  In- and out-of-plane magnetic penetration depths were measured in three
iron-based pnictide superconducting systems. All studied samples of both 122
systems show a robust power-law behavior, $\lambda (T) T^n$, with the
sample-dependent exponent n=2-2.5, which is indicative of unconventional
pairing. This scenario could be possible either through scattering in a $s_{\pm
}$ state or due to nodes in the superconducting gap. In the Nd-1111 system, the
interpretation of data may be obscured by the magnetism of rare-earth ions. The
overall anisotropy of the pnictide superconductors is small. The 1111 system is
about two times more anisotropic than the 122 system. Our data and analysis
suggest that the iron-based pnictides are complex superconductors in which a
multiband three-dimensional electronic structure and strong magnetic
fluctuations play important roles.
