The Dy-III quintuple-decker complex DyCdCdDy and hetero quadruple-decker complex DyCdY were synthesized, and their magnetic properties were compared with those of the quadruple decker complex DyCdDy. Single-crystal X-ray analysis revealed that the coordination geometries around the Dy-III ions of DyCdCdDy, DyCdY, and DyCdDy were similar. dc (direct current) magnetic measurements indicated that DyCdCdDy had very weak ferromagnetic Dy-III-Dy-III interactions but DyCdY did not. From a comparison of the magnetic relaxation times (tau) of the three complexes, the tau values of DyCdCdDy and DyCdDy, which have weak Dy-III-Dy-III interactions, are longer than that of DyCdY. In other words, weak Dy-III-Dy-III interactions effectively suppress magnetic relaxation.