1. E.L. Nagaev, Magnets with Complex Exchange Interactions Nauka, Moskow 1988 (in Russian).
2. V.V. Val’kov, G.N. Matsuleva, S.G. Ovchinnikov, Fiz. Tverd. Tela (Leningrad) 31 (1989) 60 [Sov. Phys. Solid State 31 (1989) 948].
3. V.M. Loktev, V.S. Ostrovskii, Fiz. Nizk. Temp. 20 (1994) 983 [Low Temp. Phys. 20 (1994) 775].
4. Quadrupole Phase Transitions in Magnetic Solids