Magnetic correlations in the magnetocaloric materials Mn₃GaC and Mn₃GaC_0.85N_0.15 studied by neutron polarization analysis and neutron depolarization

Partially substituting carbon by nitrogen in the antiperovskite compound Mn₃GaC increases the first order antiferromagnetic/ferromagnetic transition temperature and at the same time causes the high-temperature long-range ferromagnetism to weaken. To show that the weakening is related to the diminishing of ferromagnetic domain formation, we undertake neutron depolarization and neutron polarization analysis experiments on Mn₃GaC and Mn₃GaC_0.85N_0.15. Polarization analysis experiments show that strong ferromagnetic correlations are present at high temperatures in the paramagnetic states of both Mn₃GaC and Mn₃GaC_0.85N_0.15 and that these correlations vanish in the antiferromagnetic state. Neutron depolarization studies show that above the first order transition temperature, ferromagnetic domain formation is present in Mn₃GaC but is absent in Mn₃GaC_0.85N_0.15. The relationship between ferromagnetic domain formation and transitional hysteresis is brought forward for these two important magnetocaloric materials. (letter)