On the microstructure and exchange bias effect correlation of Ni-Mn-In Heusler alloy

Exchange bias (EB) originated from low-temperature martensites in Ni-Mn based Heusler alloys has drawn much attention, but in-depth microscopic investigation about phase structures and magnetism of low-temperature martensite and its influence on the resultant EB effect is still deficient. In this work, we fabricated Ni₅₀Mn₃₅In₁₅ ribbons with varying atomic order of the parent phase to mediate the relative fraction of martensite with different magnetic states. This provides a feasible protocol for revealing the correlation between microstructure and EB performance. X-ray diffraction (XRD) results show that the low-temperature phase is a coexistence of 5-layer modulated monoclinic martensite (5 M) and non-modulated martensite (L1₀) for all ribbons. Transmission electron microscopy (TEM) results reveal that the atomic degree of the austenite phase differentiates between ribbons, and the coexisting 5 M and L1₀ martensites in single grain. Furthermore, the magnetic domain evolution during martensitic transformation analyzed by in-situ cooling Lorentz TEM shows different magnetic configurations existed in low-temperature martensite. With decreasing the austenite L2₁ atomic order, the exchange bias is enhanced, which is ascribed to the increase of the relative volume fraction of non-ferromagnetic martensite. Our findings shed light on the micro-scale understanding of the exchange bias effect for magnetic phase transition systems and provide an effective strategy for developing high-performance magnetic materials.