Structural and physical properties of undoped and nickel-doped BaFe_2-xNi_xAs₂(x=0, 0.04) single crystals

Single crystals of undoped and nickel-doped BaFe_2-xNi_xAs₂ (x=0, 0.04) have been grown by FeAs self-flux method. The maximum dimension of the crystal is as large as ～ 1 cm along the 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) ? (1 ? = 0.1 nm), c=13.015(2) ? for BaFe_1.96Ni_0.04As₂ and a =3.9590(5) ?, c = 13.024(1) ? 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 BaFe_1.96Ni_0.04As₂ 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 due to spin density wave (SDW) instability.