  An unusual field dependent slow magnetic relaxation and two distinct
transitions were observed in a family of new rare earth-transition metal
complexes, [Ln (bipy) (H$_{2}$O)$_{4}$ M(CN)$_{6}$] $\cdot $1.5 (bipy) $ \cdot
$ 4H$_{2}$O (bipy = 2,2'-bipyridine; Ln = Gd$^{3+}$,Y$^{3+}$; M = Fe$ ^{3+}$,
Co$^{3+}$). The novel magnetic relaxation, which is quite different from those
in normal spin glasses and superparamagnets but very resembles qualitatively
those in single-molecule magnet Mn$_{12}$-Ac even if they possess different
structures, might be attributed to the presence of frustration that is
incrementally unveiled by the external magnetic field. The two distinct
transitions in [GdFe] were presumed from DC and AC susceptibility as well as
heat capacity measurements.
