In a groundbreaking advance bridging the worlds of magnetism and materials science, researchers have unveiled a metallic spin supersolid in the rare-earth compound EuCo₂Al₉ (ECA). Spin supersolids, magnetic analogues to supersolids that simultaneously exhibit solid and superfluid orders, were until now restricted to certain insulating magnets and confined to extreme sub-Kelvin regimes. This newly discovered state of matter in a metallic host not only expands the frontiers of quantum materials but also opens exciting avenues for practical applications in ultra-low-temperature refrigeration technologies.
EuCo₂Al₉ distinguishes itself as a good metal with exceptional electrical and thermal conductivity, defying conventional wisdom that spin supersolidity requires insulating environments. At the heart of the phenomenon lies the high-spin Eu²⁺ ions arrayed in a complex three-dimensional lattice comprising stacked triangular layers. The intricate interplay between Ruderman–Kittel–Kasuya–Yosida (RKKY) interactions—mediated by conduction electrons—and long-range dipolar couplings stabilizes the unusual spin-supersolid phases observed in this compound.
Neutron diffraction experiments provide definitive microscopic evidence of the spin supersolid state, revealing a coexistence of out-of-plane and in-plane magnetic orders within ECA. These concurrent orders manifest as Y and V phases in magnetization, highlighting the coexistence of solid-like magnetic rigidity and superfluid-like spin coherence across the lattice. Such direct observation confirms the theoretical predictions that have so far eluded empirical validation in metallic systems.
The persistent magnetization plateau at one-third of the saturation magnetization, a hallmark of frustrated magnetism consistent with spin supersolids, is captured exquisitely by a comprehensive RKKY–dipolar theoretical model developed by the research team. This framework not only explains the sequence of magnetic phases but also accounts for the substantial quantum fluctuations inherent in the metallic environment. These fluctuations appear enhanced by conduction electrons, challenging classic magnetic paradigms and hinting at rich underlying quantum many-body physics.
Electrical resistivity measurements present a novel transport-based probe of the spin supersolid transitions, as conduction electrons scatter off dynamic local magnetic moments. This coupling manifests in sharp anomalies in resistivity correlating with magnetic phase boundaries, providing real-time, non-invasive diagnostics of spin supersolidity. Such measurements complement neutron diffraction and magnetization data, enriching the multi-faceted observational landscape.
Remarkably, EuCo₂Al₉ achieves ultralow cooling down to 106 millikelvin through an adiabatic demagnetization process leveraging its giant magnetocaloric effect. This effect generates significant entropy changes tied to the spin-supersolid transitions, reflected in sharp features of the magnetic Grüneisen ratio – a thermodynamic quantity measuring how magnetic entropy varies with field and temperature. The synergy of large magnetic entropy and ultrahigh thermal conductivity in a metallic host creates a uniquely efficient sub-Kelvin refrigerant platform.
This discovery fundamentally shifts paradigms, demonstrating that metallic environments can not only host but also enhance spin supersolidity through conduction-electron-mediated interactions. The ability to combine solid and superfluid spin orders in a metal with high thermal conductivity bridges the gap between fundamental quantum phenomena and potential technological applications. It shows promise for high-performance refrigeration in quantum computing and cryogenic sensors, where stable and efficient ultralow temperatures are critical.
Beyond refrigeration, the presence of metallic spin supersolids could influence future studies of quantum phase transitions, magneto-transport phenomena, and spintronics devices. The coupling between itinerant electrons and local moments within a spin supersolid matrix invites exploration of unconventional mobility, magnetoresistance effects, and possibly novel quantum coherence phenomena extending over macroscopic scales.
The experimental realization of this metallic spin supersolid relied heavily on sophisticated neutron scattering techniques to resolve spatial spin textures alongside precise magnetization and transport measurements under varying magnetic fields and temperatures. Collectively, these multidisciplinary methods illuminated the delicate balance between competing magnetic orders stabilized by RKKY and dipolar couplings. The team’s theoretical insights further elucidated the pivotal role of quantum fluctuations enhanced by conduction electrons, unlocking new perspectives on entropic cooling mechanisms.
EuCo₂Al₉’s remarkable combination of electrical and thermal transport properties with complex magnetic order paves the way for engineering designer quantum materials that balance competing interactions to achieve tailored low-temperature functionalities. This tunability could inspire novel refrigeration technologies integrating magnetocaloric devices with efficient electrical control, potentially revolutionizing cryoelectronics and quantum information processing.
As the first reported metallic spin supersolid, EuCo₂Al₉ challenges the prevailing notion that spin supersolidity is confined to insulating magnets, transforming our understanding of magnetic ground states and their interplay with conduction electrons. Future studies may unveil other rare-earth or transition-metal compounds exhibiting similar phenomena, expanding the class of materials available for fundamental physics experiments and practical applications alike.
In conclusion, the breakthrough discovery of a metallic spin supersolid in EuCo₂Al₉ represents a milestone in condensed matter physics and materials science, marrying intricate spin textures with metallic conduction. Its pronounced magnetocaloric effect, quantum fluctuations, and multi-order magnetic phases provide new pathways toward efficient and effective sub-Kelvin refrigeration. This work stands at the forefront of quantum materials research, heralding both fundamental insights and transformative technologies beyond the laboratory.
Subject of Research:
Discovery of a metallic spin supersolid state and magnetocaloric effects in the rare-earth compound EuCo₂Al₉, exploring the coexistence of magnetic orders mediated by RKKY and dipolar couplings in a metallic environment.
Article Title:
Giant magnetocaloric effect and spin supersolid in a metallic dipolar magnet
Article References:
Shu, M., Xu, X., Xi, N. et al. Giant magnetocaloric effect and spin supersolid in a metallic dipolar magnet. Nature (2026). https://doi.org/10.1038/s41586-026-10144-z
Image Credits:
AI Generated
DOI:
https://doi.org/10.1038/s41586-026-10144-z
