Low-energy magnons in the altermagnet $α$-MnTe

Abstract

We report high-field electron spin resonance studies of the altermagnetic material $α$-MnTe. In magnetic fields applied parallel to the triangular Mn$^{2+}$ layers we observed a single resonance line, corresponding to an antiferromagnetic resonance (AFMR) mode. The resonance fields of this excitation exhibit an isotropic behavior with $g_\mathrm{eff}=2.01$, which is close to the free-electron $g$-factor value and agrees with the absence of orbital momenta for the Mn$^{2+}$ ions. At low temperatures, the AFMR mode is remarkably sharp ($\sim50$ mT for the full width at the half-maximum). This mode exhibits a noticeable broadening with increasing temperature, indicating the enhanced effect of magnon-magnon interactions. Based on this behavior, we estimate the strength of these interactions.

Figures (5)

• Figure 1: (a) Schematic view of the crystal structure of $\alpha$-MnTe. Big orange spheres represent Mn$^{2+}$ magnetic ions, small gray spheres Te$^{2-}$ ions. Cyan, orange, and blue lines indicate the leading exchange interactions $J_{c}$, $J_{a}$, and $J_{ac}$, correspondingly (see text for more details). Spin directions, according to the neutron diffraction data KriegnerReichlova_PRB_2017_MnTeVarious, are shown by red and blue arrows for different sublattices, respectively. The unit cell is shown by thin black lines. (b) Arrangement of Mn ions and their magnetic moments in one of the $(0,~0,~1)$ layers. The direction of the external magnetic field in this plane is given by the angle $\varphi$ with respect to the $a$ direction.
• Figure 2: Main panel: Frequency-field diagram of $\alpha$-MnTe at $T=5$ K, $H\perp c$. Points (circles for $\varphi=0^\circ$ and squares for $\varphi=15^\circ$ in-plane magnetic field directions) are the resonance-line centers; the dashed line corresponds to $g=2.01$. Upper inset: Examples of transmittance at selected frequencies. Black lines correspond to $\varphi=15^\circ$ and blue lines to $\varphi=0^\circ$. Lower inset: Schematic frequency-field diagram of $\alpha$-MnTe in magnetic field, transverse to the anisotropy axis, according to Eq. (\ref{['EQ:fHtrans']}).
• Figure 3: Resonant fields at different magnetic-field directions, for $\nu=360$ GHz and $T=5$ K. The dashed line corresponds to $g=2.01$.
• Figure 4: Temperature dependence of ESR spectra at (a) $\nu=135$ GHz ($\varphi=0^{\circ}$) and (b) at $\nu=270$ GHz ($\varphi=15^{\circ}$). Solid black lines correspond to experimental data, normalized to a value at maximal field. Dashed red lines correspond to Lorentian-based fits (see text). The spectra are offset for clarity.
• Figure 5: Resonance linewidth, obtained from spectra taken at $\nu=135$ GHz (blue circles, $\varphi=0^\circ$), $\nu=270$ GHz (green squares, $\varphi=15^\circ$), and $\nu=360$ GHz (orange diamonds, $\varphi=0^\circ$) at different temperatures (normalized to $h\nu$). Solid lines and shading correspond to Eq. (\ref{['EQ:Bose0']}) with $\Gamma_{0}/k_\mathrm{B}=27(5)$ mK. The inset shows the linewidth as a function of temperature on a linear scale.