gamma_H \propto 1/t^{\beta+\gamma-1}$, with $\beta + \gamma \simeq 1.563$ the critical exponents of 3D Ising universality class and $t \equiv (T-T_{\rm c})/T_{\rm c}$ the reduced temperature. Our adiabatic demagnetization measurements on Nd$_3$BWO$_9$ reveal a lowest temperature of 195~mK, achieved from the initial condition of 2 K and 4 T. Our findings open a new avenue for studying supercritical phenomena and magnetic cooling in rare-earth RE$_3$BWO$_9$ family and, more broadly, in Ising-anisotropic magnets such as spin ices.