Thermodynamic Approach for Deciphering Magneto-Structural Phase Transitions: Proof of Concept in Heusler Alloys
Ferromagnetic solids acquire nontrivial magnetic and caloric properties when the temperature of the structural phase transition approaches the Curie point. Deciphering magneto-structural transitions, i.e. determining their characteristic temperatures and elucidating the related properties, remains challenging. In the present paper, three types of transformational behaviour of Ni50Mn25-xCuxGa25 (x = 6.25, 6.5, 6.75, 7) and Ni50.5Mn18.5Cu6.5Ga24.5 alloys have been identified, arising from small variations in chemical composition: (i) structural martensitic transformation (MT) in the ferromagnetic phase; (ii) magneto-structural phase transition from paramagnetic austenite to ferromagnetic martensite; (iii) MT in paramagnetic phase. The temperature-dependent values of magnetization, M(T), and of magnetic susceptibility, $\chi(T)$, were measured for each alloy. A novel thermodynamic analysis was used to determine the Curie points and MT temperatures. The novelty lies in considering the interplay between structural and magnetic characteristics of the alloys through the impact of the structural transition on the spin-exchange parameter. The theoretical analysis of experimental data revealed that this impact results in a large difference ($\geq$ 50 K) between the Curie temperatures computed for the austenitic and martensitic states of each alloy. The characteristic temperatures, corresponding to the extrema of the dM(T)/dT and $\chi(T)$ functions, were calculated. The correlation of these temperatures with the Curie temperatures and the MT temperatures is not straightforward and depends strongly on the type of transformational behaviour (i) - (iii). The proposed approach provides a robust framework for extracting unmeasurable characteristic temperatures from standard magnetization data, applicable to ferromagnetic Heusler systems and other multiferroic ferromagnetic materials.