A Triangular FrustratedEu(II)–Organic Frameworkfor Sub-Kelvin Magnetic Refrigeration

Attaining sub-Kelvin temperatures remains technologically challenging and often relies on the scarce resource ³He, unless employing adiabatic demagnetization refrigeration. Herein, the active coolant typically consists of weakly coupled paramagnetic ions, whose magnetic interaction strengths are comparable in energy to the relevant temperature regime of cooling. Such interactions depend strongly on inter-ion distances, fundamentally hindering the realization of dense coolants for sub-Kelvin refrigeration. We present a magnetically concentrated triangular coordination network, Eu_0.9Ba_0.1I₂(pyrazine)₃, featuring the large <i>s</i> = 7/2 moment of Eu(II). Electron paramagnetic resonance, magnetization, and heat capacity measurements reflect antiferromagnetic correlations between the Eu(II) ions and a dominant easy-plane magnetic anisotropy. The ensuing geometric frustration prevents entropy-annihilating magnetic order down to at least 0.17 K. This remarkably low operational temperature for a low-dimensional, molecule-based magnetic refrigerant opens possibilities for on-chip cryogenic refrigeration, especially in scenarios where established inorganic refrigerants cannot be utilized.