  We investigate the effect of anti-ferromagnetic (AF) quantum criticality on
the magnetic penetration depth $\lambda(T)$ in line-nodal superconductors,
including the cuprates, the iron pnictides, and the heavy-fermion
superconductors. The critical magnetic fluctuation renormalizes the current
vertex and drastically enhances zero-temperature penetration depth
$\lambda(0)$, which is more remarkable in the iron-pnictide case due to the
Fermi-surface topology. Additional temperature ($T$) dependence of the current
renormalization makes the expected $T$-linear behavior at low temperatures
approaching to $T^{1.5}$ asymptotically. These anomalous behaviors are well
consistent with experimental observations. We stress that $\lambda(T)$ is a
good probe to detect the AF quantum critical point in the superconducting
state.
