Magnetic structures of (Co2-xNix) (OH)PO4 (x=0.1, 0.3) spin glass-like state in antiferromagnetically ordered phases

Compounds of the general formula CO2-xNix(OH)PO4 (x = 0.1, 0.3) have been synthesized under mild hydrothermal conditions. Neutron powder diffraction, susceptibility and heat capacity measurements were carried out on polycrystalline samples. The cobalt-nickel compounds are ordered as three-dimensional antiferromagnets with ordering temperatures of 70 and 64 K for x = 0.1 and x = 0.3, respectively. The magnetic study shows a spin glass-like state below 11 and 5 K for Co1.9Ni0.1 (OH)PO4 and Co1.7Ni0.3(OH)PO4, respectively. Specific heat data present peaks at 68 and 61 K for Co1.9Ni0.1 and Co1.7Ni0.3, respectively. These peaks show broad shoulders between approximately 15 and 40 K. The lack of any distinguishable anomaly below 10 K supports the spin glass nature of the low temperature transitions. Refinement of room temperature neutron diffraction data indicates that the Ni(II) ions are in octahedral co-ordination with the practical absence of these ions in the trigonal bipyramidal sites. The magnetic structures of CO2-xNix(OH)PO4 consist of ferromagnetic arrangements between the octahedral chains and trigonal bipyramidal dimers within the xz plane with the magnetic moments along the z axis. The ferromagnetic layers are disposed antiparallel to one another along the y direction establishing the three-dimensional anti ferro magnetic order (T-N approximate to 70 K for Co1.9Ni0.1 and approximate to 64 K for Co1.7Ni0.3). The different exchange pathways, the anisotropy of the Co(II) ions and the frustration of the magnetic moments in the trigonal bipyramidal geometry could be responsible for the freezing process.