Suppression of spin-torque in current perpendicular to the plane   spin-valves by addition of Dy cap layers
  We demonstrate that the addition of Dy capping layers in current perpendicular to the plane giant magneto-resistive spin-valves can increase the critical current density beyond which spin-torque induced instabilities are observed by about a factor of three. Current densities as high as 5e7 A/cm2 are measured provided that the electron current flows from the free to the reference layer. While Dy capped samples exhibit nonmagnetic 1/f noise, it is sufficiently small to be unimportant for read head operation at practical data rates. 
S. Maat, N. Smith, M.J. Carey, and J.R. Childress