Elaine Dias

MnCoGe-based intermetallics have garnered significant attention owing to their pronounced negative thermal expansion and large magnetocaloric effect (MCE), both of which are highly sensitive to chemical substitutions. Despite extensive studies, the microscopic origin underlying the tunability of these properties remains unclear. In this study, we conducted a comprehensive investigation of the local atomic environments around Mn and Co in MnCoGe1−xSnx (0≤x≤0.1) using extended x-ray absorption fine structure and other structural and magnetic characterization techniques. Our analysis reveals that the substitutional disorder introduced by Sn atoms occupying the Ge sublattice sites disrupts the cooperative lattice distortions required for the long-range displacive martensitic transformation. This disorder-induced suppression results in a systematic decrease in both the martensitic transition temperature (TM) and the magnetic ordering temperature (TC). At low doping levels, the convergence of TM and TC enhances magnetostructural coupling, thereby amplifying the MCE. However, further Sn substitution ultimately suppresses the martensitic transition entirely, leading to a decoupling of the magnetic and structural degrees of freedom and a consequent degradation of the magnetostructural response.

Ni-substituted Mn₃Ga displays a weak ferromagnetism embedded in an antiferromagnetic (AF) phase. Upon field cooling, the alloy exhibits exchange bias and an open hysteresis loop, signifying kinetic arrest at room temperature. For the first time, a kinetic arrest is seen in a compound due to the first order transition of an embedded defect phase. A systematic study of crystal structure, local structure, and magnetic properties of Mn 3 − x Ni _x Ga (x = 0, 0.25) alloys reveal the origin of ferromagnetism in Mn_2.75Ni_0.25Ga is due to the segregation of a Heusler-type environment around Ni in the cubic Mn₃Ga matrix. Upon temper annealing at 400 ^∘C, these local structural defects around the Ni phase separate into a modulated ferromagnetic (FM) Ni-Mn-Ga Heusler phase. A strong interaction between the AF host and the FM defect phase gives rise to exchange bias. The first-order transition of the defect phase seems to be responsible for the observed kinetic arrest in Mn_2.75Ni_0.25Ga.

A systematic study of crystal structure, local structure, magnetic and transport properties in quenched and temper annealed Ni 2 − x Mn 1 + x Sn alloys indicate the formation of Mn₃Sn type structural defects caused by an antisite disorder between Mn and Sn occupying the Y and Z sublattices of X₂YZ Heusler structure. The antisite disorder is caused by the substitution of Ni by Mn at the X sites. On temper annealing, these defects segregate and phase separate into L 2 1 Heusler and D 0 19 Mn₃Sn type phases.

The stoichiometric Ni₅₀Mn₂₅In₂₅ Heusler alloy transforms from a stable ferromagnetic austenitic ground state to an incommensurate modulated martensitic ground state with a progressive replacement of In with Mn without any pre-transition phases. The absence of pre-transition phases like strain glass in Ni₅₀Mn_25+x In_25−x alloys is explained to be the ability of the ferromagnetic cubic structure to accommodate the lattice strain caused by atomic size differences of In and Mn atoms. Beyond the critical value of x = 8.75, the alloys undergo martensitic transformation despite the formation of ferromagnetic and antiferromagnetic clusters and the appearance of a super spin glass state.

The volume expanding magnetostructural transition in Mn3GaC and Mn3SnC has been identified to be due to distortion of Mn6C octahedra. Despite a similar lattice volume as Mn3SnC and similar valence electron contribution to density of states as in Mn3GaC, Mn3InC does not undergo a first order magnetostructural transformation like the Ga and Sn antiperovskite counterparts. A systematic investigation of its structure and magnetic properties using probes like x-ray diffraction, magnetization measurements, neutron diffraction, and extended x-ray absorption fine structure reveals that though the octahedra are distorted resulting in long and short Mn–Mn bonds and different magnetic moments on Mn atoms, the interaction between them remains ferromagnetic. This has been attributed to the strain on the Mn6C octahedra produced due to a relatively larger size of In atoms compared to Sn and Ga. The size of In atoms constricts the deformation of Mn6C octahedra giving rise to Mn–Mn distances that favor only ferromagnetic interactions in the compound.

While the unit cell volume of compounds belonging to the Mn3Ga1−xSnxC (0 ≤x≤ 1) series shows a conformity with Vegard’s law, their magnetic and magnetocaloric properties behave differently from those of parent compounds Mn3GaC and Mn3SnC. A correlation between the observed magnetic properties and underlying magnetic and local structure suggests that replacing Ga atoms by larger atoms of Sn results in the formation of Ga-rich and Sn-rich clusters. As a result, even though the long range structure appears to be cubic, Mn atoms find themselves in two different local environments. The packing of these two different local structures into a single global structure induces tensile/compressive strains on the Mn6C functional unit and is responsible for the observed magnetic properties across the entire solid solution range.

Mn3GaC undergoes a ferromagnetic to antiferromagnetic, volume discontinuous cubic-cubic phase transition as a function of temperature, pressure, and magnetic field. Through a series of temperature dependent x-ray absorption fine structure spectroscopy experiments at the Mn K and Ga K edge, it is shown that the first order magnetic transformation in Mn3GaC is entirely due to distortions in the Mn sub-lattice and with a very little role for Mn-C interactions. The distortion in the Mn sub-lattice results in long and short Mn-Mn bonds with the longer Mn-Mn bonds favoring ferromagnetic interactions and the shorter Mn-Mn bonds favoring antiferromagnetic interactions. At the first order transition, the shorter Mn-Mn bonds exhibit an abrupt decrease in their length resulting in an antiferromagnetic ground state and a strained lattice.

A study investigating the effect of Sn substitution on the magnetocaloric properties of Mn3Ga(1−x)SnxC compounds reveals that the nature of the magnetocaloric effect (MCE) has a strong dependence on the nature of the magnetic ordering. For small amounts of Sn (x ≤ 0.2), the MCE is of the inverse type, wherein an increase in the applied field beyond 5 T gives rise to a table like temperature dependence of the entropy due to a coupling between the first order ferromagnetic (FM)–antiferromagnetic (AFM) transition and the field induced AFM–FM transition. Replacement of Ga by larger concentrations of Sn (x ≥ 0.71) results in a change of the MCE to a conventional type with very little variation in the position of (ΔSM)max with increasing magnetic field. This has been explained to be due to the introduction of local strain by A site ions (Ga/Sn), which affect the magnetostructural coupling in these compounds.