Magnetostructural correlations in antiperovskite manganese nitrides were investigated systematically for stoichiometric and solid solution Mn3Cu1−xAxN (A = Co, Ni, Zn, Ga, Ge, Rh, Pd, Ag, In, Sn or Sb). This class of nitrides is attracting great attention because of their giant negative thermal expansion, which is achieved by doping Ge or Sn into the A site as a relaxant of the sharp volume contraction on heating (spontaneous volume magnetostriction ωs) because of the magnetovolume effects. The physical background of large ωs and mechanism of how the volume contraction becomes gradual with temperature are central concerns for the physics and applications of these nitrides. An entire dataset of thermal expansion, crystal structure and magnetization demonstrates that the cubic triangular antiferromagnetic state is crucial for large ωs. The intimate relationship between ωs and the magnetic structure is discussed in terms of geometrical frustration related to the Mn6N octahedron and magnetic stress concept. The results presented herein also show that ωs depends on the number of d electrons in the A atom, suggesting the important role of the d orbitals of the A atom. Not all the dopants in the A site, but the elements that disturb the cubic triangular antiferromagnetic state, are effective in broadening the volume change. This fact suggests that instability neighboring the phase boundary is related to the broadening. The relation between the gradual volume change and the local structure anomaly is suggested by recent microprobe studies.

Electrical resistivity is systematically investigated in Mn3AgN and related compounds with an antiperovskite structure. Despite its overall metallic character, Mn3AgN features a broad maximum in the temperature-resistivity curve in the paramagnetic state and the temperature coefficient of resistance (TCR) is negative at higher temperatures. The resistivity-peak temperature was tuned to just room temperature by the partial substitution of Cu for Ag, and a TCR as low as 10−6 K−1 was achieved over a wide temperature window including room temperature. These peculiar behaviors are possibly due to collapse of coherent quasiparticle states by strong magnetic scattering.

No abstract

Giant magnetostriction of up to 2000 ppm was discovered in the tetragonally distorted ferromagnetic phase of antiperovskite Mn3CuN. This magnetostriction was associated with the rearrangement of thermoelastic martensite variants by the magnetic field, similar to that of Ni2MnGa Heusler alloy. The nitrogen-deficient Mn3CuN0.8 contracts, whereas the stoichiometric Mn3CuN expands, along the field direction. This implies a rotation of the easy axis from the longer a axis in Mn3CuN to the shorter c axis in Mn3CuN0.8. The drastic reduction of the operating magnetic field for magnetostriction in Mn3CuN0.8 suggests a possible supermagnetostriction around the compositional region of direction change in the easy axis.

The discovery of giant magnetostriction exceeding 2000 ppm in Mn3CuN has stimulated the research of manganese antiperovskites from the perspective of forced magnetostriction. We discovered that Mn3SbN exhibits a large magnetostriction of up to 450 ppm in the tetragonally distorted ferromagnetic phase below the Curie temperature TC = 360 K. The magnitude of magnetostriction is enhanced by up to 1000 ppm without reduction of TC by partial replacement of the constituent elements. The present results are examined in terms of ferromagnetic shape memory effects.