Study of the structural and magnetic properties at high temperature of Ni-Co-Mn-Al Heusler alloy prepared by an unconventional route

Ni, Co, Mn, and Al elemental powders have been ball milled in a planetary mill for various periods, up to 32 h. The obtained powder was characterized with XRD, SEM-EDS, DSC, high-temperature-XRD, and VSM techniques to study morphology, composition, thermal transitions, structural and magnetic changes occurring at high temperatures. At the end of the milling process, a particle mixture consisting of agglomerated grains with a rounded shape and an average size of about 60 mu m was formed. XRD patterns of the as-milled powders revealed the presence of a nanocrystalline solid solution mixture with (L1(0)) and (B2) structures. HT-In-situ XRD characterizations confirmed the complete formation of the B2 ordered structure from 573 K. The austenitic Heusler phase B2 remains stable during one hour of isothermal holding and its proportions increase between 648 and 773 K. Under the heating effect, crystallites size grows up greatly with increasing holding temperature, nevertheless the micro-strains decrease sharply from 0.98% at room temperature to 0.31% at 773 K. Magnetic features of the milled alloy enhance further during the heating process, making it magnetically soft with the most intriguing values of Ms, Mr, and Hc between 648 and 773 K. This is owing to the complete development of the homogeneous and ordered Heusler B2 phase and the significant increase in crystallite size with the relaxation of residual stresses resulting from the direct effects of temperature.