The effect of Al doping on the crystal structure and magnetocaloric behavior of Mn1.2Fe0.8P1−xGex compounds

We studied the influence of Al doping on the crystal structure and magnetocaloric properties of Mn1.2Fe0.8P1−xGexAly compounds prepared by spark plasma sintering, with x = 0.24, 0.25, 0.26 and y = 0, 0.004, 0.008, 0.010 or 0.12. These compounds have the same Fe2P-type structure (space group P6¯2m) as the undoped system, and we found that the addition of Al had significant effects on the hexagonal unit cell of the compound and its physical properties. As y increased, the lattice parameter a and cell volume increased, the lattice parameter c decreased, the thermal hysteresis (ΔThys) decreased, and the Curie temperature (Tc) became significantly higher than at y = 0. The variations of Tc and ΔThys correlated almost linearly with variation in the c/a ratio, indicating that Tc can be increased with Al doping. In addition, Al doping did not have any adverse effects on other magnetocaloric properties, such as the entropy change, adiabatic temperature change, and the temperature range of the coexistence of the paramagnetic and ferromagnetic phases. We also found that the magnetocaloric properties of Mn1.2Fe0.8P1−xGexAly compounds could be improved by homogenous annealing. The particular composition Mn1.2Fe0.8P0.74Ge0.26Al0.010 exhibited a Tc close to room temperature (292.2 K) with a hysteresis of ΔThys 2.1 K, making it a very promising material for magnetic refrigeration around room temperature.