Thermally induced transition from a ferromagnetic to a paramagnetic state in nanocrystalline FeAl processed by high-pressure torsion

The B2 ordered intermetallic compound FeAl shows a paramagnetic to ferromagnetic transition upon plastic deformation. The magnetic transformation is caused by the formation of a high density of antiphase boundary (APB) tubes leading to an increased number of Fe–Fe nearest neighbour pairs. In the present study it is shown that the temperature of the back transition from the ferromagnetic to the paramagnetic state depends strongly on the deformation mode. FeAl deformed by high pressure torsion (HPT) is investigated by differential scanning calorimetry, transmission electron microscopy and magnetic measurements. Based on the results of FeAl made nanocrystalline and disordered by HPT, it is concluded that the state of ferromagnetism vanishes almost completely at temperatures before re-ordering of the B2 long-range order has been encountered. This is in contrast to the findings reported for ball-milled FeAl indicating that the magnetic back transition and re-ordering occur at the same temperature. A model based on a vacancy driven change of Fe–Fe nearest neighbour configurations is proposed to explain the magnetic back transition after HPT deformation occurring at much lower temperatures.

Research highlights▶ FeAl is made nanocrystalline and ferromagnetic by HPT. ▶ Upon heating the ferromagnetism is lost before re-ordering takes place. ▶ As explanation we propose a model based on the high vacancy concentration caused by the HPT deformation. ▶ A vacancy driven change of Fe-Fe nearest neighbors could explain the loss of ferromagnetism.