Giant Magnetocaloric Effect in a Honeycomb Spiral Spin-Liquid Candidate

Unlike conventional magnetic states, which lack degeneracy, the spiral spin liquid (SSL) fluctuates among degenerate spiral configurations, with ground-state wave vectors forming a continuous contour or surface in reciprocal space. At low temperatures, the field-induced crossover from the polarized ferromagnetic state to the SSL results in a large entropy increase and decalescence, indicating its potential for magnetic cooling. However, magnetic cooling using a SSL has yet to be reported. Here, the magnetocaloric effect and cooling performance of single-crystal GdZnPO, a spin-7/2 honeycomb-lattice SSL candidate, are investigated under a magnetic field H < H_c (µ₀H_c ∼ 12 T) applied perpendicular to the honeycomb plane and below the crossover temperature (∼2 K). For H ⩾ H_c, GdZnPO enters a polarized non-degenerate ferromagnetic state. These results demonstrate that GdZnPO exhibits a giant low-temperature magnetocaloric effect near H_c, surpassing other magnetocaloric materials. This giant magnetocaloric effect is well-explained by the frustrated honeycomb spin model of GdZnPO, suggesting the stability of the SSL below H_c down to very low temperatures. Additionally, its magnetic cooling performance remains robust up to at least 4.5 K, making GdZnPO a promising candidate for magnetic refrigeration down to ∼36 mK through cycling the applied magnetic field within a narrow range.