Lattice-Expansion-Driven Stabilization of Helical Magnetic Order in Ru-Doped MnP

Abstract

The practical utilization of MnP in chiral spintronic devices is fundamentally constrained by its low helical ordering temperature ($T_{\mathrm{S}}$). Here, we demonstrate that Ru substitution in Mn${}_{1-x}$Ru${}_x$P single crystals drives a highly anisotropic lattice expansion, where the $b$-axis elongation is one-quarter that of the $a$- and $c$-axes ($\sim$ 0.04 Å). This structural distortion profoundly stabilizes the helical ground state, elevating $T_{\mathrm{S}}$ 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 ($T_{\mathrm{C}}$) from 291~K to 215~K. Synthesizing these results with reported data on Mo- and W-doped analogues reveals that $T_{\mathrm{S}}$ and $T_{\mathrm{C}}$ are governed primarily by the $b$-axis parameter, exhibiting universal linear scaling relationships ($d{T}_{\mathrm{S}}/db=1.59\times 10^4{\text{KA˚}}^{-1}$, $d{T}_{\mathrm{C}}/db=0.69\times 10^4{\text{KA˚}}^{-1}$) 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.