Structural and Magnetic Studies of a New Co(II) Thiocyanato Coordination Polymer Showing Slow Magnetic Relaxations and a Metamagnetic Transition

Reaction
of Co­(NCS)2 with 4-ethylpyridine leads to the formation
of three new compounds of composition Co­(NCS)2(4-ethylpyridine)4 (1), [(Co­(NCS)2]2(4-ethylpyridine)6 (2), and [Co­(NCS)2(4-ethylpyridine)2]n (3). In all compounds
the coordination of the Co­(II) ions is distorted octahedral. 1 consists of discrete monomeric complexes and in 2 two Co­(II) cations are linked by pairs of μ-1,3-bridging thiocyanato
ligands into dimers. In the crystal structure of 3 the
Co­(II) cations are connected into chains by the same bridge as in 2. Magnetic studies show that 1 and 2 are paramagnets down to a temperature of 2 K, while compound 3, which is the main object of this study, is an antiferromagnet
with the Néel temperature TN =
3.4 K. Its magnetic structure is built from ferromagnetic chains,
which are weakly antiferromagnetically coupled. With increasing magnetic
field a metamagnetic transition starts at ∼175 Oe, as observed
for a polycrystalline sample. Magnetic relaxations, which were observed
in the antiferromagnetic state, are retained at the metamagnetic transition.
With decreasing field 3 remains in a state, in which
except of the faster magnetic relaxation process in single chains
also a slower process coexists resulting in the appearance of a magnetic
hysteresis loop.

硫氰酸钴(II)[Co(NCS)₂]与4-乙基吡啶（4-ethylpyridine）的反应可生成三种组成为Co(NCS)₂(4-乙基吡啶)₄（标记为1）、[(Co(NCS)₂)₂](4-乙基吡啶)₆（标记为2）以及[Co(NCS)₂(4-乙基吡啶)₂]ₙ（标记为3）的新型化合物。所有化合物中的Co(II)离子均采取畸变八面体配位构型。其中1为离散单核配合物；2中两个Co(II)阳离子通过成对的μ-1,3桥联硫氰酸根配体（μ-1,3-bridging thiocyanato ligands）连接形成二聚体。在化合物3的晶体结构中，Co(II)阳离子通过与2中相同的桥联配体连接形成链式结构。磁学研究表明，1和2在低至2 K的温度下仍表现为顺磁体；而本研究的核心对象3为奈耳温度（Néel temperature）T_N=3.4 K的反铁磁体。其磁结构由弱反铁磁耦合的铁磁链构成。随着外磁场强度升高，多晶样品观测到在约175奥斯特（Oe）处发生变磁转变（metamagnetic transition）。在反铁磁态中观测到的磁弛豫现象，在变磁转变过程中依然存在。当外磁场降低时，3仍维持特定磁态：此时除单链内较快的磁弛豫过程外，还共存有较慢的弛豫过程，进而呈现出磁滞回线（magnetic hysteresis loop）。