Magnetic properties of chromium ions in oxide matrices. Part 1.—Cr₂O₃-Al₂O₃ solid solutions

Magnetic susceptibilities (77–1000 K) and e.p.r. spectra ( 100–573 K) have been measured for α-Cr₂O₃ and ten corundum-phase solid solutions α-Cr_xAl_2–xO₃ with x varying from 0.002 to 1.6. The Racah parameter B has been obtained from optical reflectance spectra. Solutions dilute in chromium obey the Curie-Weiss law and effective magnetic moments decrease from µ_eff= 3.9 µ_B at x= 0.002 to µ_eff= 3.5 µ_B at x= 0.18. Above x= 0.2 the susceptibility-temperature variation reveals long-range antiferromagnetic interactions, which remain detectable up to 700 K. E.p.r. spectra show that the ruby spectrum is replaced by the broad β-phase resonance in the range x= 0.02 to x= 0.20, and the variation of the β-phase line width with concentration (0.1 < x < 1.6) has a minimum at x= 0.8. These results and the change in the number of spins with temperature are correlated with the susceptibility and optical measurements. As x increases, exchange coupling is first felt as an influence of pair-wise M—M interaction of Cr³⁺ ions, reducing the magnetic moment, but above x= 0.20 coupling of the M—O—M superexchange type becomes important and eventually dominates the magnetic behaviour. When x reaches 1.6, the susceptibility is characteristic of an antiferromagnet with T_N= 300 K and properties are similar to those of α-Cr₂O₃.