Magnetic and Magnetocaloric Modifications Near Room Temperature in Fe0.6Al0.4 Nanoalloys under Irradiation by Swift Heavy Ions

This paper reports on the formation and properties of a kind of nanostructured magnetic material, which is ferromagnetic (FM) inclusions in a paramagnetic (PM) but well magnetized matrix. It has been argued that such FM/PM nanostructures are formed due to local chemical disordering along ion tracks in thin-film Fe0.6Al0.4 alloys irradiated with xenon ions at an energy of 160 MeV. The nonirradiated matrix, obtained by thermal annealing of the as-prepared alloy, has a PM-like behavior (with no hysteresis and remanence) at room temperature (RT). Interestingly, the irradiated samples exhibit a sharp peak in the temperature dependence of the magnetic entropy change Delta S at T similar to 320 K. The emergence of this maximum is attributed to the interfacial exchange interaction in the formed FM/PM-like nanostructures, which affects the matrix magnetization near the Curie temperature T-C of the PM-like matrix. The peak value of Delta S obtained has been compared to that theoretically predicted for plane-layered FM/PM structures upon the basis of the Landau theory for the second-order phase transitions. A discrepancy observed between the theory and experiment can be explained by occurrence of quite big superparamagnetic (SPM) clusters in a partially ordered (similar to 0.6) Fe0.6Al0.4 alloy. These entities essentially contribute the matrix magnetization at RT. The study reported here provides a better understanding of the structural, magnetic, and magnetocaloric properties of the heterogeneous FexAl1-x system, which can be viewed as particular type of nanocomposite.