Recently, several studies on the slow magnetic relaxation of Gd complexes have been reported. Although the decisive relaxation mechanism for Gd complex has not been elucidated yet, it has been assumed that magnetic relaxations of Gd complexes are driven by various kinds of relaxation mechanisms: phonon-bottleneck process, Orbach process, direct process, and Raman process. Moreover, magnetic relaxations are affected by magnetic dilution. Increasing the concentration of Gd ion increases the internal magnetic field, enhancing the magnetic relaxations. On the other hand, high Gd concentration prohibits rapid energy exchange and enhances the phonon-bottleneck effect because phonon modes available for spinlattice relaxations are saturated.