A study of magnetic phase transition in a two-dimensional lattice by heat capacity measurements

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).

Nuclear and Magnetic Structures and Magnetic Properties of the Layered Cobalt Hydroxysulfate Co5(OH)6(SO4)2(H2O)4 and Its Deuterated Analogue, Co5(OD)6(SO4)2(D2O)4

The structures (nuclear and magnetic), magnetic properties (2-300 K, 1-10(4) bar), and heat capacity of the layered ferromagnet Co5(OH)6(SO4)2(H2O)4 are reported. The crystal structure consists of brucite-like M(II)-OH layers of edge-sharing octahedra, but having two different Co sites, which are pillared by ...O3SO-Co(H2O)4-OSO3.... The absorption spectrum confirms the presence of divalent Co, and by comparison of the two isotopic materials, the assignment of the vibrational spectra is proposed. The magnetic properties are those of a ferromagnet with a Curie temperature of 14 K. Temperature and field dependence magnetization data taken on an aligned sample suggest an easy-plane magnet. The Curie temperature increases linearly with pressure at a rate of +0.12 K/kbar, suggesting small progressive and uniform modifications of the Co-Co exchange interactions. Rietveld refinement of the neutron powder diffraction data and consideration of a group analysis reveal the direction of the moments of the Co within the layer to be along the b-axis, with a maximum moment of 3.33 micro(B) per cobalt. Those of the pillars remain random. Estimation of the entropy from the heat capacity data accounts for the presence of four ordered moments of Co with spin 1/2 at the long-range ordering temperature, while the moment of the pillaring Co contributes only at lower temperature due to the increase of the internal field as the temperature is lowered. The purely 2D-magnetic ordering in an easy-plane magnet, evidenced by neutron diffraction and heat capacity, challenges the existing theories and is a rare example of a single-layer magnet.