Abstract
Martensitic and magnetic transformations in Ni48Mn39.5Sn12.5−xAlx (x=0, 1, 2, 3) Heusler alloy ribbons were investigated. It is demonstrated that both magnetic and structural transformations occur in all of the studied samples. It is also shown that substitution of Sn with Al causes the martensitic transformation (MT) and the reverse martensitic transformation (RMT) temperatures to increase to room temperature (ΔTMT=49K; ΔTRMT=43K), whereas the Curie temperature of martensite TCM decreases (ΔT=36K) and the Curie temperature of austenite TCA remains practically insensitive to Al introduction. This then allows to tune TCA and the MT temperature leading to their coincidence at ambient temperature. The austenite phase with the L21 type structure has been identified to exist in all the samples regardless of composition. On the other hand the structure of martensite has been shown to be sensitive to composition. It has been determined as the 10M martensite with (32¯) stacking sequence in Al free samples and the 4O martensite with the stacking periodicity (31¯) in Al containing samples. In addition, the splitting of the field cooling (FC) and the field heating (FH) thermo-magnetic curves at low (50Oe) magnetic field and below the TCM has been attributed to intermartensitic transition. The application of large magnetic field (50kOe) has shown the existence of two distinct ferromagnetic states with a considerable hysteresis loop. The properties of these materials make them promising for magnetocaloric applications.
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