Field induced magnetic transition in low-dimensional magnets Bi(Ni,Co)PO5

We report the magnetic properties of iso-structural compounds BiNiPO5 and BiCoPO5 which undergo antiferromagnetic ordering at 17 K and 11 K, respectively. Positive values of θCW suggest the dominant antiferromagnetic (AFM) interactions in both the compounds. Similarly, the (TN/θCW) ratio suggests that BiCoPO5 is a frustrated one dimensional antiferromagnet while BiNiPO5 is more likely a three dimensional antiferromagnet. Magnetic specific heat Cmag was found to be extended towards high temperatures in BiCoPO5 compared to BiNiPO5 supporting the low dimensional character of BiCoPO5. The magnetic isotherm (M vs. H) at 2 K shows the feature of a field induced spin-flop transition in BiCoPO5 as is expected for a strongly anisotropic AFM. We construct the field-temperature (H−T) phase diagram for BiCoPO5 from the specific heat measurements and find that the phase boundary of the PM-AFM transition can be described in terms of a mean-field power law.