Multiple magnetic transitions and the magnetocaloric effect in Gd _1-xSm_xMn₂Ge₂ compounds

The magnetic and magnetocaloric properties of polycrystalline samples of Gd_1-xSm_xMn₂Ge₂ have been studied. All the compounds except GdMn₂Ge₂ show re-entrant ferromagnetic behavior. Multiple magnetic transitions observed in these compounds are explained on the basis of the temperature dependences of the exchange strengths of the rare earth and Mn sublattices. The magnetocaloric effect is found to be positive at the re-entrant ferromagnetic transition, whereas it is negative at the antiferro-ferromagnetic transition. The Sm compounds are found to show a negative magnetocaloric effect at temperatures below the re-entrant ferromagnetic transition temperature. In SmMn ₂Ge₂, the magnetic entropy change associated with the re-entrant transition is found to decrease with field, which may be attributed to the crystal-field effect as well as the increase in the intralayer antiferromagnetic component of the Mn sublattice. The maximum value of the isothermal magnetic entropy change in this series of compounds is found to decrease with increase in Sm concentration.