Unraveling the intricate mechanism of magnetoelastic coupling in Mn3GaC type Antiperovskite materials

The first order ferromagnetic to antiferromagnetic phase transition in Mn3GaC is accompanied by a large inverse magnetic entropy change (ΔSM ~ 15 J/kg-K at 2 T, equivalent to an adiabatic temperature difference of 4 K) at about T = 178 K. When a larger atom like Sn, replaces Ga at the    A-site, the first order transition temperature increases to T = 279 K and the compound Mn3SnC transforms from a room temperature paramagnetic phase to a magnetic state wherein ferromagnetic and antiferromagnetic sublattices coexist with a complicated arrangement of spins. Consequently, the magnetocaloric effect changes from inverse to the conventional type and ΔSM is about -2 J/kg-K at H = 2 T. Utilizing various macroscopic and microscopic techniques such as x-ray diffraction, magnetization, neutron diffraction and temperature dependent XAFS, the observed difference between magnetic and magnetocaloric properties is probed into at different length scales. The magnetic properties exhibited by these compounds have been attributed to the difference in Mn-Mn bond distances that arise due to local distortions in the Mn6C octahedra of Mn3GaC type antiperovskites. As larger atoms introduced at the A-site gradually increase the unit cell volume, the type of local distortions is altered due to compressive strains on the Mn6C octahedra. These differences in the local structure of Mn atoms that depend on the type of atom present at the A-site are preserved even in solid solutions of Mn3Ga1-xSnxC, resulting in the “glassy” magnetic ground state exhibited by Mn3Ga0.45Sn0.55C.

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