Exchange-bias via nanosegregation in novel Fe(2−x)Mn(1+x)Al (x = −0.25, 0, 0.25) Heusler films

Exchange-bias has been reported in bulk nanocrystalline Fe(2)MnAl, 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−x)Mn(1+x)Al (x = −0.25, 0 and 0.25) obtained by sputtering and ex situ post-deposition annealing. We find that Fe(2)MnAl films display exchange-bias, and that varying Mn concentration determines the magnitude of the effect, which can be either enhanced (in Fe(1.75)Mn(1.25)Al) or suppressed (in Fe(2.25)Mn(0.75)Al). X-ray diffraction shows that our films present a mixed L2(1)–B2 Heusler structure where increasing Mn concentration favors the partial transformation of the L2(1) phase into the B2 phase. Scanning transmission electron microscopy (STEM) and energy dispersive X-ray spectroscopy (EDX) reveal that this composition-driven L2(1) → B2 transformation is accompanied by phase segregation at the nanoscale. As a result, the Fe(2−x)Mn(1+x)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 L2(1) 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.