Complex long-range magnetic ordering behaviors in anisotropic cobalt(II)-azide multilayer systems

The crystal structures and magnetic properties of two new Co^II molecular magnets, [Co(N₃)₂(btzb)] (1) and [Co(N ₃)₂(btze)₂] (2), are described and discussed (btzb = 1,4-bis(tetrazol-l-yl)butane and btze = 1,4-bis(tetrazol-l-yl)ethane). In the materials, (4,4) layers with μ1,3-azide bridges are cross-linked by the monolayered btzb bridging ligands or spaced by bilayered btze terminal ligands to give a 3D (1) or 2D (2) coordination network with significantly different interlayer separations (10.6 vs. 15.2 Å). The observation that the layers in 1 and 2 are almost identical have not only allowed us to determine how the interlayer separation imposes its influences on their magnetic behavior, but also helps us understand the complex magnetic behavior of each structure. In the high-temperature range (> 25 K), almost identical magnetic behaviors, typical of 2D antiferromagnetic systems, are observed for 1 and 2. At low temperature they exhibit unusual and different behaviors that combine spin canting (weak ferromagnetism), metamagnetism, and stepped hysteresis. It has been found that the interlayer separation has little influence on the ordering temperature (23 vs. 22 K), but imposes very-strong influence on the metamagnetic critical field (6500 vs. 450 Oe), the coercivity (7500 vs. 650 Oe), and the hysteresisstep size. It may also play an adjusting role in determining the canting angle. Taking into account the strong anisotropy of the systems and the interlayer dipolar interactions, we have reasonably interpreted the unusual metamagnetic and hysteresis behaviors and the differences between 1 and 2. In particularly, the stepped hysteresis loops have been explained by two weak ferromagnetic states.