Microscopic Determination of the c-axis-Oriented Antiferromagnetic Structure in LaMnSi by $^{55}$Mn and $^{139}$La NMR

We report a microscopic investigation of the magnetic structure and electronic properties of LaMnSi in its antiferromagnetic (AFM) state using nuclear magnetic resonance (NMR). Field-swept $^{55}$Mn- and $^{139}$La-NMR spectra, as well as zero-field 55Mn-NMR (ZFNMR) spectra, reveal that the Mn ordered moments are parallel to the tetragonal c axis, consistent with the C-type AFM structure and the realization of an odd-parity multipole order. The internal field at the Mn site is determined to be 19.64 T at 4.2 K, corresponding to a hyperfine coupling constant of Ahf = 6.0 T/uB. Nuclear spin-lattice relaxation rate 1/T1 exhibits a characteristic behavior of itinerant antiferromagnetism, showing metallic behavior at low temperatures and magnon-induced enhancement upon approaching the Neel temperature (TN = 295 K). These results show LaMnSi as an ideal compound to study 3d electron magnetism and odd-parity multipole order in the RT Si (R = rare-earth, T = transition metal) system, free of the complexities of 4f electrons.

💡 Research Summary

This paper presents a comprehensive nuclear magnetic resonance (NMR) investigation of the magnetic structure and electronic properties of LaMnSi in its antiferromagnetic (AFM) state. LaMnSi belongs to the RTSi family (R = rare‑earth, T = transition metal) that crystallizes in the tetragonal CeFeSi‑type structure (space group P4/nmm). Because La carries no 4f electrons, the compound provides a clean platform to study magnetism arising solely from Mn 3d electrons and to explore the possibility of odd‑parity multipole order that is theoretically allowed by the sublattice‑exchange symmetry of this structure.

Single‑crystal samples were grown by a self‑flux method (size ≈ 3.5 × 2.3 × 0.2 mm³). Electrical‑resistance measurements identified a Néel temperature (T_N = 295) K, confirming the onset of AFM order. Field‑swept NMR experiments were performed on the two NMR‑active nuclei, (^{55})Mn (I = 5/2, γ/2π = 10.554 MHz T⁻¹) and (^{139})La (I = 7/2, γ/2π = 6.0142 MHz T⁻¹), with the external magnetic field applied along the crystallographic c‑axis. Spectra were recorded at fixed frequencies of 65.15, 78.18 and 84.00 MHz. After the crystal fractured during measurements, the three pieces were stacked to obtain high‑intensity zero‑field Mn‑NMR (ZFNMR) spectra.

Both nuclei produced single, broad resonance peaks. The La peak displayed a linear relationship (f = γ_{\text{La}} H) with an intercept essentially zero, indicating that La sites experience no internal magnetic field. In contrast, the Mn peak obeyed (f = γ_{\text{Mn}}|H \pm H_{\text{int}}|). The observed Mn signal originates from sites where the external field is antiparallel to the internal hyperfine field, implying a staggered internal field oriented along the c‑axis. Zero‑field Mn‑NMR at 4.2 K gave a resonance frequency of ≈ 207 MHz, which corresponds to an internal field (H_{\text{int}} = 19.64) T. Using the ordered Mn moment from neutron diffraction ((μ_{\text{Mn}} = 3.27 μ_B)), the hyperfine coupling constant is estimated as (A_{\text{hf}} ≈ 6.0) T μ_B⁻¹, a value comparable to those reported for other c‑axis‑collinear Mn‑based compounds (e.g., BaMn₂Pn₂).

Spin‑lattice relaxation rates (1/T_1) were measured for both nuclei. For La, (1/T_1T) is temperature‑independent from 1.5 K to 20 K, obeying the Korringa law and confirming a conventional metallic state. For Mn, (1/T_1T) is also constant up to ~30 K but with a magnitude about seven times larger than that of La, reflecting the stronger hyperfine coupling at the magnetic Mn site. Above ~40 K, (1/T_1T) begins to increase, and for (T > 60) K it follows a (T^2) dependence. This behavior is characteristic of a two‑magnon Raman process dominating nuclear relaxation in itinerant antiferromagnets, indicating enhanced magnon excitations as the system approaches (T_N).

The NMR results resolve a long‑standing controversy regarding the magnetic structure of LaMnSi. Earlier neutron diffraction on polycrystals suggested a C‑type AFM arrangement with Mn moments tilted 45° from the c‑axis, whereas bulk susceptibility on single crystals favored moments parallel to c. The present data unequivocally support the latter: Mn moments are aligned parallel to the c‑axis in a C‑type AFM pattern, producing a staggered internal field at Mn sites while leaving La sites field‑free. A G‑type AFM model is inconsistent with the observed internal field magnitude and is therefore excluded.

In summary, the study establishes that LaMnSi hosts a c‑axis‑oriented C‑type antiferromagnetic order driven solely by Mn 3d electrons, exhibits itinerant antiferromagnetic spin dynamics (metallic Korringa behavior at low T, magnon‑induced (T^2) relaxation near (T_N)), and possesses a sizable hyperfine coupling that enables precise microscopic probing. Because La lacks 4f electrons, LaMnSi serves as an ideal reference system for the broader RTSi family, allowing clean investigation of odd‑parity multipole order and associated cross‑correlated phenomena (e.g., current‑induced strain, magnetoelectric effects) without the complications of 4f‑electron physics. The metallic nature further makes it a promising platform for experimental studies of current‑driven multipole responses.