Selected Examples of Spectra of Pairs

Abstract

Very detailed EPR spectra have been recorded for manganese (II) pairs in MgO and CaO. Manganese(II) enters substitutionally the host crystals: at concentrations of about 1 % appreciable numbers of isolated pairs are found which were characterized at three frequencies (15.5, 24.5, and 35.5 GHz) over the temperature range 1.2 to 300 K [7.1].A typical spectrum of Mn2+: CaO at 35.5 GHz is shown in Fig. 7.1. Signals are observed from all the five total spin states: it is apparent that on lowering temperature the low spin states increase the intensity relative to the high spin states. The signals shown in Fig. 7.1 are only part of the spectrum, the scheme of which is given in Fig. 7.2. The temperature dependence of the signal intensity of Mn:MgO was found to conform to the curves of Fig. 3.3, and the order of the levels is shown in Fig. 7.3. The levels deviate considerably from a Lande interval rule (Table 7.1), a result which was explained by including biquadratic terms. The levels on the left in Fig. 7.3 are calculated only with bilinear exchange, while those on the right, which correspond to a good approximation of the experimental data, are obtained including a biquadratic exchange constant j, such that j/J =0.05. The effect of biquadratic exchange on the energy separations is quite dramatic, according to the relation:The biquadratic term may have its origin from both an intrinsic higher order exchange and by exchange striction as discussed in Section 2.4. Beyond the departure from Lande interval rule biquadratic exchange has been considered to be responsible also for the departure of the anisotropic exchange parameters from the simple behavior expected using Eq. (3.21).