Gradual development of Γ5g antiferromagnetic moment in the giant negative thermal expansion material Mn₃Cu_1-x Ge _x N (x∼0.5)

The antiperovskite compound Mn₃ Cu_1-x Ge_x N with x∼0.5 has a giant negative thermal expansion coefficient due to the magnetovolume effect near room temperature. Competition between the Γ5g and Γ4g antiferromagnetic structures around the magnetic ordering temperature could be the driving force for such a phenomenon. In order to examine this possibility, we performed neutron-diffraction experiments on Mn₃Cu_0.5Ge_0.5N and N 14 nuclear magnetic resonance experiments on Mn₃ Cu_0.6 Ge_0.4 N. As a reference material, we also investigated Mn₃ NiN, whose spin structure is represented by a linear combination of the Γ5g and Γ4g antiferromagnetic components. We conclude that in Mn₃ Cu _1-x Ge_x N (x∼0.5), the Γ4g antiferromagnetic component is nearly absent and the competition between the Γ5g and Γ4g antiferromagnetic structures is irrelevant to the giant negative thermal expansion coefficient. The Γ5g antiferromagnetic ordered moment gradually develops with decreasing temperature. This provides a thorough justification for the analysis presented in the previous letter.