Magnetovolume Effect and Negative Thermal Expansion in Mn3(Cu1-xGex)N

Metallic manganese nitrides Mn 3 AN (A＝Zn, Ga, etc) are well known for their large magnetovolume effect (MVE), a discontinuous volume expansion at the magnetic transition.However, MVE is exceptionally absent in Mn 3 CuN.We found that MVE is induced by a small amount of Ge in the Cu site.This seems to coincide with recovery of the cubic structure.By further Ge doping, the volume expansion becomes gradual (DT～100 K) and large negative thermal expansion (NTE) is exhibited around room temperature [a＝-12×10 -6 K -1 (a: coefficient of linear thermal expansion) for Mn 3 (Cu 0.5 Ge 0.5 )N].Such a large, isotropic and non hysteretic NTE is desirable for practical applications.