Anisotropy of the upper critical field in a co-doped BaFe ₂As ₂ single crystal

The temperature dependence of the upper critical magnetic field (H _c2) in a BaFe _1.84Co _0.16As ₂ single crystal was determined via resistivity, for the inter-plane (H ab) and in-plane (H ∥ ab) directions in pulsed and static magnetic fields of up to 60 T. Suppressing superconductivity in a pulsed magnetic field at ₃He temperatures permits us to construct an H _c2-T phase diagram from quantitative H _c2(0) values and determine its behavior in low temperatures. H _c2(0) with H 7par; ab [H _c2∥(0)] and H ab [H _c2(0)] are ∼55 and ∼50T respectively. These values are ∼ 1.2{1.4 times larger than the weak-coupling Pauli paramagnetic limit (H _p - 1.84T _c), indicating that enhanced paramagnetic limiting is essential and this superconductor is unconventional. While H _c2 ∥ ab is saturated at low temperature, H _c2 with H ab (H _c2) exhibits almost linear temperature dependence towards T = 0 K which results in reduced anisotropy of H _c2 in low temperature. The anisotropy of H _c2 was ∼3.4 near T _c, and decreases rapidly with lower temperatures reaching ∼1.1 at T = 0.7 K.