Exchange-bias via nanosegregation in novel Fe_2- Mn₁₊ Al (x = -0.25, 0, 0.25) Heusler films

Exchange-bias has been reported in bulk nanocrystalline Fe₂MnAl, but individual thin films of this Heusler alloy have never been studied so far. Here we study the structural and magnetic properties of nanocrystalline thin films of Fe_2- Mn₁₊ Al (x = -0.25, 0 and 0.25) obtained by sputtering and ex situ post-deposition annealing. We find that Fe₂MnAl films display exchange-bias, and that varying Mn concentration determines the magnitude of the effect, which can be either enhanced (in Fe_1.75Mn_1.25Al) or suppressed (in Fe_2.25Mn_0.75Al). X-ray diffraction shows that our films present a mixed L2₁-B2 Heusler structure where increasing Mn concentration favors the partial transformation of the L2₁ phase into the B2 phase. Scanning transmission electron microscopy (STEM) and energy dispersive X-ray spectroscopy (EDX) reveal that this composition-driven L2₁ → B2 transformation is accompanied by phase segregation at the nanoscale. As a result, the Fe_2- Mn₁₊ Al films that show exchange-bias (x = 0, 0.25) are heterogeneous, with nanograins of an Fe-rich phase embedded in a Mn-rich matrix (a non-negative matrix factorisation algorithm was used to give an indication of the phase composition from EDX data). Our comparative analysis of XRD, magnetometry and X-ray magnetic circular dichroism (XMCD), shows that the Fe-rich nanograins and Mn-rich matrix are composed of a ferromagnetic L2₁ phase and an antiferromagnetic B2 phase, respectively, thus revealing that exchange-coupling between these two phases is the cause of the exchange-bias effect. Our work should inspire the development of single-layer, environmentally friendly spin valve devices based on nanocomposite Heusler films.