Lowering of the L10 ordering temperature of FePt nanoparticles by He+ ion irradiation

Arrays of FePt particles (diameter 7nm) with mean interparticle distances of 60nm are prepared by a micellar technique on Si substrates. The phase transition of these magnetic particles towards the chemically ordered L10 phase is tracked for 350kV He+ ion irradiated samples and compared to a nonirradiated reference. Due to the large separation of the magnetically decoupled particles the array can be safely annealed without any agglomeration as usually observed for more densely packed colloidal FePt nanoparticles. The He+ ion exposure yields a significant reduction of the ordering temperature by more than 100K.