Quantum phase transitions in frustrated ferro- and ferrimagnets

We discuss the ground state of the one-dimensional frustrated quantum Heise nberg ferrimagnet. In the limit of weak frustration the ground state has ma croscopic total spin and exhibits magnetic long-range order. In the opposit e limit of strong frustration the ground state is a singlet (S = 0). We fin d indications that this transition to a singlet state is of first order. We compare the ferrimagnet with the corresponding ferromagnet. (C) 2000 Elsev ier Science B.V. All rights reserved.