Metamagnetic transitions in frustrated spin-ladders

Variational calculations of the magnetization curve at zero temperature are reported for two models of frustrated ladder spin systems with ferro-and antiferromagnetic interactions. The ground state of the models is either ferro-or antiferromagnetic depending on model parameters. The character of the transition from the ferro-to the antiferromagnetic state differs from that of the corresponding transition in the XXZ model and is characterized by the appearance of bound multimagnon states. The existence of these states is shown to result in magnetization jumps at certain external field values. The region of the phase diagram where such jumps occur was determined, and the corresponding critical field values were found.