Pressure-induced changes in the magnetic and magnetocaloric properties of R Mn2 Ge2 (R=Sm,Gd)

We have studied the variation of magnetic and magnetocaloric properties of polycrystalline compounds SmMn2 Ge2 and GdMn2 Ge2 as a function of applied hydrostatic pressure. The magnetic transition temperatures are found to change considerably with pressure. The temperature regime of existence of antiferromagnetic (AFM) ordering is found to increase with pressure, in both the compounds. In SmMn2 Ge2, the sign of the magnetocaloric effect at the low-temperature ferromagnetic (FM)-AFM transition changes with pressure. The isothermal magnetic entropy change in this compound is found to increase by about 20 times as the pressure is increased from the ambient value to 6.8 kbar. Effect of pressure in GdMn2 Ge2 is less compared to that in SmMn2 Ge2. The variations in the magnetic and magnetocaloric properties are attributed to the changes in the magnetic state of the Mn sublattice under pressure. The difference in R-Mn coupling in Sm and Gd compounds is also found to play a role in determining the magnetic and magnetocaloric properties, both at ambient as well as under applied pressures.