  Muon spin relaxation/rotation (muSR) is a vital technique for probing the
superconducting gap structure, pairing symmetry and time reversal symmetry
breaking, enabling an understanding of the mechanisms behind the unconventional
superconductivity of cuprates and Fe-based high-temperature superconductors,
which remain a puzzle. Very recently double layered Fe-based super-conductors
having quasi-2D crystal structures and Cr-based superconductors with a quasi-1D
structure have drawn considerable attention. Here we present a brief review of
the characteristics of a few selected Fe- and Cr-based superconducting
materials and highlight some of the major outstanding problems, with an
emphasis on the superconducting pairing symmetries of these materials. We focus
on muSR studies of the newly discovered superconductors ACa2Fe4As4F2(A = K, Rb,
and Cs), ThFeAsN, and A2Cr3As3(A = K, Cs), which were used to determine the
superconducting gap structures, the presence of spin fluctuations, and to
search for time reversal symmetry breaking in the superconducting states. We
also briefly discuss the results of muSR investigations of the
superconductivity in hole and electron doped BaFe2As2.
