Distributed Magnetic Entropy Response with Broadened Working Temperature Enabled by Rapidly Solidified HoErMn Alloy Microwires

High-performance magnetocaloric materials with a wide working temperature span are urgently needed for deep-cryogenic applications such as hydrogen liquefaction and magnetic refrigeration. In this study, HoErMn alloys with identical composition were fabricated as fully crystallized as-cast rods and rapidly solidified microwires (melt -extraction) to investigate microstructure-driven modulation of magnetic phase transitions and magnetocaloric response. Microstructure analyses reveal that the as-cast alloy consists of a multiphase crystalline structure, whereas the microwires exhibit an amorphous matrix containing dispersed nanocrystals. Both samples undergo continuous second-order magnetic phase transitions; however, the microwires display an elevated magnetic glass freezing behavior, pronounced FC/ZFC bifurcation, and broadened transition behavior, indicative of enhanced magnetic frustration and inhomogeneous magnetic exchange interactions originating from structural disorder and local compositional fluctuations. The as-cast sample attains a slightly higher peak magnetic entropy change (-ΔS_M^max = ∼10.2 J·kg⁻¹·K⁻¹ at 5 T), while the microwires preserve a comparable-ΔS_M^max (= ∼9.5 J·kg⁻¹·K⁻¹) but exhibit a substantially broadened entropy-change profile. Consequently, the microwires achieve a superior relative cooling power (RCP = ∼ 551 J·kg⁻¹) compared with the bulk counterpart (RCP = ∼ 490 J·kg⁻¹). The distributed magnetic phase-transition landscape induced by the amorphous/nanocrystalline composite architecture effectively expands the operational temperature span, offering a microstructure-engineering pathway for high-performance deep-cryogenic magnetocaloric materials.