The relation between the microstructure and the magnetic properties of Fe₂P-type (Mn,Fe)₂(P,Si,B) based materials has been systematically investigated by changing the annealing temperature and time. X-ray diffraction, Scanning Electron Microscopy and Energy Dispersive X-ray Spectroscopy measurements show that the alloys contain the main Fe₂P-type phase and two impurity phases of (Fe,Mn)₅Si₃-type and Fe₂MnSi-type. Boron appears to facilitate the formation of the Fe₂P-type phase during the arc-melting progress. Upon increasing the annealing temperatures from 1123 to 1423 K, the Curie temperature (T_C) decreases from 302.0 to 270.5 K in the Mn_1.15Fe_0.85P_0.55Si_0.45 alloys and the magnetic-entropy change (ΔS_M) increases linearly with annealing temperature. For the Mn_1.15Fe_0.85P_0.52Si_0.45B_0.03 alloys annealed at 1423 K for different times, T_C decreases from 263.8 and 232.8 K with increasing annealing time and ΔS_M reaches a maximum value after annealing for 48 h. The differences in the annealing temperature and time influence the Si content in the Fe₂P-type phase of the alloys and determine T_C, the thermal hysteresis and the magneto-elastic transition.

The lattice dynamics in MnFe0.95Si0.50P0.50 were investigated experimentally using Fe57 nuclear inelastic scattering and inelastic x-ray scattering across the first-order magnetic transition which occurs close to room temperature. The lattice dynamics characterization was supported by a macroscopic magnetic characterization, an x-ray diffraction study, and a hyperfine interactions characterization using Mössbauer spectroscopy. The Fe specific and the x-ray generalized density of phonon states were obtained both in the ferromagnetic and in the paramagnetic state. A prominent shift, 2meV at 20meV, in the x-ray generalized density of phonon states across the first-order magnetic transition, that involves vibrations with essentially Fe character, is revealed corroborated by a change in the local environment quantified in the isomer shift and the quadrupole splitting. Above 35meV the vibrational modes are practically insensitive to the magnetic transition. The entropy change induced by a 1T magnetic field across the magnetic transition, ∼10J/K/kg, is only a fraction of the Fe vibrational entropy change, 62(21)J/K/kg.