Ion mass dependence of irradiation-induced local creation of ferromagnetism in ${\text{Fe}}_{60}{\text{Al}}_{40}$ alloys

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.

Figure 1

(Color online) Hysteresis loops investigated by means of longitudinal magneto-optic Kerr effect of an ${\text{Fe}}_{60}{\text{Al}}_{40}$ alloy before and after being irradiated with three different fluences of 45 keV ${\text{Xe}}^{+}$. In addition, the average damage within a depth of 20 nm is specified.

Figure 2

(Color online) MOKE amplitude (a) as a function of ion fluence and (b) as a function of ion damage for five different ion species (${\text{He}}^{+}$, ${\text{Ne}}^{+}$, ${\text{Ar}}^{+}$, ${\text{Kr}}^{+}$, and ${\text{Xe}}^{+}$). The primary energy is adjusted to achieve the maximum number of displacements per atom in a 10 nm depth of the ${\text{Fe}}_{60}{\text{Al}}_{40}$ alloy. The irradiation causes the transition from the paramagnetic B2 structure to the ferromagnetic A2 structure.

Figure 3

(Color online) The GIXRD spectra of the ${\text{Fe}}_{60}{\text{Al}}_{40}$ $\left(\text{at}\text{.}\%\right)$ alloy after being annealed (i.e., virgin) and homogeneously irradiated by using either ${\text{He}}^{+}$ (2 keV, 0.2 dpa) or ${\text{Xe}}^{+}$ (45 keV, 0.3 dpa). The Miller indices of the ${\text{Fe}}_{60}{\text{Al}}_{40}$ $\left(\text{at}\text{.}\%\right)$ alloy are also indicated. Note that SLPs denote the superlattice peaks.

Figure 4

(Color online) (a) AFM and [(b)–(d)] MFM images of the FeAl sheet after ${\text{Xe}}^{+}$ irradiation. The squares in (a) indicate the $7.5\times 7.5\mu {\text{m}}^2$ regions that have been irradiated. For the series of MFM images, different magnetic fields have been applied: (b) 0 Oe, (c) $-400\text{Oe}$, and (d) 400 Oe. The arrows in (c) and (d) indicate the direction of the applied magnetic field.