Single-crystal studies and electronic structure investigation of the room-temperature semiconductor NaMnAs

Synthesis of single-crystalline NaMnAs is reported, and we confirm its high quality by x-ray photoemission spectra. Magnetometry results are consistent with previous findings of antiferromagnetic order. The electronic structure was further studied using optical transmittance and by theoretical modeling using local-density approximation (LDA) extended to LDA $+ U$ for the purpose of determining the Heisenberg model parameters. Optical transmittance measurements confirm theoretical predictions that NaMnAs is a semiconductor. The Néel temperature was determined from temperature dependence of susceptibility, in agreement with our Monte Carlo simulations.

Figure 1

Unit cell of NaMnAs with magnetic moment directions refined by Bronger et al. [34].

Figure 2

Optical microscope image of typical NaMnAs single crystal. The sample thickness on the order of $100\mu \mathrm{m}$. The $c$ axis is pointing perpendicular to the plane whereas $a$ axes are parallel with the long edges of the sample.

Figure 3

(a) Symmetric $2\theta /\omega$ scan of the (001) NaMnAs single crystals, and (b) $Q_z$ scan across the (205) and (206) directions, respectively. Dashed lines represent the calculated diffraction intensities using the crystal structure from Ref. [34]. In the calculation we assumed an ideal kinematically diffracting crystal lattice.

Figure 4

XPS of the Mn $2p$ core level. The fit (solid line) corresponds to seven Voigt profiles (P1–P7) assuming a ranged Shirley background. Parameters of peaks P1–P7 are are given in Table 1.

Figure 5

UPS data covering the VBM region measured with a photon energy $\hslash \omega =21.2$ eV. The position of the VBM at 0.19 eV is estimated by the leading edge method (see the text). In the inset the signal intensity around $E=0$ (analyzer Fermi edge) is shown in detail.

Figure 6

Susceptibility measurements (temperature-dependent magnetization at $B=1$ T). The shift of maximum in the inset, upon thermal cycling, is likely an artifact of the signal-enhancement method.

Figure 7

Magnetization isotherm of NaMnAs single crystals with the magnetic field oriented on the basal plane.

Figure 8

The inset: The spectral dependence of the optical transmission $T$. The main plot: The same data replotted as $-lnT\left(E\right)$ fits in the interval of 1.2–1.4 eV by a direct (red, dashed) or indirect band-gap model allow to estimate the band gap.

Figure 9

Total spin-up and Mn-projected DOS from LDA $+ U$ calculations with $U=0,3$, and 5 eV (magnetic moments on Mn sites are 3.77, 4.27, and 4.51 Bohr magneton, respectively). Note that spin-down DOS is the same (and the Mn1 and Mn2 projected DOS swaps).

Figure 10

Calculated Heisenberg exchange parameters $J_{ij}$ plotted as a function of distance between Mn sites $i$ and $j$. The arrow indicates $J_{ij}$ corresponding to the coupling between nearest neighbors belonging to different layers.

Figure 11

Normalized temperature-dependent average magnetic moment on one Mn sublattice (blue triangles) from Monte Carlo simulations (see the text). The fit (also described in text) allows to more easily read off the Néel temperature $T_N$ (to be compared with the peak position in Fig. 6).

Figure 12

Visualization of Heisenberg exchange parameters $J_{ij}$ in the direct space. The two strongest couplings are shown on the left (here, only Mn atoms are displayed) and the two strongest interlayer couplings are shown on the right. Note that among the latter, there is only one NN but four next-NNs. Energies are given in meV.