Slow magnetic relaxation in lanthanide ladder type coordination polymers

A series of four isostructural lanthanide-containing one dimensional coordination polymers with picolinic (pic) and glutaric (glu) acids, Ln(glu)(pic)(H2O)(2), where Ln = Gd-III (1), Tb-III (2), Dy-III (3) and Er-III (4) were synthesized under hydrothermal conditions and structurally characterized by powder and single-crystal X-ray diffraction, elemental analysis, IR spectroscopy and thermogravimetric measurements. These compounds are isostructural to the previously reported Sm(glu)(pic)(H2O)(2) and the structure consists in pairs of lanthanide ions double bridged by carboxylic groups which are connected along c by the glutaric acid ligands as a ladder type 1D coordination polymer. The magnetic properties of these compounds were studied by static magnetization and AC magnetic susceptibility measurements in the temperature range 1.7-300 K. Weak dominant ferromagnetic interactions between lanthanide cations were found in the compounds, except in compound 3. Compounds 1, 3 and 4 reveal frequency dependent AC susceptibility and slow relaxation of the magnetization under applied external static field. These data classify the Gd-III, Dy-III and Er-III compounds as molecular magnets. In spite of the ladder chain structure of the compounds this magnetic behaviour is not due to a single chain but instead it is ascribed to single ion anisotropic effects in the case of 3 and 4 and possibly to a phonon-bottleneck effect of the spin-lattice relaxation in 1.