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

Traditional vapor-compression refrigeration technologies, ubiquitous in air conditioners, refrigerators, and freezers, have faced severe criticism due to their reliance on refrigerants with high global warming potential. Magnetic refrigeration offers a compelling alternative, utilizing magnetocaloric materials whose temperature changes when subjected to alternating magnetic fields, thereby eliminating the need for harmful chemical refrigerants. However, the field’s progress has been hampered by a fundamental tradeoff: materials that exhibit a large magnetocaloric cooling effect typically suffer from irreversible hysteresis losses, leading to rapid degradation over repeated thermal cycles. On the other hand, magnetocaloric materials engineered for durability generally exhibit diminished cooling performance. This inherent compromise has thwarted efforts to realize practical magnetic cooling devices with superior efficiency and longevity.

The research team’s innovative materials design approach targets this impasse by finely tuning the covalent bonding environment within intermetallic crystals. Their case study focused on the gadolinium-germanium compound Gd₅Ge₄, a well-known magnetic refrigerant displaying a strong magnetocaloric response coupled to a coupled magnetic-structural phase transition. When exposed to a magnetic field, the unpaired electron spins of Gd align, raising the material’s temperature through an adiabatic process. This magnetic ordering triggers a concomitant structural change, characterized by significant shifts in lattice parameters and interatomic distances, particularly between germanium atoms that connect structural slabs within the material. These atomic-scale distortions produce hysteresis, manifesting as energy losses that degrade refrigerated cooling upon cycling.

To overcome these challenges, the team employed a strategic chemical substitution, partially replacing germanium atoms with tin. This carefully controlled substitution modulates the covalent character of the bonds connecting the slabs, reducing the extent of geometric rearrangements during the phase transition. The result is a flattened energy landscape around the transition point, which suppresses hysteresis and its associated losses. Such precise bond chemistry control stabilizes the crystal lattice framework during repeated magnetization and demagnetization cycles, enabling durable performance without sacrificing the magnitude of the cooling effect.

Experimental validation of this design strategy revealed remarkable performance improvements. The partially substituted Gd₅(Ge₁₋ₓSnₓ)₄ compound exhibited a reversible adiabatic temperature change that more than doubled, increasing from approximately 3.8 K to 8 K under cycling conditions. This enhancement marks a significant leap forward in magnetic refrigerant functionality, as it combines both an intensified magnetocaloric response and enhanced cyclic stability. These features are crucial for translating laboratory-scale discoveries into reproducible, long-lasting refrigeration devices suitable for commercial and industrial deployment.

From a fundamental perspective, this research sheds light on the crucial interplay between electronic bonding, crystal structure, and magnetic order in determining magnetocaloric properties. By controlling covalent bonding networks, the energy barrier associated with the structural phase transition can be tuned, effectively minimizing irreversibility. This concept challenges conventional wisdom which often viewed magnetic and structural transitions as inseparable and difficult to decouple, offering a new paradigm for materials design across related fields such as spintronics and solid-state cooling technologies.

The implications of this research extend beyond room-temperature cooling applications. Given that the developed magnetocaloric materials operate effectively at cryogenic temperatures, they are highly promising candidates for next-generation hydrogen liquefaction technologies. The need for low-environmental-impact liquefaction methods is rapidly increasing alongside global efforts to adopt hydrogen as a clean energy carrier. The ability of this material system to deliver large cooling effects reliably under cyclic operation could significantly improve energy efficiency in hydrogen liquefiers, reducing carbon footprints associated with fuel production and storage.

Looking forward, the team envisions expanding the bond chemistry tuning approach to a broader class of intermetallic compounds, potentially unlocking magnetocaloric systems with customizable characteristics tailored for diverse cooling and gas liquefaction challenges. Integrating advanced characterization techniques such as synchrotron X-ray diffraction and neutron scattering, alongside computational modeling, will facilitate accelerated discovery and optimization. This strategy holds promise for the creation of an entirely new generation of magnetic refrigerants that combine energy efficiency, long-term stability, and reduced reliance on problematic refrigerants.

This research was enabled by extensive interdisciplinary collaboration, harnessing expertise in materials science, crystallography, magnetism, and chemical physics. Contributions came from senior researchers and emerging scientists across multiple prestigious institutions, supported by multiple international funding agencies including Japan’s JSPS and JST as well as Germany’s DFG. Such collective efforts exemplify the increasingly global nature of frontline scientific innovation, where cross-border knowledge exchange accelerates solutions for pressing technological and environmental challenges.

Beyond magnetic refrigeration, the concept of controlling covalent bonds to tune energy landscapes around phase transitions represents a versatile design principle. Analogous challenges encountered in thermoelectric materials, shape-memory alloys, and battery electrode materials could also potentially benefit from similar chemical engineering approaches. This could open exciting cross-disciplinary avenues towards materials with finely tuned phase stability and durability, enabling more efficient energy conversion and storage technologies essential for a sustainable future.

In summary, this landmark study demonstrates that precise atomic-scale control of bonding within magnetocaloric materials can decisively break the historical tradeoff between cooling efficacy and cyclic durability. Such achievements unlock new horizons for magnetic cooling technology as a powerful, environmentally friendly alternative to conventional refrigeration. By enabling large temperature swings without hysteresis losses, this approach paves the way for robust, energy-saving devices with transformative potential for everyday climate control, hydrogen energy infrastructure, and beyond.

Subject of Research:
Magnetic cooling materials; intermetallic compounds; magnetocaloric effect; covalent bonding; phase transition tuning.

Article Title:
Control of Covalent Bond Enables Efficient Magnetic Cooling

News Publication Date:
December 18, 2025

Web References:
DOI: 10.1002/adma.202514295

Image Credits:
Tang Xin, National Institute for Materials Science; Sepehri Navid Hossein Sepehri-Amin, National Institute for Materials Science; Tadakatsu Ohkubo, National Institute for Materials Science; Yoshio Miura, Kyoto Institute of Technology; Shintaro Kobayashi, Japan Synchrotron Radiation Research Institute; Takuo Ohkochi, University of Hyogo; Konstantin Skokov, Technical University of Darmstadt

Keywords

Magnetocaloric effect, magnetic refrigeration, Gd₅Ge₄, covalent bond tuning, hysteresis elimination, energy-efficient cooling, cryogenic temperature, hydrogen liquefaction, phase transition control, intermetallic compounds, cyclic stability, sustainable refrigeration.