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

Xin-Wei Wu, Deng-lu Hou, Li Ma, Cong-mian Zhen, De-wei Zhao, Guoke Li

Abstract

The practical utilization of MnP in chiral spintronic devices is fundamentally constrained by its low helical ordering temperature ($T_{\rm S}$). 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 $T_{\rm S}$ 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 ($T_{\rm C}$) from 291~K to 215~K. Synthesizing these results with reported data on Mo- and W-doped analogues reveals that $T_{\rm S}$ and $T_{\rm C}$ are governed primarily by the $b$-axis parameter, exhibiting universal linear scaling relationships ($dT_{\rm S}/db = 1.59 \times 10^4\ \text{KÅ}^{-1}$, $dT_{\rm C}/db = 0.69 \times 10^4\ \text{KÅ}^{-1}$) 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.

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 (). Here, we demonstrate that Ru substitution in MnRuP single crystals drives a highly anisotropic lattice expansion, where the -axis elongation is one-quarter that of the - and -axes ( 0.04 Å). This structural distortion profoundly stabilizes the helical ground state, elevating 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 () from 291~K to 215~K. Synthesizing these results with reported data on Mo- and W-doped analogues reveals that and are governed primarily by the -axis parameter, exhibiting universal linear scaling relationships (, ) far greater than those associated with the - or -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 -axis engineering as a robust, generalizable paradigm for stabilizing helimagnetism in MnP.

Paper Structure

This paper contains 4 sections, 4 figures, 1 table.

Table of Contents

  1. INTRODUCTION
  2. Experimental
  3. Results and discussion
  4. Conclusion

Figures (4)

  • Figure 1: Structural characterization of Mn$_{1-x}$Ru$_x$P single crystals. (a) The crystal structure of MnP (space group Pnma; $a = 5.260$ Å; $b = 3.174$ Å; $c = 5.919$ Å), where the exchange parameters are depicted on a distorted NiAs-type lattice. (b) XRD patterns collected from the side facets of needle-like Mn$_{1-x}$Ru$_x$P single crystals for various Ru content $x$. (c) Representative Rietveld refinement profile of the XRD pattern for powder obtained by grinding single crystals. The inset shows an optical image of a typical Mn$_{1-x}$Ru$_x$P ($x = 0.00$, 0.01, 0.05, 0.07, 0.10) single crystal. (d) Evolution of the lattice parameters $a$, $b$, and $c$ with increasing $x$ from 0.00 to 0.10.
  • Figure 2: Magnetic properties of Mn$_{1-x}$Ru$_x$P single crystals. (a, b) Zero-field-cooled (ZFC) magnetization curves measured under an applied magnetic field of 100 Oe; (c, d) isothermal magnetization curves measured at 5 K. The data were acquired along the [010] crystallographic direction for panels (a) and (c), and along [101] for panels (b) and (d).
  • Figure 3: Scaling of magnetic phase transitions across chemically doped MnP. (a) (b) Magnetic field--temperature ($H$--$T$) phase diagrams of Mn$_{1-x}$Ru$_x$P with the magnetic field applied along the [010] and [101] crystallographic directions, respectively. (c) $T_{\rm C}$ and $T_{\rm S}$ as a function doping concentration $x$ in the Mn$_{1-x}$M$_x$P system (M = Ru, Mo, W) . (d) $T_{\rm S}$ and $T_{\rm C}$ plotted against the $b$-axis lattice parameter, integrating data from chemically doped MnP and hydrostatic pressure experiments on MnP.
  • Figure 4: Phase stability of the helical phase of MnP based on first-principles calculations. (a) Energy difference between the helical and ferromagnetic phases (left axis) as a function of Ru substitution level $x$. Calculations were performed using lattice parameters corresponding to different $x$ values, without explicitly substituting Mn atoms with Ru in the supercells. (b) Theoretical $R$--$R'$ phase diagram based on the Bertaut--Kallel model, where $R = J_1/J_2$ and $R' = J_1'/J_2$. The red and blue curves represent the boundaries separating the helical--ferromagnetic and helical--antiferromagnetic regions, respectively.