Magnetic phase transitions in ns-laser irradiated FeAl systems: the role of open volume defects


Magnetic phase transitions in ns-laser irradiated FeAl systems: the role of open volume defects

Liedke, M. O.; Bali, R.; Hübner, R.; Gradauskaite, E.; Ehrler, J.; Wang, M.; Potzger, K.; Zhou, S.; Wagner, A.

Fe60Al40 alloys exhibit disorder dependent magnetic phase transitions (MPT), e.g., a ferromagnetic disordered A2-phase turns into a paramagnetic ordered B2-phase [1]. The ordered B2-phase, formed due to annealing up to 500°C in vacuum can be reversed to the disordered A2-phase via ion-irradiation [2]. It has been shown that the physical origin of MPT is related to the so-called anti-site disorder (ASD), i.e., variations in the number of Fe-Fe nearest neighbors due to disordering of the system [3]. However, variations of the lattice parameter, secondary phases, and changes in the concentration and size of open volume defects may play an important role as well. Here, an excimer UV ns-laser has been utilized to induced defects and examine the role of ASD and defects onto magnetic properties of Fe60Al40. Samples of 40 nm thick Fe60Al40 films with different initial order levels were exposed to a range of laser fluences: (i) Ne+ irradiated fully-disordered (A2- Fe60Al40), and (ii) vacuum annealed ordered alloys (B2- Fe60Al40) and (iii) as-grown semi-disordered (A2/B2- Fe60Al40). It is seen that for laser pulses of fluences below 100 mJ·cm^-2 cause subtle changes to the magnetization depending on the Fe60Al40 initial state, whereas for fluences above 150 mJ·cm-2, strong increase in ferromagnetism is observed for all Fe60Al40 initial states. The laser irradiated samples were probed with the Positron Annihilation Spectroscopy (PAS) to analyze for the existence of vacancies and/or phase separation. Although the low fluence region shows nearly no variation in vacancy defect concentration, a slight increase in the number of Al atoms around defect sites is found. For the high fluence regime, it is seen that a large variation in vacancy defects occurs, followed by pronounced phase separation. Structural analysis of the phase separated films shows strong migration of Al atoms leaving behind Fe-enriched regions, consistent with the PAS spectra.
[1] M. O. Liedke et al., J. Appl. Phys. 117, 163908 (2015)
[2] J. Fassbender, et. al., Phys. Rev. B 77, 174430 (2008)
[3] R. Bali, et al., Nano Lett. 14, 435 (2014)

Keywords: positron; positron annihilation spectroscopy; MOKE; ns-laser; magnetic phase transition; order; disorder

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    The International Workshop on Positron Studies and Defects 2017 (PSD-17), 03.-08.09.2017, Dresden, Deutschland

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