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

Abstract We studied the influence of Al doping on the crystal structure and magnetocaloric properties of Mn 1.2 Fe 0.8 P 1 − x Ge x Al y 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 Fe 2 P-type structure (space group P 6 ¯ 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 (Δ T hys ) decreased, and the Curie temperature ( T c ) became significantly higher than at y  = 0. The variations of T c and Δ T hys correlated almost linearly with variation in the c / a ratio, indicating that T c 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 Mn 1.2 Fe 0.8 P 1 − x Ge x Al y compounds could be improved by homogenous annealing. The particular composition Mn 1.2 Fe 0.8 P 0.74 Ge 0.26 Al 0.010 exhibited a T c close to room temperature (292.2 K) with a hysteresis of Δ T hys 2.1 K, making it a very promising material for magnetic refrigeration around room temperature.