Low-temperature aging induced enhancement of atomic order and magnetocaloric effect in Ni-Co-Mn-Sn alloy

Low-temperature aging represents a promising strategy for tuning transformation and magnetofunctional properties of ferromagnetic shape memory alloys without altering chemical composition. However, kinetic mechanisms and property evolution associated with such treatments remain underexplored. In this study, effects of aging at 523 K from 0 to 1200 min on martensitic transformation (MT), magnetocaloric effect (MCE), and atomic order of bulk Ni₄₇Co₅Mn₄₀Sn₈ alloy were systematically investigated. As aging time increased, MT temperature initially decreased rapidly, followed by a slower decline, with a total drop of ∼7 K. This behavior reflects a diffusion-limited ordering process, where atomic rearrangement slows as defect mobility and driving force decline. Meanwhile, austenite Curie temperature increased by 4.2 K, resulting in an ∼11.2 K expansion of ferromagnetic austenite region. Enhanced field-induced transformation behavior was observed. At 7.0 T, aged sample (1200 min) exhibited improved MCE performance, with 32.9 J/(kg·K) magnetic entropy change, 426.0 J/kg refrigeration capacity and 288.2 J/kg net refrigeration capacity, representing increases of ∼4.8 %, ∼12.4 %, and ∼12.0 %, respectively. Moreover, critical field to reach saturation magnetic entropy change decreased from ∼6.0 T to ∼2.5 T, indicating improved low-field efficiency. XRD revealed an increase in I(111)/I(220) (from 0.076 to 0.233), consistent with Rietveld refinement results showing reduced anti-site defects (MnZ from 0.0583 to 0.0541, SnY from 0.0356 to 0.0185). TEM dark-field imaging based on (111) superlattice diffraction further confirmed the expansion of L2₁-ordered domains after aging. These results confirm that improvement in atomic order during low-temperature aging effectively enhances magnetostructural coupling and low-field MCE performance in Ni-Co-Mn-Sn alloys.