Mn-Induced Thermal Stability of L1₀ Phase in Fept Magnetic Nanoscale Ribbons

Magnetic nanoscale materials exhibiting the L1₀ tetragonal phase such as FePt or ternary alloys derived from FePt show most promising magnetic properties as a novel class of rare earth free permanent magnets with high operating temperature. A granular alloy derived from binary FePt with low Pt content and the addition of Mn with the nominal composition Fe₅₇Mn₈Pt₃₅ has been synthesized in the shape of melt-spun ribbons and subsequently annealed at 600 °C and 700 °C for promoting the formation of single phase, L1₀ tetragonal, hard magnetic phase. Proton-induced X-ray emission spectroscopy PIXE has been utilized for checking the compositional effect of Mn addition. Structural properties were analyzed using X-ray diffraction and diffractograms were analyzed using full profile Rietveld-type analysis with MAUD (Materials Analysis Using Diffraction) software. By using temperature-dependent synchrotron X-ray diffraction, the disorder-order phase transformation and the stability of the hard magnetic L1₀ phase were monitored over a large temperature range (50-800 °C). A large interval of structural stability of the L1₀ phase was observed and this stability was interpreted in terms of higher ordering of the L1₀ phase promoted by the Mn addition. It was moreover found that both crystal growth and unit cell expansion are inhibited, up to the highest temperature investigated (800 °C), proving thus that the Mn addition stabilizes the formed L1₀ structure further. Magnetic hysteresis loops confirmed structural data, revealing a strong coercive field for a sample wherein single phase, hard, magnetic tetragonal L1₀ exists. These findings open good perspectives for use as nanocomposite, rare earth free magnets, working in extreme operation conditions.