Hard Magnets Based on Layered Cobalt Hydroxide: The Importance of Dipolar Interaction for Long-Range Magnetic Ordering

The synthesis of Co5(OH)8(C7H15CO2)2·4H2O, Co5(OH)8(O2CC6H12CO2)·5H2O, Co5(OH)8(C12H25SO4)2·(3H2O·2NH3), and Co5(OH)8(C2N3)2·6H2O, their characterization by electron microscopy, XRD, TGA, IR, UV−vis, and XANES, and their magnetic properties are reported. They belong to a family of layered compounds having a triangular magnetic lattice. The basal spacing is 22.8, 16.2, 25.0, and 11.5 Å for the carboxylate, dicarboxylate, sulfate, and cyanide, respectively. From the powder X-ray and crystal electron diffraction data, the compounds are inferred to adopt the structure of the monoclinic form of Zn5(OH)8X2·solvent. XANES confirms that only divalent cobalt is present in the compounds and visible absorption spectra display bands originating from both octahedral and tetrahedral coordinated CoII and none from CoIII. The magnetic data show that all the compounds behave as two sublattice ferrimagnets which are characterized by a minimum in the temperature dependence of the moments and long-range ordering observed by spontaneous magnetization in small dc applied field, out-of-phase components in the ac magnetization and hysteresis loop. The saturation magnetization, approaching 3 μB at 2 K in field of 5 T, is in good agreement with the proposed structure consisting of three CoII in octahedral coordination in one sublattice and two CoII in tetrahedral coordination for the other. The Curie temperature attains 58 K and coercive field approaches 12 000 Oe at 2 K. The long-range ordering is driven by dipolar interaction between layers which have large effective moment resulting from short-range intralayer interactions. Interestingly, the Curie temperatures are independent of the chemical and physical nature of the anions, as expected for the proposed dipolar mechanism. The large coercive fields result from the synergy of crystalline shape and single ion anisotropies and to the alignment of the moments perpendicular to the layers.