ESR studies of the Cr3+ pair in MgO

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Abstract
Next nearest neighbor Cr3+ pairs in MgO single crystals have been studied by ESR techniques. The spectra associated with the spin states S = 1 and S = 2 were observed. The appropriate spin Hamiltonian for the system is H = β H⋅g⋅S + J S1⋅S2 − j (S1⋅S2)2 + Ds{Sz2 − (1/3) S (S + 1) }, where the z axis is along 〈100〉. The axially symmetric D tensors were measured to be Ds = 0.360 cm−1 and Ds = 0.105 cm−1 for S = 1 and S = 2 states, respectively. From the study of intensity change of the resonance line as a function of temperature, the isotropic exchange coupling constant J = 83.3 cm−1 and j = 17.9 cm−1 were also determined. In addition, the temperature dependence of Ds was obtained and was attributed to thermal expansion of the crystal lattice.