Magnetic field effects in the spin dynamics of ferro- and antiferromagnetic Ising-type chains with s=12

We study the influence of a magnetic field on the eigenvalue spectrum and the excitation spectrum probed by dynamic form factors (DFF) in s=12 Ising-type ferro- and antifer-romagnetics. At zero temperature, we derive exact expressions for the DFF. On this basis and finite-chain results, the physical original of thermally induced central peaks in DFF is traced back to particular transitions between excited states. Because the energies of these states differ in their field dependence, we find a field-driven splitting of the thermally induced central peaks into distinct resonances, each associated with a specific transition between two excited states. The antiferromagnets CsCoCl3, CsCoBr3, and the ferromagnet CoCl22D2O are typical Ising-type systems where these novel magnetic field effects could be observed by neutron and light scattering techniques. © 1982 The American Physical Society.