Magnetic investigations of monocrystalline [Co(NCS)₂(L)₂]: new insights into single-chain relaxations

A large single crystal of a compound from the family of coordination polymer [Co(NCS)2(L)2]n chains was synthesized and its magnetic properties are reported. [Co(NCS)2(4-(3-phenylpropyl)pyridine)2]n is ferromagnetic with Tc = 3.39 K. Single-ion ab initio calculations predict an almost Ising-type magnetic anisotropy and the direction of the magnetic easy-axis nearly along the Co-Npy bond of the apical pyridine-based co-ligand. Both predictions are confirmed by single-crystal magnetic measurements. The magnetic relaxation of the single crystal sample significantly differs from the powder sample data, and clearly shows the presence of two separate relaxation processes. The process dominant below 3.2 K demonstrates a single chain magnet (SCM) behaviour, with a crossover between single-wall and two-wall processes, in spite of the fact that the system is ferromagnetically ordered. The faster process that dominates just below Tc is attributed to spin waves. Micromagnetic Monte Carlo simulations of the investigated compound show that the dipolar field cancels for some chains located at the border between 3-dimensional domains. Such chains are responsible for the measured ac signal, and demonstrate the SCM behaviour. The quantitative analysis of the SCM relaxation time is supported by preparing and examining a corresponding diamagnetically diluted compound, [CoxCd1-x(NCS)2(4-(3-phenylpropyl)pyridine)2]n (x = 0.013), which behaves as a field-induced single-ion magnet. The relaxation pathways for single Co(ii) spins are determined to be Raman, direct, and quantum tunneling processes, which were included in an improved approach to describe the magnetic relaxation in the Co(ii)-based SCM compound.