Upper Critical Fields and Anisotropy of BaFe_1.9Ni_0.1As₂ Single Crystals

Temperature dependence of the upper critical magnetic field (H_c2) near T_c of 20K in a BaFe_1.9Ni_0.1As₂ single crystal is determined via magneto-resistance measurements, for the out-plane (H⊥ab) and in-plane (H || ab) directions in magnetic fields of up to 8T. The upper critical fields at zero temperature estimated by the Werthamer-Helfand-Hohenberg (WHH) formula are μ₀ H_c2^||(0)= 137T and μ₀ H_c2^⊥(0) = 51T, both exceeding the weak-coupling Pauli paramagnetic limit (μ₀H_p=1.84T_c). However, the WHH formula could overestimate the μ₀H_c2^|| (0) value. The anisotropy of upper critical fields is around 3 in the temperature range close to T_c. The result is very similar to the Co-doped 122 superconductor BaFe_2-xCo_xAs₂, indicating that electron-doped 122 superconductors exhibit similar superconducting properties.