Thermodynamic properties in the approach to the quantum critical point of the spin-ladder material Na2Co2(C2O4)3(H2O)2

Slow Magnetic Relaxation in {[CoCxAPy)] 2.15 H2O}n MOF Built from Ladder-Structured 2D Layers with Dimeric SMM Rungs

We present the magnetic properties of the metal-organic framework {[CoCxAPy]·2.15 H2O}n (Cx = bis(carboxypropyl)tetramethyldisiloxane; APy = 4,4`-azopyridine) (1) that builds up from the stacking of 2D coordination polymers. The 2D-coordination polymer in the bc plane is formed by the adjacent bonding of [CoCxAPy] 1D two-leg ladders with Co dimer rungs, running parallel to the c-axis. The crystal packing of 2D layers shows the presence of infinite channels running along the c crystallographic axis, which accommodate the disordered solvate molecules. The Co(II) is six-coordinated in a distorted octahedral geometry, where the equatorial plane is occupied by four carboxylate oxygen atoms. Two nitrogen atoms from APy ligands are coordinated in apical positions. The single-ion magnetic anisotropy has been determined by low temperature EPR and magnetization measurements on an isostructural compound {[Zn0.8Co0.2CxAPy]·1.5 CH3OH}n (2). The results show that the Co(II) ion has orthorhombic anisotropy with the hard-axis direction in the C2V main axis, lying the easy axis in the distorted octahedron equatorial plane, as predicted by the ab initio calculations of the g-tensor. Magnetic and heat capacity properties at very low temperatures are rationalized within a S* = 1/2 magnetic dimer model with anisotropic antiferromagnetic interaction. The magnetic dimer exhibits slow relaxation of the magnetization (SMM) below 6 K in applied field, with a tlf ≈ 2 s direct process at low frequencies, and an Orbach process at higher frequencies with U/kB = 6.7 ± 0.5 K. This compound represents a singular SMM MOF built-up of Co-dimers with an anisotropic exchange interaction.

Coexistence of spin ordering on ladders and spin dimer formation in a new-structure-type compound Sr2Co3S2O3

We report on the syntheses and characterizations of single crystalline and polycrystalline Sr₂Co₃S₂O₃ with a novel crystal structure type. It contains Co-O 2-leg rectangular ladders and necklace ladders. The two ladders share common legs and construct a hybrid spin ladder. A rare meridional heteroleptic octahedral coordination is found for the Co²⁺ ions in the 2-leg ladder. Within the necklace ladders, the Co²⁺ ions are in trans-octahedral coordination. An antiferromagnetic order is observed at T_N ~ 267 K, while a broad maximum in magnetic susceptibility is found below T_N. This relatively high ordering temperature among Co-based ladder compounds is related to the highly anisotropic mer-coordination of the Co²⁺ ions. The trans-octahedrally coordinated Co²⁺ ions, on the other hand, corresponds to the possible short-range magnetic correlations through dimers with an effective . This results in a rare situation that spin ordering and spin dimers coexist down to 2 K.

Magnetic metal-organic frameworks

The purpose of this critical review is to give a representative and comprehensive overview of the arising developments in the field of magnetic metal-organic frameworks, in particular those containing cobalt(II). We examine the diversity of magnetic exchange interactions between nearest-neighbour moment carriers, covering from dimers to oligomers and discuss their implications in infinite chains, layers and networks, having a variety of topologies. We progress to the different forms of short-range magnetic ordering, giving rise to single-molecule-magnets and single-chain-magnets, to long-range ordering of two- and three-dimensional networks (323 references).