Structural and physical properties of undoped and nickel-doped BaFe2-xNixAs2 (x=0, 0.04) single crystals

Single crystals of undoped and nickel-doped BaFe2-xNixAs2 (x = 0, 0.04) have been grown by FeAs self-flux method. The maximum dimension of the crystal is as large as similar to 1 cm along ab plane. The crystalline topography of a cleaved crystal surface is examined by scanning electron microscope (SEM). By x-ray powder diffraction (XRD) experiments using pure silicon as an internal standard, precise unit cell parameters (tetragonal at room temperature) are determined: a = 3.9606(4) angstrom (1 angstrom=0.1 nm), c = 13.015(2) angstrom for BaFe1.96Ni0.04As2 and a = 3.9590(5) angstrom, c = 13.024(1) angstrom for BaFe2As2. DC magnetization and transport measurements are performed to check superconducting transition (T-c = 15 K for x = 0.04) and other subtle anomalies. For BaFe1.96Ni0.04As2 crystal, the resistance curve at normal state shows two distinct anomalies associated with spin and structure transitions, and its magnetization data above 91 K exhibit a linear temperature dependence clue to spin density wave (SDW) instability.