Cross relaxation studies in the Cr3+-doped magnetically dilute paramagnetic Al2O3-Fe2O3 system

Moessbauer measurements are made in undoped and Cr3+-doped Al2O3-0.04 mol% 57Fe2O3 systems at 4.2 K. The cross relaxation between Fe3+ and Cr3+ ions, dispersed in the diamagnetic Al2O3 lattice, is studied. The spin-spin relaxation time of the Cr3+ ions is reduced with increasing Cr3+ concentration and these faster relaxing Cr3+ spins are found to enhance the Fe3+-relaxation process, destroying the paramagnetic hyperfine structure observed in the 57Fe Moessbauer spectra of undoped Al2O3-0.04 mol% 57Fe2O3. It is concluded that at higher Cr3+ concentrations, cross relaxation does not take place by the mutual energy exchange between a Cr3+ ion and an Fe3+ ion but a new kind of pair cross-relaxation process occurs where a Cr3+ ion and an Fe3+ ion first form an exchange coupled pair and then two such pairs cross relax. (author)