  The MnAs compound shows a first-order transition at T$_C\approx42$ C, and a
second-order transition at T$_t\approx120$ 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
$-196$ up to $140$ C, using the $\gamma-\gamma$ perturbed angular correlations
method with the radioactive probe $^{77}$Br$\rightarrow^{77}$Se, produced at
the 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 1st 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 (L)APW+lo method (\textsc{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.

Materials Science