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 T_N = 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-ClPhCO ₂)₄(BTDA-TCNQ)] (1′), which has a higher T _N (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, [{(Ru ₂)_1-x(Rh₂)_x(2-MeO-4-ClPhCO ₂)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 T_N to lower temperatures, the magnitude of the shift being dependent on the doping ratio x (T_N = 5.9 K for Rh-3%, T_N = 3.7 K for Rh-5%, and T_N was not observed above 1.8 K for Rh-16%). Drying a doped compound increased its T_N, as was found for 1′: T_N = 9.9 K for Rh-3%′, T_N = 9.2 K for Rh-5%′, and T_N was not observed above 1.8 K for Rh-16%′. T_N had a linear relationship with the doping ratio x of the [Rh₂] species in both the fresh and dried compounds. The T_N 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.