Effects of ion-beam irradiation on the L1 ₀ phase transformation and their magnetic properties of FePt and PtMn Films

In this paper, we illustrate how to modify the structure and magnetic properties of L1 ₀ FePt and PtMn films using ion-beam irradiation. Highly ordered L1 ₀ FePt and PtMn phases were achieved directly by using 2 MeV He-ion irradiation without conventional post-annealing. A high ion-beam current density (∼μA/cm ²) was used to achieve direct beam heating on samples. This irradiation-induced heating process provides efficient microscopic energy transfer and creates excess point defects, which significantly enhances the diffusion and promotes the formation of the ordered L1 ₀ phase. In-plane coercivity of FePt films greater than 5700 Oe could be obtained after disordered FePt films were irradiated with a He-ion dose of 2.4×10 ¹⁶ ions/cm2. The direct ordering of FePt took place by using ion-irradiation heating at a temperature as low as 230°C. In PtMn-based spin valves, an L1 ₀ PtMn phase, a large exchange field and a high giant magnetoresistance (GMR) ratio (11%) were simultaneously obtained by using He-ion irradiation. On the other hand, Ge-ion and O-ion irradiation completely destroyed the ferromagnetism of FePt and the GMR of PtMn-based spin valves, respectively.