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

TL;DR

This work uses field-swept $^{55}$Mn- and $^{139}$La-NMR along with zero-field $^{55}$Mn-NMR on a LaMnSi single crystal to resolve the Mn-AFM structure. The measurements reveal Mn moments aligned with the tetragonal $c$ axis, consistent with a $q=0$ C-type AFM and an odd-parity multipole order, while the La site experiences no internal field. A large Mn internal field of $H_{ m int} = 19.64$ T yields a hyperfine coupling of $A_{ m hf} \\approx 6.0 \, \\mathrm{T}/\\mu_B$, and spin-lattice relaxation shows itinerant-metallic behavior for La and magnon-enhanced relaxation near $T_{\rm N} = 295$ K for Mn. Collectively, these results establish LaMnSi as a clean, 3d-electron system for probing odd-parity multipole order and current-driven multipole phenomena in RTSi, free from 4f-electron complexities.

Abstract

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.

Figures (4)

• Figure 1: (Color online) (a) Crystal structure of LaMnSi drawn by VESTAVESTA. (b) Temperature dependence of the in-plane electrical resistance of LaMnSi single crystal. The inset shows an enlarged view around $T_{\rm N}$.
• Figure 2: (Color online) (a) Field-swept NMR spectra obtained at 4.2 K for three different resonance frequencies. The peak position of the $^{139}$La-NMR signal is indicated by red squares, and that of the $^{55}$Mn-NMR signal is indicated by yellow-green triangles. Both were observed as broad single peaks. The vertical offset between spectra is scaled according to the frequency interval. The inset shows the extrapolation of the linear fit to zero magnetic field, along with the expected resonance frequency of the $^{55}$Mn-ZFNMR signal originating from the other Mn sublattice with an oppositely oriented internal field. (b) Schematic illustration explaining the relation between the resonance field and resonance frequency in $^{55}$Mn-NMR. The Mn site highlighted in red corresponds to the $^{55}$Mn-NMR signal observed in (a). (c) Temperature dependence of the $^{55}$Mn-ZFNMR spectrum from 4.2 K to 125 K. The inset shows the temperature dependence of the resonance frequency of $^{55}$Mn-ZFNMR (light-green triangles) and the ordered Mn moment (black circles)RMnSi_neutron_Welter1994.
• Figure 3: (Color online) $T$ dependence of the nuclear spin-lattice relaxation rate $1/T_{\rm 1}T$ obtained from $^{55}$Mn-ZFNMR and $^{139}$La-NMR measurements. The dashed and dash-dotted lines represent a temperature-independent behavior following the Korringa law and a $T^2$-dependent behavior, respectively. The inset illustrates the two-magnon Raman process, which accounts for the $T^2$-dependent behavior.
• Figure 4: (Color online) Three types of possible magnetic structures of LaMnSi drawn by VESTAVESTA. (a) Magnetic structure suggested by neutron diffraction experiments on polycrystalline samples, in which the ordered moments are tilted by 45° from the $c$ axis.RMnSi_neutron_Welter1994 (b) Magnetic structure suggested by magnetic susceptibility measurements on single-crystal sample, in which the ordered moments are parallel to the $c$ axis.LaMnSi_Tanida2022 This structure is consistent with the present NMR results. (c) G-type AFM structure compatible with both the magnetic susceptibilityLaMnSi_Tanida2022 and NMR results.