Abstract
The spin supersolid—a magnetic analogue of the supersolid that simultaneously exhibits solid and superfluid orders—has emerged as a promising sub-Kelvin refrigerant with strong low-energy fluctuations and associated entropic effects1. However, the stringent prerequisites have so far confined its presence to certain magnetic insulators. Here we report the discovery of a metallic spin supersolid in a rare-earth compound EuCo2Al9 (ECA), which is a good metal with excellent electrical and thermal conductivity. The high-spin Eu2+ ions form a three-dimensional lattice with stacked triangular layers, in which the spin-supersolid state is stabilized through a mechanism involving both Ruderman–Kittel–Kasuya–Yosida (RKKY) and dipolar couplings. Neutron diffraction shows microscopic evidence of spin supersolidity, demonstrating the coexistence of out-of-plane and in-plane spin orders in this alloy. Our RKKY–dipolar model successfully captures the metallic spin-supersolid Y and V phases in ECA, along with the 1/3 magnetization plateau. The observed nonclassical magnetization behaviours within these phases point to significant quantum fluctuations, probably enhanced by the conduction electrons. The resistivity measurements provide a transport probe for the spin-supersolid transitions, because of scattering of conduction electrons from local moments. Through the adiabatic demagnetization process, ECA achieves ultralow cooling to 106{ hinspace}mK, exhibiting a giant magnetocaloric effect that manifests sharp anomalies in the magnetic Gr{“u}neisen ratio. ECA emerges as one of the first metallic spin supersolids, combining low cooling temperature, large magnetic entropy and ultrahigh thermal conductivity for high-performance sub-Kelvin refrigeration.