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Magneto-structural correlations in Fe60Al40 thin films

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


Magnetic materials become more significant for future data storage devices and spintronic applications. In certain alloy thin films like Fe60Al40, nano-sized ferromagnetic structures can be created by means of focused ion irradiation.[1,2,3] Fe60Al40 shows a disorder induced phase transition from the thermodynamically stable, chemically ordered B2 to the metastable A2 phase going along with an evolution of ferromagnetism and an increase of the lattice parameter (Figure 1). This can be explained with the higher local number of Fe-Fe nearest neighbors in the disordered state. The correlation between structural and magnetic properties in the phase transition regime, consisting of A2 and B2 phases, is uncertain as well as the influence of the ion type or the temperature treatment.
The effects of ion implantation on the structural and magnetic properties of 250 nm thick Fe60Al40 films, possessing A2 and B2 structure respectively, have been investigated by means of X-ray diffraction (XRD) and Vibrating sample magnetometry. From XRD measurements, the order parameter S and the peak shift due to the change of the lattice parameter have been derived and correlated with the magnetization. The irradiation of paramagnetic B2 Fe60Al40 with H+, He+ or Ne+ ions with different fluences at low temperatures led to an increase of the saturation magnetization (MS) which was expected to be directly related to the number of displacements per atom (dpa) by using the simulation program TRIM [4], independent on the ion species. However, unlike than expected, the induced magnetization differed but correlated directly with the measured lattice parameter. A significant change of lattice parameter and MS did not appear for proton irradiation at elevated temperatures (250 °C) where the ordered B2 phase was retained. Upon low temperature (LN2) hydrogen implantation of disordered A2 Fe60Al40 films, on the other hand, unlike for helium or neon irradiation, the lattice parameter and the saturation magnetization decreased indicating a little ordering. This might offer the possibility of H+ irradiation induced ordering of chemically disordered alloy thin films well below the ordering temperature.
Furthermore, the studies show that the structural and magnetic properties of 250 nm thick Fe60Al40 films are directly linked with each other (Figure 2) and do not depend on the type of treatment. The chemical disorder induced evolution of ferromagnetism comes along with an abrupt disappearance of the (100)-superlattice peak represented by the order parameter dropping to 0. Nevertheless, the role of defects remains uncertain since ion irradiation leads besides the structural disordering also to an increase of the defect concentration and a temperature treatment to structural ordering and an annealing of defects. However, as described beforehand, H+ implantation causes little ordering but also an increase of the open volume defect concentration, which was characterized by means of Positron annihilation spectroscopy. This offers the opportunity to differentiate between structural disorder and defect concentration.
Given the fact that the proton implanted films follow the shown general behavior a dependence on the structural order only can be assumed.

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