Degenerate hubbard model in a magnetic field. Application to Jahn‐Teller systems

The Hubbard model in the strong coupling limit is used to study the behaviour of substances containing ions with orbitally degenerate ground state (Jahn‐Teller ions) in a magnetic field. In such substances the orbital occupancy and the magnetic structure appear to be closely connected. It is shown that in these systems a transition of first order may occur between different anti‐ferromagnetic phases as well as jumplike spin‐flip transitions (even for an isotropic exchange interaction S i S j ). The field dependence of the magnetic moment may be nonlinear, typical of meta‐magnetic substances. Both a simple model problem and a specific case of pervoskites with Jahn‐Teller ions (such as KCuF 3 ) are studied. The behaviour of quasi one dimensional systems is also considered.