Concomitant interfacial spin fractal transformation and exchange bias in a magnetic shape memory alloy

Small-angle neutron scattering is used to study the exchange bias effect in Mn50Ni40Sn10. The martensitic transformation is found to be responsible for the nanometer-scale spin clusters (SCs) inside the low-temperature ferromagnetic (FM) martensite phase. The magnetic field dependences of the SCs and FM domains exhibit an asymmetry that characterizes the exchange bias. We find that the surface geometries of the two magnetic phases are strongly susceptible to magnetization changes. While the FM domains and SCs are dominated by diffuse and fractal surfaces around the coercive fields, this configuration is reversed in the saturation regime, where the FM and SC surfaces become fractal and diffuse, respectively. These geometry changes are driven by the spin reorientation inside the heterogeneous FM/SC domain walls, where exchange anisotropy arises. Our work reveals the microscopic mechanism underlying the exchange bias effect in magnetic shape memory alloys.