Magnetostructural Phase Transition in Fe₆₀V₄₀ Alloy Thin Films

The tuning and control over the intrinsic magnetic properties, such as saturation magnetization (Ms) and Gilbert damping (α), can be obtained via systematic rearrangement of the lattice at the nanoscale. In certain binary alloys such as B2 Fe60Al40 [1] and B2 Fe50Rh50 [2], the Ms can be tuned by inducing chemical disorder. The disorder caused by the randomization of site occupancies of the atoms can be achieved by ion-irradiation. Here we explore a magnetic phase transition in Fe60V40 thin films caused by ordering of the lattice structure from short-range order to the crystalline state.
Fe60V40 films (~ 40 nm) were grown onto SiO2/Si substrate. The as-grown films are weakly ferromagnetic with low Ms of 17 kA/m; whereas irradiation with 25 keV Ne+-ions at fluences of ~ 5 × 1015 ions/cm2 leads to a drastic increase of Ms to ~ 750 kA/m, as shown in Fig. 1. X-ray diffraction as well as transmission electron microscopy reveal a structural short-range order in the as-grown films, that transform to A2 Fe60V40 with increasing Ne+-fluence. The A2 region appears to nucleate at the film top surface and with increasing fluence, it propagates deeper down into the film. The effect of the phase transition on the dynamic behaviour has been investigated using ferromagnetic resonance (Fig. 2). The low value of Gilbert damping (~ 0.002) has been obtained from the frequency dependence of the resonance spectra. Furthermore, the transition can be tracked using Conversion electron Mössbauer spectroscopy to shed light on the variation of local magnetic ordering during the transition. The tunable film structure as well as low damping form the basis for further investigations on nanomagnets embedded within Fe60V40 thin films.

Funding by the DFG - 322462997 (BA 5656/1-2 | WE 2623/14-2) is acknowledged. Ion-irradiation was performed at the Ion Beam Centre of the HZDR.

References
[1] J. Ehrler, et al., New J. Phys., 22, 073004 (2020).
[2] B. Eggert, et al., RSC Adv.,10, 14386 (2020).