Tuning the magnetic and structural properties of Fe60Al40 thin films by ion irradiation


Tuning the magnetic and structural properties of Fe60Al40 thin films by ion irradiation

Ehrler, J.; Bali, R.; Böttger, R.; Zhou, S.; Grenzer, J.; Potzger, K.

Magnetic materials are significant for future data storage device and spintronic applications.
Recently it has been shown that nano-sized ferromagnetic structures can be generated within non-ferromagnetic B2-Fe60Al40 thin films via local ion-irradiation. Ion-irradiation leads to disordering, forming the ferromagnetic A2 phase. The mechanism of this disorder induced effect has been variously attributed to the increase in the number of Fe-Fe nearest neighbors due to disorder and the corresponding increase of the lattice parameter (a0) [1,2,3]. Considering a0 as an indicator for the disorder as the driving parameter for the ferromagnetic order, 250 nm thick B2-Fe60Al40 films irradiated with various ion species, i.e. H+, He+ and Ne+, have been investigated with respect to the correspondence between a0 and the saturation magnetization (MS).
The films have been prepared by magnetron sputtering and irradiated at the Ion Beam Center at the Helmholtz-Zentrum Dresden - Rossendorf. The irradiation-induced displacements per atom (dpa) were calculated with the help of the program TRIM [4]. The films have been irradiated at low
temperature (LN2) with an energy of 17 keV and a fluence of 4E17 ions/cm² for H+ ions and with 3.52E16 ions/cm² at 27 keV for He+. According to
TRIM, these implantation parameters induce the same dpa for the H+ and He+ irradiations. MS was measured by Vibrating sample magnetometry and the order parameter (S) as well as a0 were derived from X-ray diffraction.
The non-irradiated B2-ordered Fe60Al40 films (S=1) possess an MS of 5 kA/m and a0 of 2.89 Å. Despite similar ion-induced disorder, films irradiated with H+ and He+ showed an increased MS of 314 kA/m and 551 kA/m respectively. This suggests deviations from the TRIM calculations, in particular for the lighter H+ ion. Nevertheless, study of a vast number of films irradiated with H+, He+ as well as Ne+ at low and room temperature respectively shows a fundamental correlation between structural and magnetic properties of the Fe60Al40 films, independent of the implanted ion species or temperature treatment (see Figure 1).
The films exhibit a sharp transition at a0=2.91 Å, where MS rapidly increases, tracked exactly by a decrease of S, suggesting that MS depends on the increase of the Fe-Fe nearest neighbor interactions (as indicated by decreasing S) only being independent on a0. DFT calculations will help further understanding of this correlation.

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  • Lecture (Conference)
    Moscow International Symposium on Magnetism 2017, 01.-06.07.2017, Moscow, Russia

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