Second-order effects on the EPR spectra of exchange-coupled pairs of paramagnetic ions

The paper gives a spin-hamiltonian formalism for a pair of exchange-coupled paramagnetic ions, operating within the single S-multiplet of total spin and in which coupling among the different multiplets is taken into account with a second-order perturbative approximation. Arbitrary values of the coupled spins S₁ and S₂ are considered and a tensorial form for the various contributions to the pair energy is assumed, without restrictions on principal values and directions. That allows us to point out some new EPR aspects related principally to the role of local and exchange anisotropies.