Ion mass dependence of irradiation-induced local creation of ferromagnetism inFe60Al40alloys

Abstract

Ion irradiation of ${\text{Fe}}_{60}{\text{Al}}_{40}$ alloys results in the phase transformation from the paramagnetic, chemically ordered B2 phase to the ferromagnetic, chemically disordered A2 phase. The magnetic phase transformation is related to the number of displacements per atom (dpa) during the irradiation. For heavy ions (${\text{Ar}}^{+}$, ${\text{Kr}}^{+}$, and ${\text{Xe}}^{+}$), a universal curve is observed with a steep increase in the fraction of the ferromagnetic phase that reaches saturation, i.e., a complete phase transformation, at about 0.5 dpa. This proves the purely ballistic nature of the disordering process. If light ions are used (${\text{He}}^{+}$ and ${\text{Ne}}^{+}$), a pronounced deviation from the universal curve is observed. This is attributed to bulk vacancy diffusion from the dilute collision cascades, which leads to a partial recovery of the thermodynamically favored B2 phase. Comparing different noble gas ion irradiation experiments allows us to assess the corresponding counteracting contributions. In addition, the potential to create local ferromagnetic areas embedded in a paramagnetic matrix is demonstrated.