Bending strain tailored exchange bias in epitaxial NiMn/ γ ′-Fe₄N bilayers

The strain tunable exchange bias has attracted much attention due to its practical applications in flexible and wearable spintronic devices. Here, the flexible epitaxial NiMn/γ′-Fe4N bilayers are deposited by facing-target reactive sputtering. The maximum strain-induced change ratios of exchange bias field HEB and coercivity HC (|ΔHEB/HEB| and |ΔHC/HC|) are 51% and 22%, respectively. A large strain-induced |ΔHEB/HEB| appears in a thicker ferromagnetic layer, but a large |ΔHC/HC|) appears in a thinner ferromagnetic layer. At a compressive strain, the antiferromagnetic anisotropy of the tetragonal NiMn layer increases, resulting in an increased HC of NiMn/γ′-Fe4N bilayers. The bending-strain induced changes of anisotropy magnetoresistance and planar Hall resistance are also observed at low magnetic fields. The bending-strain tailored magnetic properties can be ascribed to the distributions of ferromagnetic and antiferromagnetic anisotropies.