1. Sh. U. Abdullaev, U. V. Valiev, and B. Yu. Sokolov, Opt. Spektrosk. 87(3), 419 (1999) [Opt. Spectrosc. 87, 386 (1999)].
2. A. K. Zvezdin, V. M. Matveev, A. A. Mukhin, and A. I. Popov, Rare Earth Ions in Magnetic-Ordered Crystals (Moscow, 1985).
3. U. V. Valiev, T. Asilov, and R. A. Salyukov, Prib. Tekh. Éksp. 4, 87 (1994).
4. V. Nekvasil, Phys. Status Solidi B 94, K41 (1979).
5. E. L. Ivchenko, E. V. Maksimov, and V. N. Medvedev, Opt. Spektrosk. 47(6), 1096 (1979) [Opt. Spectrosc. 47, 609 (1979)].