Ferrimagnetic and metamagnetic layered cobalt (II)–hydroxides: first observation of a coercive field greater than 5 T

The synthesis, characterization by XRD, UV-vis, IR and TGA and the magnetic properties of four layered compounds, namely1 Co5(OH)8(H2O)2 (NO3)2;2 Co5(OH)8(O2 CC6H4CO2) · 2H2O;3 Co2 (OH)3(NO3);4 Co4(OH)2(O2 CC6H4CO2)3 · (NH3)1.5 (H2O)2.5,are reported. 1 and 2 are characterized by triple–deck layers consisting of both octahedral and tetrahedral cobaltous ions and 3 and 4 contain single–deck layers of only octahedral cobaltous ions. 1 and 2 behave as ferrimagnets which are characterized by a minimum in the χT versus T plot, spontaneous magnetization, imaginary AC–susceptibility and hysteresis loop. The Curie temperatures are 30 and 40 K and the coercive fields at 4.2 K are 850 and 1750 Oe, respectively. 3 and 4 behave as metamagnets which are characterized by maxima in the susceptibility at the Néel temperature of 10 and 38 K, respectively, and by a critical field (antiferromagnetic ↔ paramagnetic) of ca. 1 kOe for 3 at 4.5 K and greater than 50 kOe for 4 at 2 K. The tricritical temperature, separating the region of reversible and non–reversible M versus H, for 4 is established at 22.5 K. The long–range magnetic ordering for the two structural types is discussed on the basis of dipolar interactions between layers. The intralayer interactions are ferromagnetic in all cases whereas the interlayer interactions are ferromagnetic for 1 and 2 and antiferromagnetic for 3 and 4. The results indicate that the Curie or Néel temperature is weakly dependent on the interlayer distance and its observation does not depend on the existence of covalent bonds between the layers. The large coercive fields observed are due to the alignment of the moments perpendicular to the layers and the synergy between crystalline shape and single–ion anisotropies.