Randomly packed Ni2MnIn and NiMn structural units in off-stoichiometric Ni2Mn2−y

Abstract

Ni 2 Mn 2−y In y alloys transform from the martensitic L1 0 antiferromagnetic ground state near y = 0 to the austenitic ferromagnetic L2 1 Heusler phase near y = 1 due to doping of In impurity for Mn. The offstoichiometric alloys prepared by rapid quenching are structurally metastable and dissociate into a mixture of L2 1 (Ni 2 MnIn) and L1 0 (NiMn) phases upon temper annealing. Despite this structural disintegration, the martensitic transformation temperature remains invariant in the temper annealed alloys. Investigations of the local structure of the constituent atoms reveal the presence of strongly coupled Ni 2 MnIn and NiMn structural units in the temper annealed as well as the rapidly quenched off-stoichiometric Ni 2 Mn 2−y In y alloys irrespective of their crystal structure. This random packing of the L2 1 and L1 0 structural units seems to be responsible for invariance of martensitic transition temperature in the temper annealed alloys as well as the absence of strain glass transition in rapidly quenched alloys.

Antisite disorder and phase segregation in Mn2NiSn

“…Recent EXAFS studies on temper annealed Ni 2 Mn 2−x In x alloys have shown that complete phase separation is present at least at the local structural level even if the alloy presents a single-phase structure in XRD [30]. Therefore, to check the extent of phase separation and the changes in the local structural environment due to temper annealing, the LT EXAFS data recorded at the Mn, Ni and Sn K edges in Ni 1.25 Mn 1.75 Sn and NiMn 2 Sn TA alloys are compared with the respective EXAFS data recorded in the RQ alloys (see figure 8).…”

Antisite disorder and phase segregation in Mn$_{2}$NiSn

“…and c. indicates the presence of disorder around the Mn and Sn atoms. Such structural defects are responsible for phase separation and the phenomenon of shell ferromagnetism in the Ni 2 Mn 2−x In x alloys [30]. Upon temper annealing, these alloys phase separate into proportionate amounts of Heusler (Ni 50 Mn 25 In 25 ) and L1 0 (Ni 50 Mn 50 ) phases [31].…”

“…Recent EXAFS studies on temper annealed Ni 2 Mn 2−x In x alloys have shown that complete phase separation is present at least at the local structural level even if the alloy presents a single-phase structure in XRD [30]. Therefore, to check the extent of phase separation and the changes in the local structural environment due to temper annealing, the LT EXAFS data recorded at the Mn, Ni and Sn K edges in Ni 1.25 Mn 1.75 Sn and NiMn 2 Sn TA alloys are compared with the respective EXAFS data recorded in the RQ alloys (see Fig.…”

Factors influencing the martensitic transformation in Ni–Mn–Z (Z = Ga, Sn) Heusler alloys

“…Recently, these Mn-rich alloys have been shown to be metastable against thermal annealing and they phase separate into stoichiometric Heusler and tetragonal L1 0 phases [21]. Martensitic transformation in these Ni 2 Mn 1+x Z 1−x alloys is conjectured to be due to the presence of martensitic NiMn structural units randomly distributed among the Heusler structural units [22]. Theoretical calculations have suggested the weakening of the interatomic covalent hybridization due to additional atoms to be responsible for the increase in the T M [23].…”