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Direct measurement of the temperature dependence of the magnetic penetration depth in Ba(Fe$_{1-x}$Ni$_x$)$_2$As$_2$ superconductors

The temperature dependence of the in-plane magnetic penetration depth $λ_{ab}$ of Ba(Fe$_{1-x}$Ni$_x$)$_2$As$_2$ single crystals is determined directly from the shielding magnetic susceptibility, measured in the Meissner region with the field parallel to the ab layers. The doping levels studied cover the underdoped, optimally-doped and overdoped regimes. At temperatures below 0.5Tc a well-defined power-law behavior $λ_{ab}(T)-λ_{ab}(0)=AT^n$ (with n approximately 2.5) is observed. At lower temperatures (T<0.3Tc) the data are still consistent with n=2 and $A\propto T_c^{-3}$, as predicted by the strong pair-breaking scenario proposed by Gordon et al., Phys. Rev. B 81, 180501(R) (2010). The temperature dependence of the superfluid density $ρ_s\proptoλ_{ab}^{-2}$ presents a marked positive curvature just below Tc which is a sign of two-gap superconductivity. The analysis of $ρ_s(T)$ in terms of a two-gap model allowed to estimate parameters like the in-band and inter-band couplings, the relative weight of each band, and their dependence with the doping level. A comparison with $ρ_s(T)$ data obtained by using other techniques in compounds with a similar composition is also presented.