Effect of Carbon Doping on the Structure and Magnetic Phase Transition in (Mn,Fe₂(P,Si))

Given the potential applications of (Mn,Fe₂(P,Si))-based materials for room-temperature magnetic refrigeration, several research groups have carried out fundamental studies aimed at understanding the role of the magneto-elastic coupling in the first-order magnetic transition and further optimizing this system. Inspired by the beneficial effect of the addition of boron on the magnetocaloric effect of (Mn,Fe₂(P,Si))-based materials, we have investigated the effect of carbon (C) addition on the structural properties and the magnetic phase transition of Mn _1.25Fe _0.70P _0.50Si _0.50C _z and Mn _1.25Fe _0.70P _0.55Si _0.45C _z compounds by x-ray diffraction, neutron diffraction and magnetic measurements in order to find an additional control parameter to further optimize the performance of these materials. All samples crystallize in the hexagonal Fe ₂P -type structure (space group P-62m), suggesting that C doping does not affect the phase formation. It is found that the Curie temperature increases, while the thermal hysteresis and the isothermal magnetic entropy change decrease by adding carbon. Room-temperature neutron diffraction experiments on Mn _1.25Fe _0.70P _0.55Si _0.45C _z compounds reveal that the added C substitutes P/Si on the 2c site and/or occupies the 6k interstitial site of the hexagonal Fe ₂P -type structure.