Electron spin resonance linewidths of Co2+ in magnesium oxide

Electron spin resonance linewidths of Co²⁺ in single-crystal MgO at 9 GHz were examined experimentally and theoretically for a range of Co²⁺ concentration from 310 to 9900 ppm. In contrast to the behaviour expected from dipolar broadening, the experimental peak-to-peak linewidth for the octed hyperfine lines were about twelve to fifty times less than the calculated dipolar linewidths according to temperature and concentrations. The peak-to-peak linewidths as determined experimentally increase from 0.8 mT at 310 ppm Co²⁺ to 4.0 mT at 9900 ppm Co²⁺ at a polar angle _H= O° and at 20 K. Peak-to-peak linewidths independent of polar angle but strongly dependent on temperature and slightly dependent on concentrations were observed. The ratio of moments M ₄ ^1/4 /M ₂ ^1/2 derived from the experimental data lay between 1.35 to 1.44 and the lineshapes were markedly Lorentzian in the range of temperature measured. The data suggest that Co²⁺ entered the lattice substitutionally, occupying magnesium sites, that the linewidths were determined, after exchange energy, over the whole concentration range examined and that the exchange energy whose values lay between 6 and 77 GHz, varied linearly with concentration.