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.

After almost 20 years of intensive research on magnetocaloric effects near room temperature, magnetic refrigeration with first-order magnetocaloric materials has come close to real-life applications. Many materials have been discussed as potential candidates to be used in multicaloric devices. However, phase transitions in ferroic materials are often hysteretic and a metric is needed to estimate the detrimental effects of this hysteresis. We propose the coefficient of refrigerant performance, which compares the net work in a reversible cycle with the positive work on the refrigerant, as a universal metric for ferroic materials. Here, we concentrate on examples from magnetocaloric materials and only consider one barocaloric experiment. This is mainly due to lack of data on electrocaloric materials. It appears that adjusting the field-induced transitions and the hysteresis effects can minimize the losses in first-order materials. This article is part of the themed issue 'Taking the temperature of phase transitions in cool materials'.