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 fe rro- or antiferromagnetic depending on model parameters. The character of t he transition from the ferro- to the antiferromagnetic state differs from t hat of the corresponding transition in the XXZ model and is characterized b y 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, an d the corresponding critical field values were found. (C) 2001 MAIK "Nauka/ Interperiodica".