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 [Ru2II,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 [Rh2II,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.