Mixed exchange-coupled soft α-(Fe80Pd20) and hard L10FePd phases in Fe64Pd36 thin films studied by first order reversal curves

[Display omitted] •Mixed exchange-coupled magnetically hard-soft FePd alloy.•Exchange interaction through inter-phase boundaries.•First order reversal curves used to analyse the exchange coupling. Fe64Pd36 thin films have been fabricated by thermal evaporation onto single-crystal Si(100) substrates. After suitable thermal treatments, we demonstrate that two different magnetic phases can coexist in the same alloy, one α-(Fe80Pd20) which is magnetically soft, and one L10 FePd which is magnetically hard, giving rise to exchange coupling between them through their interfaces, without needing to artificially nanostructure the material. The structural and magnetic properties of as-prepared and furnace annealed samples are therefore studied by means of X-ray diffractometry, conversion electron Mössbauer spectroscopy, atomic and magnetic force microscopy and high sensitivity magnetometry. As prepared samples are characterised by different phases having soft magnetic properties: α-(Fe80Pd20) and disordered (fcc) FePd phases with different compositions. Furnace annealing in vacuum at 600°C for one hour promotes the transformation of the fcc phases into the L10 one, while α-(Fe80Pd20) remains largely unaffected. The structural transformations are accompanied by an increase of the coercive field without any significant changes of the saturation magnetisation value. In the sample annealed at 600°C, magnetisation reversal occurs in two steps: one at lower fields, attributed to the soft phase, and one at much larger fields, attributed to the hard phase and to the fraction of the soft one coupled to it. The exchange coupling between the two phases is demonstrated and thoroughly analysed by means of first order reversal curves.