Room-temperature antiferromagnetic resonance in NaMnAs

Abstract

We report on antiferromagnetic resonance experiments in bulk tetragonal NaMnAs -- a room-temperature antiferromagnetic semiconductor. Our results corroborate previous ab initio studies, which propose that NaMnAs is an easy-axis antiferromagnet with the Néel vector oriented along the tetragonal axis. At $ B = 0 $, we find a single antiferromagnetic resonance line at 7 meV and associate it with a doubly degenerate ($ k = 0 $) magnon mode. Its energy softens considerably with increasing $ T $, but remains clearly visible in the data up to room temperature. From the experimental data, we estimate the single-ion anisotropy of the Mn ions in NaMnAs to be $ D \approx 0.2 $ meV, a value that is relatively large compared to other manganese-based antiferromagnets.

Figures (9)

• Figure 1: Schematic spin structure of NaMnAs magnetic crystal cell with only the magnetic Mn2+ ions visible. The spins of Mn2+ are aligned parallel to the tetragonal axis (c axis). The colored lines represent nearest neighbor exchange interactions of different orders.Jn indicates the nearest neighbor couplings, with $n$ being the order of the coupling.
• Figure 2: Low-temperature (T=4.2 K) transmission spectrum of a bulk NaMnAs crystal, several hundred microns thick, in the THz-infrared spectral range. The wave vector is aligned with the tetragonal axis in (a) and tilted by 35$^{\circ}$ in (b). Vertical dashed lines show positions of IR active phonons.
• Figure 3: Infrared magneto-transmission of NaMnAs bulk crystal measured with the magnetic field applied along the tetragonal axis ($B_\|$) shown as a stack plot and false-color plot in (a) and (b), respectively. The data were collected using a superconducting coil (up to 16T) and a resistive coil (from 16T to 30T). Each spectrum is normalized by the average of spectra over the whole range of $B_\|$ scanned. Panel (c) shows extracted positions of both AFMR branches, compared to theoretical expectations based on the Kittel's formula (\ref{['eq:faraday_afmr']}) for $g = 1.99$ (solid lines). The size of circles represents the error bar.
• Figure 4: Panel (a): Color-plot of infrared magneto-transmission taken on a NaMnAs bulk crystal in the Voigt configuration, with $B$ applied perpendicular to the tetragonal axis ($B_\perp$). Each spectrum is normalized by the average of spectra over the scanned magnetic field range (up to 16T). The field-dependence \ref{['eq:voigt_afmr']} is shown as solid white line. The white dashed line shows the magnon energy at zero field and also the expected lower branch magnon energy. Panel (b): Extracted minima of the magneto-transmission along with the expected dependence of the magnon mode following Eq. \ref{['eq:voigt_afmr']} for $g=2$.
• Figure 5: THz magneto-transmission of bulk NaMnAs, measured in the Faraday configuration at temperatures $T=5$, 50, 100, 150, 200, 250 and 295 K in panels (a-g), respectively. The magnetic field was applied along the tetragonal axis of the material. To plot the data, each individual spectrum collected at the magnetic field $B$ was normalized by the spectrum averaged over the interval $B\pm\delta B$ for $\delta B=5$ T, or shorter, respecting the range of magnetic fields explored ($0\leq B\leq16$ T). Then, the derivative of spectra with respect to the energy (frequency) was performed. With increasing $T$ the characteristic AFMR feature monotonically redshifts, from 7.0 meV at 5 K down to 5.4 meV at room temperature. The AFMR energies at $B=0$ were plotted as a function of $T$ in Fig. \ref{['fig:namnas_points_temp']}, along with the corresponding effective $g$ factor, evaluated using Eq. \ref{['eq:faraday_afmr']}.