EPR of exchange-coupled pairs of Eu²⁺ in CaO and SrO

The EPR spectra of isolated Eu2+ ions have been studied in crystals of CaO and SrO. In SrO, there is no crystal field splitting of the 8S7/2 level of the Eu2+. The g values and hyperfine constants are the same in both crystals. Crystals of CaO and SrO containing 1%-2% Eu exhibit in addition a large number of weak lines at both lower and higher fields than the single ion spectra observed in more lightly doped crystals. These resonances arise from coupled pairs of Eu2+ ions, and their analysis provides direct information about the exchange interactions between the Eu2+ ions. The temperature dependence of these intensities shows that the nearest neighbor exchange interaction J nn is ferromagnetic and its magnitude is approximately 1°. In the [100] direction, some of the resonances due to next-nearest neighbor pairs have been identified. This exchange interaction Jnnn is very small, less than 0.1°. Its sign cannot be determined from the temperature dependence of the intensities of the resonances. These values for the exchange interactions are in agreement with those inferred from the magnetic properties of the europium chalcogenides. © 1964 The American Institute of Physics.