Spin-Chain Incipient Magnetocaloric Effect and Rare-Earth Controlled Switching in the Haldane-Chain System, R2BaNiO5

We have experimentally investigated the magnetocaloric effect (MCE) of a prototype spin-frustrated one-dimensional spin-chain system, the famous Haldane-chain system, R2BaNiO5 (R = Nd, Gd, Er, Dy). The significant MCE is observed far above long-range ordering, even in the paramagnetic region, which is attributed to the change in magnetic entropy due to short-range spin correlation arising from (low-dimensional) magnetic frustration. Such a spin-chain incipient MCE above long-range ordering is rarely reported. Interestingly, multiple magnetocaloric switching from conventional to inverse MCE (and vice versa) are observed below long-range magnetic ordering, as a function of temperature and magnetic field, for the R = Nd, Dy, and Er members. However, such MCE switching is absent in the Gd member, which is an S-state atom (orbital moment L = 0). Our systematic investigation of this series demonstrates that the interplay between crystal-electric field (CEF), strong spin-orbit coupling (SOC) and rare earth anisotropy of R-ions play an important role in spin reorientation, leading to multiple MCE switching due to intriguing changes in magnetic and lattice entropy. The maximum change of entropy for Er, Gd, Dy and Nd is 7.8, 6.8, 4.0 and 1.0 J Kg-1 K-1 respectively. Our study presents a pathway for tuning MCE switching and the MCE effect over large temperature regions in d-f coupled spin-frustrated and spin-chain oxide systems.

💡 Research Summary

The authors present a comprehensive experimental study of the magnetocaloric effect (MCE) in the one‑dimensional Haldane‑chain family R₂BaNiO₅, where R = Nd, Gd, Er, and Dy. Using polycrystalline samples prepared by solid‑state reaction, they measured heat capacity C(T) from 1.8 K to 80 K under magnetic fields up to 14 T and derived the magnetic entropy change ΔS_M by integrating the field‑induced heat‑capacity difference divided by temperature.

A key finding is the observation of a sizable “incipient” MCE well above the long‑range antiferromagnetic ordering temperatures (T_N) for all members, indicating that short‑range spin correlations in the low‑dimensional frustrated chains contribute significantly to magnetic entropy even in the paramagnetic regime.

Dy₂BaNiO₅ (T_N ≈ 58 K) displays a complex behavior: below ~30 K the magnetic entropy changes sign, giving rise to an inverse MCE (negative ΔS_M) that coexists with a conventional (positive) MCE at higher temperatures. Two field‑induced metamagnetic transitions at ≈ 4.5 T and 6.5 T are identified, corresponding to spin‑flop and subsequent canting toward ferromagnetism. These order‑order transitions produce large entropy jumps (≈ ± 3 J kg⁻¹ K⁻¹) and enable magnetic‑field‑tunable switching between conventional and inverse MCE.

Er₂BaNiO₅ orders weakly at T_N ≈ 32 K; a broad heat‑capacity anomaly around 10 K reflects the splitting of the Er³⁺ Kramers doublet by exchange fields. The magnetic entropy reaches a maximum of 7.8 J kg⁻¹ K⁻¹ under 14 T, and a noticeable MCE persists up to 90 K, far above T_N, again underscoring the role of short‑range correlations.

Gd₂BaNiO₅, with Gd³⁺ being an S‑state ion (L = 0), shows only a conventional MCE. The maximum ΔS_M is 6.8 J kg⁻¹ K⁻¹ at 25 K under 14 T, and no sign reversal or switching is observed, highlighting the importance of crystal‑electric‑field (CEF) effects and spin‑orbit coupling (SOC) in the other members.

Nd₂BaNiO₅ (T_N ≈ 48 K) exhibits an inverse MCE below ~26 K (ΔS_M ≈ −2.4 J kg⁻¹ K⁻¹) that switches to a conventional MCE above this temperature (ΔS_M ≈ +1 J kg⁻¹ K⁻¹). The inversion is attributed to thermal population of higher‑lying Nd³⁺ Kramers doublets and the consequent change in Nd‑Ni exchange interactions.

Across the series, the authors argue that the interplay of CEF splitting, strong SOC, and rare‑earth anisotropy governs spin reorientation and the balance between magnetic and lattice entropy. The presence of multiple exchange pathways (R‑O‑Ni, Ni‑O‑Ni, R‑R) generates magnetic frustration, which together with low dimensionality yields the observed incipient MCE and the temperature‑field‑controlled switching phenomena.

The work demonstrates that Haldane‑chain oxides with d‑f coupling can provide sizable, tunable MCE over a broad temperature window, overcoming the narrow‑window limitation of conventional magnetocaloric materials. By selecting appropriate rare‑earth ions and exploiting CEF/SOC effects, one can design materials that exhibit both conventional and inverse MCE, offering new routes for efficient low‑temperature refrigeration and adiabatic demagnetization applications.