Spin dynamics in several different types of ferromagnetic metal (FM)/10-nm-thick n-type GaAs quantum well (QW) junctions is studied by means of time-resolved Kerr rotation measurements.Compared with the MnGa/insitu doped 10-nm-thick n-type GaAs QW junction,the spin lifetime of the MnGa/modulation-doped 10-nm-thick n-type GaAs QW junction is shorter by a factor of 5,consistent with the D’yakonov-Perel’spin relaxation mechanism.Meanwhile,compared with the spin lifetime of the MnAs/in-situ doped 10-nm-thick n-type GaAs QW junction,the MnGa/in-situ doped 10-nm-thick n-type GaAs QW junction is of a spin lifetime longer by a factor of 4.2.The later observation is well explained by the Rashba effect in the presence of structure inversion asymmetry,which acts directly on photo-excited electron spins.We demonstrate that MnGa-like FM/in-situ doped 10-nm-thick n-type GaAs QW junctions,which possess relatively low interfacial potential barriers,are able to provide long spin lifetimes.