Magnetism of spinel microcrystals in a Cr3+ -doped cordierite glass : an E. S. R. study

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Abstract
We have used Electron Spin Resonance to characterize the Cr3+ clusters which grow, during various heat treatments, in a 0.8 % Cr2 O3-doped cordierite glass. We found useful to compare the magnetic properties of these clusters to those of powdered MgCr2O 4. In MgCr2O4 the room-temperature E.S.R. spectrum consists of a single exchange-narrowed dipolar line. An approximate value of the exchange field between the two sublattices at T = 0 K is He ∼ 2 × 106 G, explaining the disappearance of the E.S.R. signal below TN = 15 K. Room-temperature E.S.R. results confirm that the Cr3+ clusters built up during a 100 H-825 °C or a 6 H-850 °C heating have a MgCr2xAl2(1-x) O4 composition, and precise the x value (x ∼ 0.9 ) . At low temperature these clusters are antiferromagnetic fine grains, with a high anisotropy field (Ha ≳ 10 kG). After a 4 h-875 °C + 2 h-900 °C heating, the Cr3+ concentration in the clusters is smaller, whereas after 2 h-875 °C + 10 min-1 050 °C heating, the Cr3+ ions are randomly distributed in MgAl2O4