Cross-relaxation to fast relaxing ions in alumina

Abstract

Measurements of the spin-lattice relaxations times of Cr³⁺ and Fe³⁺ ions in Al₂O₃ are reported for crystals in which Fe²⁺ ions are also known to be present. Marked cross-relaxation to Fe²⁺ is observed when the Fe²⁺ level splitting equals that of Cr³⁺ or Fe³⁺ and a quadrupole-electric-field interaction Hamiltonian is used to explain the results. Other cross-relaxation mechanisms are discussed. It is also shown that an analysis of the temperature dependence of the measured relaxation time and its minimum gives a value for the zero-field splitting of Fe²⁺, even though the Fe²⁺ concentration is very small.