Paramagnetic Ce3+ optical emitters in garnets: Optically detected magnetic resonance study and evidence of Gd-Ce cross-relaxation effects

Paramagnetic ${\mathrm{Ce}}³⁺$optical emitters have been studied by means of optically detected magnetic resonance (ODMR) via ${\mathrm{Ce}}³⁺$ spin-dependent emission in cerium-doped garnet crystals which were both gadolinium free and contain gadolinium in a concentration from the lowest (0.1%) to 100%, i.e., to the superparamagnetic state. It has been shown that the intensity of photoluminescence excited by circularly polarized light into ${\mathrm{Ce}}³⁺$ absorption bands can be used for selective monitoring the population of the ${\mathrm{Ce}}³⁺$ ground-state spin sublevels. Direct evidence of the cross-relaxation effects in garnet crystals containing two electron spin systems, i.e., the simplest one of ${\mathrm{Ce}}³⁺$ ions with the effective spin $S=\frac{1}{2}$ and the system of ${\mathrm{Gd}}³⁺$ ions with the maximum spin $S=\frac{7}{2}$, has been demonstrated. Magnetic resonance of ${\mathrm{Gd}}³⁺$ has been found by monitoring ${\mathrm{Ce}}³⁺$ emission in cerium-doped garnet crystals with gadolinium concentrations of 0.1 at. %, 4%--8%, and 100%, which implies the impact of the ${\mathrm{Gd}}³⁺$ spin polarization on the optical properties of ${\mathrm{Ce}}³⁺$. Strong internal magnetic fields in superparamagnetic crystals were shown to modify the processes of recombination between UV-radiation-induced electron and hole centers that lead to the recombination-induced ${\mathrm{Ce}}³⁺$ emission. Observation of spikes and subsequent decay in the cross-relaxation-induced ODMR signals under pulsed microwave excitation is suggested to be an informative method to investigate transient processes in the many-spin system of ${\mathrm{Ce}}³⁺, {\mathrm{Gd}}³⁺$, and electron and hole radiation-induced centers.