  The structural transitions, magnetic properties, and electronic structures of
Co(Fe)-doped MnNiSi compounds are investigated by x-ray powder diffraction
(XRD), differential scanning calorimetry (DSC), magnetic measurements, and
first-principles calculations. Results indicate that all samples undergo a
martensitic transition from the Ni2In-type parent phase to TiNiSi-type
orthorhombic phase at high temperatures. The substitution of Co(Fe) for Mn in
Mn1-xCoxNiSi (x = 0.2, 0.3, 0.4) and Mn1-yFeyNiSi (y = 0.26, 0.30, 0.36, 0.46,
0.55) samples decreases the structural transition temperature (Tt) and Curie
temperature of martensite (TCM). The martensite phases show a typical
ferromagnetic behavior with saturation field (HS) being basically unchanged
with increasing Co(Fe) content, while the saturation magnetization (MS) shows a
decreasing tendency. The theoretically calculated moments are in good agreement
with the experimentally measured results. The orbital hybridizations between
different 3d elements are analyzed from the distribution of density of states.
