Magnetism and the defect state in the magnetocaloric antiperovskite Mn₃GaC_1−δ

Magnetic and spectroscopic techniques were used to study the intermetallic antiperovskite Mn3GaC. An anti ferromagnetic-ferromagnetic magnetostructural transition at 160 K underlies a remarkable magnetocaloric effect; these phenomena are suppressed in the substoichiometric composition Mn3GaC1-delta. X-ray absorption spectroscopy (XAS) data reported for three compositions Mn3GaC1-delta, delta = 0, 0.10, 0.22, are the basis for drawing inferences concerning the mechanism controlling magnetic order as a function of carbon stoichiometry. While the temperature dependence of the Mn3GaC carbon K edge reveals no observable change across the first-order magnetic transition, a clear splitting of the carbon absorption bands is observed that increases with increasing carbon deficiency. The room temperature Mn and Ga K edges indicate no significant variation with C content. FEFF 8.2 code calculations are in good qualitative agreement with data for the stoichiometric sample, but do not predict the changes in XAS observed in C-deficient samples. These results and the Goodenough-Anderson-Kanamori rules are the basis for a phenomenological model that attributes the carbon content dependence of the low temperature transition to the promotion of weak near-neighbour 90 degrees Mn-Mn pairs in the carbon-deficient compound over the stronger 180 degrees Mn-C-Mn interaction, locking in dominant ferromagnetism at low temperatures.