Electron Spin Resonance Study of the Antiferromagnetism of Chromia Alumina

Chromia alumina mixtures calcined between 500° and 1400°C were studied by electron spin resonance (ESR) in the temperature range from 125° to 550°K. The low-chromia content chromic ions are paramagnetic while at high-chromia contents the ESR spectra exhibit antiferromagnetic behavior. For Cr/Al ratios greater than ⅔ the 1400°C calcination samples produce sharp Néel points which increase with chromia content, while the samples with Cr/Al ratios less than ½ show broad Néel points which become less well resolved with decreasing chromia content. The spectra of high Cr/Al ratio samples calcined at 900°C and below are superpositions of a βw phase resonance which appears paramagnetic and a βn phase resonance which has a fairly sharp Néel point several degrees below the bulk chromia value, and the latter is attributed to clumps of chromia in the sample.