Thermodynamic approach for deciphering magneto-structural phase transitions: Proof of concept in Heusler alloys

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Ferromagnets acquire nontrivial magnetic and caloric properties when a structural phase transition temperature approaches the Curie point. Deciphering magneto-structural transitions, i.e., determining their critical temperatures, remains challenging. In this paper, a novel thermodynamic analysis was employed to determine the Curie points and structural martensitic transformation (MT) temperatures of five metamagnetic Heusler samples, Ni 50 Mn 25- x Cu x Ga 25 ( x = 6.25, 6.5, 6.75, 7) and Ni 50.5 Mn 18.5 Cu 6.5 Ga 24.5 . Three types of transformational behavior were identified, arising from small compositional variations: (I) MT in the ferromagnetic state; (II) magneto-structural phase transition from paramagnetic austenite to ferromagnetic martensite; (III) MT in paramagnetic state. For the first time, the calculated effect of structural transition on spin-exchange parameter is validated against experimental results. Computation of magnetization M(T) and susceptibility χ(T) curves is enabled by modeling the interplay between magnetic and structural degrees of freedom. Critical temperatures can be identified from the extrema of the dM(T)/dT and χ(T) functions for behaviors (I) and (III). Instead, it is possible to calculate virtual Curie temperatures by fitting the theoretical model to experimental curves for case (II). In addition, the analysis revealed how variations in the spin-exchange parameter result in a large difference (≥ 50 K) between the Curie temperatures of austenitic and martensitic phases, thereby determining the width of the compositional range over which behavior (II) is observed. This approach provides a robust framework for extracting critical temperatures from standard magnetization data. It is applicable to ferromagnetic Heusler systems and other multiferroic ferromagnets, offering guidance for materials design.