Anisotropic H _c2 up to 92 T and the signature of multiband superconductivity in Ca ₁₀(Pt ₄As ₈)((Fe _1-xPt _x) ₂As ₂) ₅

The upper critical fields H _c2(T) of single crystals of the superconductor Ca ₁₀(Pt _4-δAs ₈)((Fe _0.97Pt _0.03) ₂As ₂) ₅ (δ0.246) are determined over a wide range of temperatures down to T=1.42 K and magnetic fields of up to μ ₀H92 T. The measurements of anisotropic H _c2(T) curves are performed in pulsed magnetic fields using radio-frequency contactless penetration depth measurements for magnetic field applied both parallel and perpendicular to the ab plane. Whereas a clear upward curvature in Hc2(T) along Hc is observed with decreasing temperature, the Hc2ab(T) along Hab shows a flattening at low temperatures. The rapid increase of the Hc2c(T) at low temperatures suggests that the superconductivity can be described by two dominating bands. The anisotropy parameter γ _HHc2ab/Hc2c is ∼7 close to T _c and decreases considerably to ∼1 with decreasing temperature, showing rather weak anisotropy at low temperatures.