An Intensity Standard for Electron Paramagnetic Resonance Using Chromium-Doped Corundum (Al₂O₃: Cr³⁺)

We report on the preparation of a standard reference material (SRM), made of single-crystal chromium-doped corundum (synthetic ruby), for use as an intensity standard in electron paramagnetic resonance (EPR) experiments. The SRM can be used to measure, by comparison, the number of spins in an unknown test sample. We selected the above material for the standard because its magnetic resonance properties are well understood and because it is physically and chemically stable under common laboratory conditions. To prepare samples which gave satisfactory EPR signals, it was necessary to perform annealing and chemical etching after cutting. This removed strains and surface damage. After treatment, the Cr³⁺ resonance lines were sharper and the intensities were in good agreement with the theoretically predicted values. To aid in application, the theoretical resonance fields and line intensities were calculated and tabulated for arbitrary orientations of the sample, at several commonly used microwave frequencies. The concentration of Cr³⁺ in the samples was determined by measurement of the static susceptibility. A quantitative EPR intensity experiment, based on measuring the microwave power absorbed during resonance, gave the same concentration. This proves, that within experimental error the EPR experiment detects all the Cr³⁺ ions in the sample, which makes the ruby a useful SRM.