Magnetic Dimensional Crossover from Two- to Three-Dimensional Heisenberg Magnetism in a Cu–W Cyano-Bridged Bimetal Assembly

In this work, we synthesized a cyano-bridged Cu–W bimetal assembly, [CuII(pyrimidine)2]4­[CuII(H2O)2]2[WV(CN)8]4·4H2O (1), which has a monoclinic crystal structure (P21/n space group, a = 15.7365(3) Å, b = 21.1555(4) Å, c = 27.1871(5) Å, β = 91.8630(7)°, and Z = 4). In this compound, Cu and W sites form two-dimensional (2-D) layers along the ab plane, while the other Cu sites are bridged between the 2-D layers, constructing a three-dimensional (3-D) structure. The magnetic susceptibility measurement showed that ferromagnetic interaction operates in the magnetic spins of the present compound. The field-cooled-magnetization (FCM) curve indicates that the magnetization gradually increases in the temperature range of ca. 40–8 K, and the spontaneous magnetization appears at a Curie temperature of 8 K. To understand the anomalous magnetization increase in the temperature range of ca. 40–8 K, we measured the magnetic heat capacity (Cmag). The Cmag vs T plots have a broad peak around 18 K and a sharp peak at 8 K. Such a type of Cmag vs T plots indicates a dimensional crossover from a 2-D to a 3-D Heisenberg magnetic model. This is because 1 has a pseudo 2-D network structure; that is, the magnitude of the intralayer superexchange interaction is much larger than that of the interlayer superexchange interaction. Such a magnetic dimensional crossover is a rare and intriguing issue in the field of magnetic substances.