Non-Heisenberg couplings and ferromagnetic instability in a random antiferromagnetic spin-1 chain

A random antiferromagnetic spin-1 chain with both Heisenberg and biquadrati c nearest neighbor couplings is found to exhibit a novel ferromagnetic-like instability at low energies, if there is competition between the Heisenber g and biquadratic couplings, even though each individual nearest neighbor c oupling is such as to produce a singlet ground state for the isolated pair. This instability leads to the generation of ferromagnetic couplings, and t hus to the formation of weakly coupled large moments at low temperatures. A perturbative calculation (up to 4th order) is performed to investigate whe ther such a situation is physically realizable in spin-1 systems. (C) 1998 American Institute of Physics. [S0021-8979(98)25111-5].