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.