Impact of F and S doping on (Mn,Fe)2(P,Si) giant magnetocaloric materials

The quarternary (Mn,Fe)2(P,Si)-based materials with a giant magnetocaloric effect (GMCE) at the ferromagnetic transition TC are promising bulk materials for solid-state magnetic refrigeration. In the present study we demonstrate that doping with the light elements fluorine and sulfur can be used to adjust TC near room temperature and tune the magnetocaloric properties. For F doping the first-order magnetic transition (FOMT) of Mn0.60Fe1.30P0.64Si0.36Fx (x = 0.00, 0.01, 0.02, 0.03) is enhanced, which is explained by an enhanced magnetoelastic coupling. The magnetic entropy change |ΔSm| at a field change (Δμ0H) of 2 T markedly improved by 30% from 14.2 Jkg−1K−1 (x = 0.00) at 335 K to 20.2 Jkg−1K−1 (x = 0.03) at 297 K. For the F doped material the value of |ΔSm| for Δμ0H = 1 T reaches 11.6 Jkg−1K−1 at 294 K, which is consistent with the calorimetric data (12.4 Jkg−1K−1). Neutron diffraction experiments reveal enhanced magnetic moments by F doping in agreement with the prediction of DFT calculation. For S doping in Mn0.60Fe1.25P0.66-ySi0.34Sy (y = 0.00, 0.01, 0.02, 0.03, 0.04) three impurity phases have been found from microstructural analysis, which reduce the stability of the FOMT in the main phase and decrease TC, e.g. the |ΔSm| reduces from 7.9(12.6) Jkg-1K-1 (332 K) for the undoped sample to 3.4(6.2) Jkg-1K-1 (313 K) for the maximum doped sample for Δμ0H = 1(2) T. Neutron diffraction experiments combined with first-principles theoretical calculation, distinguish the occupation of F/S dopants and the tuning mechanism for light element doping, corresponding to subtle structural changes and a strengthening of the covalent bonding between metal and metalloid atoms. It is found that the light elements F and S can effectively regulate the magnetocaloric properties and provide fundamental understanding of (Mn,Fe)2(P,Si)-based intermetallic compounds.