A Triangular Frustrated Eu(II)–Organic Framework for Sub-Kelvin Magnetic Refrigeration

Attaining very-low temperatures remains technologically challenging and often relies on the scarce resource 3He, unless employing adiabatic demagnetisation 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 realisation of dense coolants for sub-Kelvin refrigeration. We present a magnetically concentrated triangular coordination network, Eu0.9Ba0.1I2(pyrazine)3, featuring the large s = 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, that is, a remarkably low working temperature for a relatively dense magnetic refrigerant.