Thermal mixing between spin systems in ${\mathrm{Al}}_2$${\mathrm{O}}_3$ by a resonant rf electric field

Spin thermal mixing due to the irradiation of an rf electric field has been observed, for the first time, between a two-level system with Δm=±2 and a dipolar system of ${}_{}{}^{27}{}_{}{}^{}\mathrm{Al}$ nuclei in ${\mathrm{Al}}_2$${\mathrm{O}}_3$. The thermal mixing is accomplished in a time much shorter than the spin-lattice relaxation time by applying strong rf electric field with amplitude of about 22 kV/cm at room temperature. Experimental results are sufficiently interpreted by the theory developed by assuming a concept of spin temperature in the rotating frame. A technique of double-quantum adiabatic demagnetization in the rotating frame is used in the experiments.