Unconventional features of the low-temperature properties of a dimerized quantum mixed-spin chain

When the strength of the bonds between consecutive pairs in an alternating mixed-spin $(S,s)$ chain is different (but of the same antiferromagnetic nature), and there is no crystal-field anisotropy, the $\chi_M T$ product — $\chi_M$ being the molecular susceptibility per unit cell and $T$ the temperature — smoothly decreases upon cooling, reaches a minimum and then increases at low temperatures.

When the bimetallic chain $^1_\infty$[LCu$^{II}$Co$^{II}$(NCS)$_2$] was synthesized, an antiferromagnetic-ferromagnetic bond alternation was realized between the $(S,s)=(3/2,1/2)$ Co and Cu ions. Surprisingly, the $\chi_M T$ values decreased upon cooling, reaching a pseudo-plateau, and then rapidly decreasing (instead of increasing) at low temperatures. They gave two possible reasons for the decrease, being either due to the zero-field splitting caused by the crystal-field anisotropy on the Co(II) ion ground state; or the ferromagnetic nature of the dimer interactions.

I was able to clarify the picture and attribute the cause of the novel magnetic behavior to the ferromagnetic nature of the dimer interactions. They make the system behave effectively as a spin-1 chain with antiferromagnetic interactions, for which the $\chi_M T$ vs $T$ curve decreases at low temperature.