  Based on the first principle calculation, we propose a new class of
high-$T_c$ half-heusler ferromagnets NiMnZ (Z = Si, P, Ge, As). The structural
and magnetic properties are investigated through the calculation of the
electronic structure, phase stability, equilibrium lattice constant, magnetic
exchange interaction $J_{ij}$ and Curie temperature $T_c$. It is found that all
alloys show half-metallicity and ferromagnetism at temperatures much higher
than room temperature in a wide range of lattice expansion (compression). At
the equilibrium lattice constant, $T_c$ of 715K, 840K, 875K and 1050K are
predicted by Monte Carlo simulation for NiMnP, NiMnAs, NiMnGe and NiMnSi,
respectively. Following these results, these alloys are strongly expected to be
promising candidates for spintronic applications.
