A Giant Exchange Bias Effect Due to Enhanced Ferromagnetism Using a Mixed Martensitic Phase in NiMnGa Spun Ribbons.

The structure of a material is an important factor in determining its physical properties. Here, we adjust the structure of the NiMnGa spun ribbons by changing the wheel speed to regulate the exchange bias effect of the material. The characterization results of micromorphology and structure show that as the wheel speed increases, the martensite lath decreases from 200 nm to 50 nm, the structure changed from the NM to a NM and 10M mixed martensitic structure containing mainly NM, then changed to NM and 10M where 10M and NM are approaching. Meanwhile, H first increased and then decreased as the wheel speed increased. The optimum exchange bias effect (H = 7.2 kOe) occurs when the wheel speed is 25 m∙s, mainly attributed to the enhanced ferromagnetism caused by part of 10M in NM martensite, which enhanced the exchange coupling of ferromagnetism and antiferromagnetism. This work reveals the structural dependence of exchange bias and provides a way to tune the magnitude of the exchange bias of Heusler alloys.