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

Abstract

Compounds of the general formulaCo2−xNix(OH)PO4 (x = 0.1, 0.3) have been synthesized under mild hydrothermal conditions. Neutronpowder diffraction, susceptibility and heat capacity measurements were carriedout on polycrystalline samples. The cobalt–nickel compounds are ordered asthree-dimensional antiferromagnets with ordering temperatures of 70 and 64 K forx = 0.1 and x = 0.3, respectively. The magnetic study shows a spin glass-like state below 11 and 5 K forCo1.9Ni0.1(OH)PO4 and Co1.7Ni0.3(OH)PO4, respectively. Specific heat data present peaks at 68 and 61 K forCo1.9Ni0.1 and Co1.7Ni0.3, respectively. These peaks show broad shoulders between approximately 15 and 40 K. Thelack of any distinguishable anomaly below 10 K supports the spin glass nature of the lowtemperature transitions. Refinement of room temperature neutron diffraction dataindicates that the Ni(II) ions are in octahedral co-ordination with the practicalabsence of these ions in the trigonal bipyramidal sites. The magnetic structures ofCo2−xNix(OH)PO4 consistof ferromagnetic arrangements between the octahedral chains and trigonal bipyramidal dimers within thexz plane with the magneticmoments along the z axis. The ferromagnetic layers are disposed antiparallel to one another along they direction establishing the three-dimensional antiferromagnetic order(TN≈70 Kfor Co1.9Ni0.1 and ≈64 K for Co1.7Ni0.3). The different exchange pathways, the anisotropy of the Co(II) ions and the frustration ofthe magnetic moments in the trigonal bipyramidal geometry could be responsible for thefreezing process.