Thermal stability and structural ordering of thick Co$_2$MnAl films grown on MgO and Si substrates

Abstract

While most studies on Co$2$MnAl (CMA) focus on the ultrathin regime for spintronic junctions, applications scaling with magnetic volume--such as Anomalous Nernst Effect (ANE) energy harvesters, integrated power inductors, and magnetic MEMS--require significantly thicker films. In this work, 750 nm thick CMA films were grown on MgO (001) and Si substrates to establish a roadmap for their thermal integration. Structural analysis reveals that B2 ordering is optimized on MgO at 600 $^\circ$C, which is important for functional devices. In contrast, samples on Si substrates exhibit a disorder transition toward the A2 phase, closely linked to diffusion-driven degradation processes. Above 500 $^\circ$C, Si interdiffusion into the Heusler layer triggers the formation of interfacial silicides and Kirkendall voids, leading to microcracking, electrical resistivity jumps, and a final fully disordered A2 phase. Despite the Mn-poor stoichiometry (Co$2.25$Mn$0.7$Al$1.05$), the films maintain a robust saturation magnetization of ~600 emu/cm$^3$, consistent with recent theoretical models for off-stoichiometric Heusler alloys. These results define the processing windows for integrating thick CMA layers into multifunctional CMOS-compatible platforms.