Cone-spiral magnetic ordering dominated lattice distortion and giant negative thermal expansion in Fe-doped MnNiGe compounds
Negative thermal expansion (NTE) has emerged as one of intense research topics to meet the demands of precision industry for compensating positive thermal expansion (PTE) properties. The adjustment of NTE behavior is the key for tailoring thermal expansion. Chemical modification and particle size effect have been regarded as effective means to tune NTE behavior, and the crystallographic contribution is usually the upper limit of NTE. Here, we report a new way to tune the NTE behavior involving lattice distortion dominated by magnetic structure in hexagonal MnM’Ge-based (M’: Ni, Co) alloys. The achieved maximal linear NTE reaches ∆L/L ~ -23690 × 10−6 (ᾱ = -121.5 × 10−6/K) in a temperature interval as wide as ~195K (80-275K) for Fe-doped MnNiGe alloys. This value is 3.3 times larger than that of corresponding average crystallographical contribution, and exceeds almost all NTE materials reported to date. Neutron powder diffraction and first-principles calculations were carried out. The results revealed that the Fe-doped MnNiGe shows an incommensurate cone-spiral magnetic ordering, and the lattice distortion during phase transition is more significant than that of MnCoGeIn with a linear ferromagnetic ordering. The larger lattice distortion favors cleavage breaking of hexagonal phase along c-axis. As a result, texture effect along (110) crystal plane occurs during molding process, which greatly enhances the amplitude of isotropic in-plane linear NTE. The present study provides a new strategy for exploring adjustable NTE behavior.