Field-induced first-order antiferromagnetic-ferromagnetic transitions in RMn2Ge2 compounds and their relation to the magnetostriction of the Mn sublattice

A model for field-induced first-order antiferromagnetic-ferromagnetic trans itions is presented. The model is based on a free-energy expression includi ng Mn-Mn and R-Mn exchange interactions, as well as harmonic and anharmonic contributions to the lattice deformation energy. The Mn-Mn interaction is assumed to be linearly dependent on the unit-cell dimensions, giving rise t o magnetostrictive phenomena. The magnetostriction appears to be crucial fo r the occurrence of the first-order antiferromagnetic-ferromagnetic transit ions. From the model, an expression for the critical field B-c, correspondi ng to the first-order transition, has been obtained and used for a descript ion of the temperature dependence of B-c. The model is tested experimentall y by magnetization measurements on several Sm1-yRyMn2Ge2 compounds and appe ars to give a good description of the measured B-c vs T relations. Implicat ions of the present work for magnetoresistance in bulk compounds and thin f ilms are briefly discussed. [S0163-1829(99)13113-8].