Remarkable Magnetic Performances and Thermal Analysis of Nanocomposite Melt-Spun MnAl(Ga) Alloys.

Several potential applications of rare-earth-free magnets have been identified in emerging technological domains such as renewable energy and e-mobility in smart cities, with emphasis on electric bikes and scooters, as well as in the field of autonomous vehicles. In all these applications, with operability at high temperatures, there is considerable interest in nanocomposite magnetic alloys based on MnAl binary systems. The reason for such interest is represented by the occurrence of the τ-MnAl phase, which is magnetic and structurally compatible with L10 tetragonal phases. While the research on MnAl and MnAl-derived magnets has been extensive, alloys with the addition of post-transitional metals with an orthorhombic structure, such as Ga, are less often investigated. The purpose of such additions is to promote and preserve the formation and stability of the τ-MnAl phase, which has promising magnetic properties. The paper illustrates the thermal stability and optimized magnetic properties of τ-MnAl in an alloy based on an off-equiatomic MnAl system with 4 at% Ga addition. A thorough thermal analysis involving a differential scanning calorimetry study is reported, powered by kinetic analysis using two complementary iso-conversional methods: Friedman and Ozawa-Flynn-Wall. Structural characterization is performed using XRD, with the results supported through full-profile MAUD analysis (version 2.99, University of Trento, Italy), revealing that the hexagonal ε phase, predominant in the as-cast sample, transforms massively into the tetragonal τ phase, which becomes predominant upon annealing. Magnetic measurements are employed to fully characterize the alloy's magnetic properties. It is shown that the phase transformation creates very good conditions for achieving good remanence values of about 210 kA/m and large intrinsic coercivities of about 446 kA/m at ambient temperatures.