Hyperfine interactions in MnAs studied by perturbed angular correlations of $\gamma$-rays using the probe Br${}^{77}\to$ Se${}^{77}$ and first-principles calculations for MnAs and other Mn pnictides

The MnAs compound shows a first-order transition at $T_C\approx 42\hspace{0.5em}{}^{°}$C, and a second-order transition at $T_t\approx 120\hspace{0.5em}{}^{°}$C. The first-order transition, with structural (hexagonal-orthorhombic), magnetic (FM-PM), and electrical conductivity changes is associated to magnetocaloric, magnetoelastic, and magnetoresistance effects. We report a study in a large temperature range from $-196\hspace{0.5em}{}^{°}$ up to $140\hspace{0.5em}{}^{°}$C, using the $\gamma -\gamma$ perturbed angular correlations method with the radioactive probe ${}^{77}\text{Br}\to {}^{77}$Se, produced at the On-Line Isotope Mass Separator (ISOLDE)-CERN facility. The electric field gradients and magnetic hyperfine fields are determined across the first- and second-order phase transitions encompassing the pure and mixed phase regimes in cooling and heating cycles. The temperature irreversibility of the first-order phase transition is seen locally at the nanoscopic scale sensitivity of the hyperfine field, by its hysteresis, detailing and complementing information obtained with macroscopic measurements (magnetization and X-ray powder diffraction). To interpret the results hyperfine parameters were obtained with first-principles spin-polarized density functional calculations using the generalized gradient approximation with the full potential (linear) augmented plane wave plus local orbitals method (wien2k code) by considering the Se probe at both Mn and As sites. A clear assignment of the probe location at the As site is made and complemented with the calculated densities of states and local magnetic moments. We model electronic and magnetic properties of the chemically similar MnSb and MnBi compounds, complementing previous calculations.

Figure 1

Diagram of the $\gamma -\gamma$ decay cascade of ${}^{77}\mathrm{Se}$, with the properties of the relevant intermediate isotope and of the decay from the parent isotope ${}^{77}\mathrm{Br}$ by processes of electron capture and positron emission.

Figure 2

PAC spectra and Fourier transforms of the first five measurements in chronological order. The fits are represented by the red lines.

Figure 3

PAC spectra and Fourier transforms of the last five measurements in chronological order. Between $21.2^{°}$ and $25\hspace{0.5em}{}^{°}$C the sample was heated to $100\hspace{0.5em}{}^{°}$C, so that the $35\hspace{0.5em}{}^{°}$C measurement is made on cooling. The fits are represented by the red lines.

Figure 4

PAC spectra and Fourier transforms. Measurements made when heating the sample. The fits are representedby the red lines.

Figure 5

PAC spectra and Fourier transforms. Measurements made when cooling the sample. The fits are represented by thered lines.

Figure 6

PAC spectra and Fourier transforms at $-196\hspace{0.5em}{}^{°}$C. The fits are represented by the red lines.

Figure 7

Temperature dependence of the magnetization, measured at $B=0.01$ T, signaling the strong thermal hysteresis at the first-order transition.

Figure 8

Hyperfine field of the main fraction vs. temperatures, excluding the value at $-196\hspace{0.5em}{}^{°}$C. Circles for measurements when heating, squares for cooling. The open symbols show the results of the first experiment, for comparison. The lines are the first (when heating) and last (when cooling) measurements at the orthorhombic phase.

Figure 9

XRD fractions of the total area corresponding to the peaks of the hexagonal phase, at three $2\theta$ intervals.

Figure 10

Total DOS for MnAs, MnSb, and MnBi in units of $\text{states}/(\text{eV}f.u.)$. The up states are represented by the red solid line and the down states by the blue dashed line. Energy is in eV, relative to the Fermi energy (dashed line).

Figure 11

Density of states of MnAs in the hexagonal phase divided in the most important contributions for the majority (upper figure) and minority (lower figure) states.