Giant Magnetocaloric Effect in a High-Spin Shastry-Sutherland Dipolar Magnet

The Shastry-Sutherland lattice is a prototypical frustrated quantum magnet. It is notable for its exactly solvable dimer-singlet ground state and hosts a wealth of magnetic phenomena under external fields. Here, this work investigates the high-spin (S = 7/2) Eu-based magnet Eu2MgSi2O7 (EMSO) using low-temperature magnetothermal measurements and Monte Carlo simulations, revealing a giant magnetocaloric effect (MCE) in this Shastry-Sutherland compound. The entropy change peak value is found to be 55.0 J kg-1 K-1 under a field change of B = 0-4 T, approximately 1.5 times larger than the commercial Gd3Ga5O12 (GGG). Adiabatic demagnetization refrigeration achieves a lowest temperature of 151 mK, deeply into the sub-Kelvin regime. Furthermore, a distinctive cooling effect persists below about 1 T, a characteristic absent for conventional magnetic coolants. A dipolar Shastry-Sutherland model is introduced as a minimal model to describe this system; in particular, the experimentally revealed 1/3 magnetization pseudo-plateau can be ascribed to the presence of dipolar couplings between Eu2+ ions, further stabilized by the thermal fluctuations, explaining the persistent cooling effect. This work establishes EMSO as a novel platform for exploring the dipolar Shastry-Sutherland system and for sub-Kelvin adiabatic demagnetization refrigeration.

💡 Research Summary

The Shastry‑Sutherland (SS) lattice is a paradigmatic frustrated geometry that, in its spin‑½ incarnation, hosts an exact dimer‑singlet ground state, a cascade of magnetization plateaux, and possible quantum spin‑liquid phases. However, the magnetocaloric potential of high‑spin SS systems has remained largely unexplored. In this work the authors investigate Eu₂MgSi₂O₇ (EMSO), a rare‑earth oxide in which Eu²⁺ ions (S = 7/2, μ_eff ≈ 7 μ_B) form a quasi‑two‑dimensional SS network. Because the Eu‑Eu distances are short (nearest‑neighbor d₁ ≈ 3.73 Å, next‑nearest d₂ ≈ 4.23 Å) the magnetic dipolar interaction is comparable in magnitude to the Heisenberg exchanges (J ≈ 0.5 K, J′ ≈ 1.0 K) and must be taken into account. The authors therefore introduce a “dipolar Shastry‑Sutherland” (DSS) model that contains antiferromagnetic intra‑ and inter‑dimer exchanges together with the full long‑range dipolar tensor evaluated via Ewald summation.

Low‑temperature specific‑heat measurements reveal a sharp zero‑field peak at T ≈ 0.7 K, signalling a magnetic ordering transition. With increasing field the peak shifts to lower temperature and broadens, indicating strong spin fluctuations persisting well below 1 K. Magnetization isotherms measured from 0.4 K to 1.8 K display a pronounced 1/3 magnetization pseudo‑plateau around μ₀H ≈ 0.5 T. The differential susceptibility shows a characteristic peak‑dip‑peak structure, confirming the presence of an up‑up‑down (UUD) configuration that is stabilized by thermal fluctuations (“order‑by‑thermal‑disorder”).

The magnetocaloric effect (MCE) is quantified from both isothermal magnetization and heat‑capacity data. For a field change Δμ₀H = 0–2 T the magnetic entropy change reaches –ΔS_max = 36.7 J kg⁻¹ K⁻¹; for Δμ₀H = 0–4 T it reaches 55.0 J kg⁻¹ K⁻¹, which is about 1.5 times larger than that of the commercial refrigerant Gd₃Ga₅O₁₂ (GGG) under the same field swing. Quasi‑adiabatic demagnetization experiments starting from T_i = 1.8 K and μ₀H_i = 8 T cool the sample down to a record low temperature of 151 mK. Remarkably, the minimum temperature is not attained at zero field but at μ₀H_c ≈ 1.13 T, suggesting a field‑driven quantum critical point (QCP). Below this field the isentropes are unusually flat, producing a “persistent cooling” regime that is absent in conventional refrigerants.

Monte‑Carlo simulations of the DSS Hamiltonian reproduce all key experimental observations. The inclusion of dipolar couplings dramatically enhances the stability and flatness of the 1/3 plateau, converting the pseudo‑plateau of the pure SS model into a true UUD plateau at sufficiently low temperature. Energy minimization shows that, unlike the classical SS model where the UUD state vanishes at T = 0, the DSS model possesses a finite field window where the UUD state is the ground state, followed by a canted orthogonal phase before full polarization. The simulated magnetic Grüneisen ratio exhibits a sign‑changing peak‑dip structure at the QCP, in agreement with experiment.

The authors argue that the giant MCE and the persistent low‑field cooling stem from the large magnetic entropy density inherent to the high‑spin Eu²⁺ ions combined with the entropy‑driven stabilization of the 1/3 plateau. The dipolar interaction not only introduces additional frustration but also selects a highly degenerate manifold that can be lifted by thermal fluctuations, thereby creating a broad region of enhanced entropy at low fields. Compared with GGG, LiGdF₄, and even the spin‑½ SS material Yb₂Be₂GeO₇, EMSO delivers a larger entropy change (≈ 60 J kg⁻¹ K⁻¹ in volumetric units) and reaches a lower base temperature, establishing it as a superior candidate for sub‑Kelvin adiabatic demagnetization refrigeration.

In summary, this work provides the first experimental demonstration of an entropy‑driven 1/3 magnetization pseudo‑plateau in a high‑spin dipolar SS magnet, links it directly to an unprecedented magnetocaloric performance, and introduces EMSO as a versatile platform for both fundamental studies of frustrated dipolar systems and practical ultra‑low‑temperature cooling technologies.