Evidence for itinerant electron-local moment interaction in Li-doped $α$-MnTe

TL;DR

This work investigates how Li-doping tunes itinerant-electron coupling to Mn$^{2+}$ local moments in the semiconducting altermagnet α-MnTe. By combining inelastic neutron scattering with linear spin-wave theory and a local-moment sum-rule analysis, the authors show that Li-induced carriers broaden spin waves and modestly renormalize the exchange bandwidth while preserving robust Mn$^{2+}$ local moments with $S=\tfrac{5}{2}$. The results point to Ruderman–Kittel–Kasuya–Yosida (RKKY)–type mediation and magnon–electron damping as key factors, suggesting altermagnetic spin dynamics are strongly influenced by itinerant carriers. Overall, the study highlights the crucial role of itinerant carriers in the magneto-transport and spin dynamics of altermagnets and provides a quantitative framework for itinerant-local moment coupling in MnTe-based systems, with implications for spintronic functionality.

Abstract

We use inelastic neutron scattering to study the impact of Li doping on the semiconducting altermagnet $α$-MnTe. Introducing Li results in a spin reorientation from in-plane to out-of-plane and increases the density of itinerant carriers. While the spin waves in Li-doped $α$-MnTe remain largely Heisenberg-like, there is significant spin wave broadening across the entire Brillouin zone, signaling enhanced magnon damping induced by itinerant carriers. By extracting the local dynamic susceptibility and applying the total moment sum rule, we find that both undoped and Li-doped $α$-MnTe exhibit the full expected Mn$^{2+}$ local moment of $\approx5.9~μ_\mathrm{B}$ with $S=5/2$. These results demonstrate that Li-doped $α$-MnTe hosts robust local-moment magnetism whose interactions are mediated via Ruderman-Kittel-Kasuya-Yosida-type interactions, highlighting the importance of itinerant carriers in magneto-transport and spin dynamic properties of altermagnets.

Figures (9)

• Figure 1: (a,b) Parallel and top views of the lattice and magnetic structures of $\alpha$-MnTe, highlighting magnetic moment directions and exchange couplings. The dashed lines indicate the altermagnetic exchange interactions $J_{10}$ and $J_{11}$. (c,e) Schematics of magnetic domain configurations in undoped and Li-doped $\alpha$-MnTe, respectively. The edges of the hexagons are parallel to $\bm{a}$ and equivalent crystallographic directions in panel (b). (d) Parallel view of the lattice and magnetic structures of Li-doped $\alpha$-MnTe, illustrating Li atoms (occupancy exaggerated) occupying Te-site defects. (f) Magnetization $M$ of Li-doped $\alpha$-MnTe versus temperature $T$. (g) Longitudinal resistivity $\rho_{xx}$ versus temperature $T$ for undoped and Li-doped $\alpha$-MnTe. (h) Transverse resistivity $\rho_{xy}$ of Li-doped $\alpha$ MnTe. (i) Carrier density $n$ and mobility $\mu$ of undoped and Li-doped $\alpha$-MnTe. (j,k) Calculated spin waves of undoped $\alpha$-MnTe for three domains within (j) and outside (k) the nodal planes. The calculations use exchange parameters from Ref. liu_chiral_2024. The inset in (k) displays the intensity contributions from the three domains at the point of altermagnetic splitting. The solid and dashed lines denote different branches of the spin waves, and the vertical dashed line separates the splitting dominated by single-ion anisotropy ($D_z$) and altermagnetism ($|J_{10}-J_{11}|$). (l,m) Calculated spin waves of Li-doped $\alpha$-MnTe, with the single-ion anisotropy replaced by the value reported in Ref. yumnam_MagnonGapTuning_2024.
• Figure 2: (a,b) INS spectra acquired at base temperature ($T<10$ K) with an incident energy of $E_i = 50$ meV for Li-doped [high-flux (HF) mode] and undoped $\alpha$-MnTe, shown along high-symmetry directions within the nodal plane. The black solid and dashed curves represent calculated spin wave branches, which are degenerate in the Li-doped sample. (c-e) Energy-axis line cuts at selected $\bm{Q}$ points marked in panel (a), including both HF and high-resolution (HR) modes for Li-doped $\alpha$-MnTe. Dashed lines indicate Gaussian fits. Horizontal bars mark the instrumental energy resolution at the Gaussian peak position for each configuration. (f) Schematic of the Brillouin zone indicating high-symmetry points. The nodal planes are shown in gray, and the $\Gamma$-A–L–M plane is highlighted in red.
• Figure 3: (a,b) INS spectra along $[1.33,0,L]$ and (c,d) along the $[H,0,H]$ direction for undoped and Li-doped $\alpha$-MnTe. (e) Schematic of the $\Gamma$-A-L-M plane of the Brillouin zone, where splitting is expected to occur in undoped $\alpha$-MnTe. Green and purple lines highlight the $[1.33,0,L]$ and $[H,0,H]$ directions, respectively. Arrows indicate the direction of constant-$\bm{Q}$ line cuts. (f) Comparison of line cuts for the dispersion along the $[1.33,0,L]$ and (g) $[H,0,H]$ directions. Points represent the measured data, and error bars represent 1$\sigma$ uncertainties. Solid and dashed curves represent fits to LSWT models where $J_{10}$ and $J_{11}$ are nonzero and zero, respectively. For Li-doped $\alpha$-MnTe, the nonzero values of $J_{10}$ and $J_{11}$ were fixed to the refined values from undoped $\alpha$-MnTe. Data for Li-doped $\alpha$-MnTe was shifted vertically by 0.075 units for clarity. (h,i) Wavevector dependence of the energy full-width at half-maxima (FWHM) near the zone center $\bm{Q}_\Gamma=(1,0,1)$ along $\Gamma$-A and $\Gamma$-M directions, respectively.
• Figure 4: (a-c) Constant-energy maps of the INS spectra in the $HK$ plane for undoped (left) and Li-doped $\alpha$-MnTe (right) between 33.5 meV and 34.5 meV. The BZ is outlined by a thin grey line. The observed splitting in undoped $\alpha$-MnTe is indicated by red arrows. (d) Energy dependence of the local dynamic susceptibility extracted from INS data, with effective moments obtained from the total moment sum rule.
• Figure S1: Energy resolutions calculated using PyChop for the three experimental configurations used to measure the spin excitation data presented in this Letter. HF: high-flux mode; HR: high-resolution mode.