EVALUATION BY EPR OF THE EXCHANGE INTERACTIONS COUPLING ANISOTROPIC SPINS AT SYMMETRY-RELATED SITES IN PARAMAGNETIC CRYSTALS

The EPR spectra of paramagnetic single crystals having two or more anisotropic ions per unit cell with effective spin S = 1/2, located at symmetry related but magnetically nonequivalent positions, and coupled by exchange interactions are analyzed. The spectra collapse in the presence of the exchange, and EPR data can provide information on the spectral differences and on the coupling between those magnetic ions, A method that allows evaluation of exchange interaction parameters between nonequivalent ions from single-crystal EPR spectra, measured as a function of microwave frequency and magnetic field orientation, is discussed. This method can be very selective, allowing an evaluation of effective exchange couplings between different sets of nonequivalent ions within the range of 0.01 K less than or equal to \J\/k less than or equal to 1 K, from EPR data taken at room temperature. (C) 1995 Academic Press, Inc.