Paramagnetic resonance measurements 1 on pairs of Mn 2+ ions in MgO have indicated values for the isotropic Mn-Mn exchange interactions which are surprisingly large compared with those estimated from bulk measurements on MnO. We have therefore made a more detailed investigation of the pair spectrum. The results suggest that there is an appreciable biquadratic contribution to the exchange of the form -j(S a -S ) in addition to the usual bilinear term, j(S a «S 6 ). This not only helps to explain the discrepancy, but may also have significant effects on the magnetic properties of MnO.The measurements have been made on crystals of MgO containing about 1 at.% Mn using wavelengths 0.85 and 1.25 cm in the temperature range 4 to 200°K. Only nearest-neighbor pairs have been examined in the present experiments, i.e., Mn 2+ ions occupying lattice sites such as (0, 0, 0) and (1/2,1/2, 0). Following previous work, 1 ' 2 we first assume that the isotropic exchange is of the usual form, j(S a «S"), where S a = gb = 5/2 for Mn 2+ and J is positive (antiferromagnetic). This gives total spin states S = 0,1,2,3,4,5, and the interval rule describing the spacing between adjacent states iswhere S is the spin of the higher state. We have confirmed previous results 1 on the S = 1 state and have also identified all the expected transitions belonging to S = 2, 3, and 4 from the positions and anisotropies of the lines in the spectrum. (The spectrum from S = 5 is obscured by stronger absorption due to lower S states and due to isolated Mn 2+ ions.) These positions are found to be accurately predicted by anisotropic interaction parameters of the expected form. 2 Having identified the transitions in this way, the energy intervals predicted by Eq. (1) have been investigated by measuring, as a function of temperature, the intensity of transitions belonging to different total spin states both relative to each other and relative to standard comparison substances. The results are given in the first two columns of Table I. It can be seen that the expected interval rule [Eq. (1)] is not obeyed. Much better agreement with experiment would be obtained if the isotropic exchange were of the form 5C=J(S fl .S 6 ) 2 -j(S fl .sV, ex which gives a modified interval rule(2)

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