Magnetic Sponge Phenomena Associated with Interchain Dipole–Dipole Interactions in a Series of Ferrimagnetic Chain Compounds Doped with Minor Diamagnetic Species

The donor/acceptor ionic chain (i.e., the D(+)A(-) chain) [Ru2(2-MeO-4-ClPhCO2)4(BTDA-TCNQ)]·2.5(benzene) (1; 2-MeO-4-ClPhCO2(-) = 2-methoxy-4-chlorobenzoate; BTDA-TCNQ = bis(1,2,5-thiadiazolo)tetracyanoquinodimethane) is a ferrimagnetic chain with S = 3/2 from [Ru2(II,III)](+) (i.e., D(+)) and S = 1/2 from BTDA-TCNQ(•-) (i.e., A(-)), with J ≈ -100 K, in which long-range antiferromagnetic ordering at TN = 11 K occurs because interchain antiferromagnetic interactions are critical. Compound 1 undergoes a reversible crystal-to-crystal structural transformation with the elimination/absorption of the crystallization solvent to form the dried compound [Ru2(2-MeO-4-ClPhCO2)4(BTDA-TCNQ)] (1'), which has a higher TN (14 K). This change is clearly caused by the shortening of the interchain distances because the exchange coupling parameter for the chain is the same in both 1 and 1'. The chain compounds in 1 can be doped with minor diamagnetic [Rh2(II,II)] species, [{(Ru2)(1-x)(Rh2)(x)(2-MeO-4-ClPhCO2)4}(BTDA-TCNQ)]·2.5(benzene) (x = 0.03 for Rh-3%; x = 0.05 for Rh-5%; x = 0.16 for Rh-16%), which shifts the TN to lower temperatures, the magnitude of the shift being dependent on the doping ratio x (TN = 5.9 K for Rh-3%, TN = 3.7 K for Rh-5%, and TN was not observed above 1.8 K for Rh-16%). Drying a doped compound increased its TN, as was found for 1': TN = 9.9 K for Rh-3%', TN = 9.2 K for Rh-5%', and TN was not observed above 1.8 K for Rh-16%'. TN had a linear relationship with the doping ratio x of the [Rh2] species in both the fresh and dried compounds. The TN linear relationship is associated with the magnitude of the effective magnetic dipole (i.e., the average correlation length) in the chains caused by the [Rh2] defects as well as naturally generated defects in the synthetic process and with the interchain distances affected by the crystal-to-crystal transformations. These results demonstrate that slightly modifying the short-range correlation lengths, which changes the magnetic dipole magnitudes, strongly affects the bulk antiferromagnetic transition, with key dipole-dipole interactions, in low-dimensional anisotropic systems.