An unconventional phenomena is observed at the first-order magnetic transitions in (Mn,Fe)₂(P,Si) materials. Here, we show that the first crossing of the transition upon cooling is associated with an abnormal temperature increase. While differential scanning calorimetry can detect this recalescence-like event, purposely-designed probes were employed to quantify it. Recalescence at a magnetic transition is extremely rare. But in (Mn,Fe)₂(P,Si), it is even more remarkable by its amplitude, with the temperature rising up to +4.0 K. In (Mn,Fe)₂(P,Si), this phenomenon is associated with irreversible burst-like evolution of the microstructure (increase in defect concentration and micro-cracking) and of the crystal structure.

The crystal structure, magnetic and magnetocaloric properties of (Mn,Fe,T)1.95P0.50Si0.50 (T = Co, Ni and Cu) compounds have been investigated. All the compounds crystallize in the Fe2P-type hexagonal structure. All these transition metal substitutions for either Mn(3g) or Fe(3f) weaken the ferromagnetic ordering, while showing complex effects on the energy barrier for nucleation in the first-order magnetic phase transition and result in complex behaviors in thermal/magnetic hysteresis. A first-principles density functional theory calculations show that Co, Ni and Cu atoms occupy 3f, 3f and 3g sites, respectively.

Single phase Mn_0.66Fe_1.29P_1-xSi_x (0 ≤ x ≤ 0.42) compounds were synthesized using the melt-spinning (rapid solidification) technique. All the compounds form in the Fe₂P-type hexagonal structure, except a Co₂P-type orthorhombic structure of the Si-free Mn_0.66Fe_1.29P compound. The compounds with 0.24 ≤ x ≤ 0.42 present a FM-PM phase transition, while the compounds with lower Si content show an AFM-PM phase transition. In the Mn_0.66Fe_1.29P_1-xSi_x compounds, T_C and ΔT_hys are not only Si content dependent, but also magnetic field dependent. By increasing the Si content from x = 0.24 to 0.42, T_C increases from 195 to 451 K and ΔT_hys is strongly reduced from ∼61 to ∼1 K. T_C increases and ΔT_hys decreases with increasing magnetic field, ΔT_C/ΔB is about 4.4 K/T. Mn_0.66Fe_1.29P_1-xSi_x compounds show large saturation magnetic moments with values up to 4.57 μ_B/f.u. A large MCE with a small thermal hysteresis is obtained simultaneously in Fe-rich Mn_0.66Fe_1.29P_1-xSi_x melt-spun ribbons.