Effect of heat treatment on structural and magnetic features of nanostructured Ni-Co-Mn-Al Heusler alloy obtained by planetary ball mills

Abstract

This paper investigates the structural, microstructural and magnetic properties observed at room temperature and after high-temperature heat treatment of Ni-Co-Mn-Al Heusler alloy, produced by high-energy ball milling. A number of characterization techniques were employed to study morphology, structural composition, thermal transitions, phase changes and magnetic shifts, including: room and high temperature XRD analysis, SEM-EDXS, DSC, TEM and VSM. The obtained alloy is a homogeneous mixture of agglomerated rounded grains with an average size of around 60 µm, composed mainly of a metastable martensitic phase (L1₀) and a minority austenitic phase (B2). High-temperature XRD characterization confirms the complete formation of the ordered B2 Heusler phase from 573 K. Crystallite size increases with temperature, but microdeformations drop significantly due to relaxation of residual stresses. Magnetic measurements show that magnetization saturation, and coercivity values increase after each high-temperature heat treatment, making the alloy’s magnetic behavior softer. The increase in saturation magnetization values indicates that a greater number of magnetic domains are aligned in the same direction within the material, enabling it to be more magnetized. This improvement in magnetic performance makes the material more suitable for various applications such as transformers, electric motors or magnetic sensors.