Lifetime for 5D4 → 7F5 emission in heavily Tb3+-doped superparamagnetic glasses

We have investigated a relationship between the Verdet constant and the lifetime of the 5D4 level in heavily Tb3+-doped superparamagnetic glasses. Around 40 K the lifetime for 5D4 → 7F5 emission showed its maximum value in the same way as that of the Verdet constant. The correspondence was attributed to the enlarged size of magnetic clusters which enhanced the diffusion of the excitation energy among Tb3+ ions. Also, we observed that the lifetime was most strongly dependent on the applied magnetic field at the temperature where the enlargement of clusters appeared.