by penetration depth measurements.

We report measurements of London penetration depth and magnetization on single crystals of FeS superconductor with a transition temperature of 4 K. From magnetization measurements, FeS was confirmed to be a type-II superconductor with H_c1(0) = 58 ± 4 Oe, λ_ab(0) = 900 ± 90 nm, and ξ(0) = 90 ± 12 nm. The low temperature (T < 0.3T_c) behavior of penetration depth deviates from a conventional exponential predicted by BCS theory and fits well to a power law with an exponent n = 2.2 ± 0.1 which suggests the presence of nodes in the system. The superfluid density was calculated and was found to fit best to a multi-gap s+d wave model with the d-wave gap having slightly larger weight (p=0.6). Δ(0)/k_BT_c for the d-wave gap is higher than that of the BCS weak coupling model, implying strong coupling in the system. This gap structure is consistent with recent theoretical and experimental studies.