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 transitions is presented. The model is based on a free-energy expression including 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 to magnetostrictive phenomena. The magnetostriction appears to be crucial for the occurrence of the first-order antiferromagnetic-ferromagnetic transitions. From the model, an expression for the critical field B-c, corresponding to the first-order transition, has been obtained and used for a description of the temperature dependence of B-c. The model is tested experimentally by magnetization measurements on several Sm1-yRyMn2Ge2 compounds and appears to give a good description of the measured B-c vs T relations. Implications of the present work for magnetoresistance in bulk compounds and thin films are briefly discussed. [S0163-1829(99)13113-8].