Square-shape magnetocaloric effect in Mn3GaC.

We have studied the antiferro-ferromagnetic first-order transition which occurs in the giant magnetocaloric compound Mn3GaC by heat capacity at zero field and at H = 5T. At H = 0 we have found transitions at T-t1 = 155.5 K and T-t2 = 165.2K in addition to the ferro-paramagnetic transition at T-C = 248 K. The first one has a large anomalous entropy Delta S/R = 0.36 and a strong field dependence. Direct measurements of the temperature change upon adiabatic application of magnetic field gave Delta T-S congruent to -5K, practically independent of the initial temperature and field, over a wide temperature range below T-t1. The results are explained in terms of the magnetic phase diagram. (c) 2004 Elsevier B.V. All rights reserved.