Anisotropic lattice expansion and stabilization of helimagnetism in Ru-substituted MnP

The practical utilization of MnP in chiral spintronic devices is fundamentally constrained by its low helical ordering temperature (TS). Here, we demonstrate that Ru substitution in Mn₁₋ₓRuₓP single crystals drives a highly anisotropic lattice expansion, where the b-axis elongation is one-quarter that of the a- and c-axes (~ 0.04 Å). This structural distortion profoundly stabilizes the helical ground state, elevating TS from 51~K to 215~K and the critical field along the [010] direction at 5~K from 2.3 to 30.0~kOe, while suppressing the Curie temperature (TC) from 291~K to 215~K. Synthesizing these results with reported data on Mo- and W-doped analogues reveals that TS and TC are governed primarily by the b-axis parameter, exhibiting universal linear scaling relationships (dTS/db = 1.59 × 10⁴\ KÅ⁻¹, dTC/db = 0.69 × 10⁴\ KÅ⁻¹) far greater than those associated with the a- or c-axes. First-principles calculations reveal that the lattice expansion selectively attenuates ferromagnetic coupling while preserving antiferromagnetic interactions between nearest-neighbor Mn atoms, thereby enhancing magnetic frustration and stabilizing helimagnetism. These findings establish chemical pressure via directed b-axis engineering as a robust, generalizable paradigm for stabilizing helimagnetism in MnP.