An unusual field-dependent slow magnetic relaxation and two distinct transitions were observed in a family of rare-earth–transition-metal complexes [Ln (bipy) $\left(H_{2}O{)}_{4}M\right(\mathrm{CN}{)}_6]\cdot 1.5$ (bipy) $\cdot 4H_{2}O$ $(bipy={2,2}^{\prime }$-bipyridine; ${\mathrm{Ln}=\mathrm{Gd}}^{3+},Y^{3+};$ $M={\mathrm{Fe}}^{3+},$ ${\mathrm{Co}}^{3+}).$ The magnetic relaxation, which is quite different from those in normal spin glasses and superparamagnets but qualitatively resembles those in the single-molecule magnet ${\mathrm{Mn}}_{12}-\mathrm{Ac}$ 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.

Received 14 July 2000

DOI:https://doi.org/10.1103/PhysRevB.63.054431