Evidence of a Surface-Mediated Magnetically Induced Miscibility Gap in Co-Pt Alloy Thin Films

(100) and (111) oriented single-crystal ${\mathrm{CoPt}}_{3}$ films were deposited over a range of growth temperatures from $\ensuremath{-}$50 to $800{}^{\ensuremath{\circ}}$C. The Curie temperature is increased by $200{}^{\ensuremath{\circ}}$C over the value expected for the homogeneous alloy in the as-deposited films (of both orientations) grown near $400{}^{\ensuremath{\circ}}$C. We interpret this as evidence for a previously unobserved, surface-mediated, magnetically driven miscibility gap in vapor-deposited ${\mathrm{CoPt}}_{3}$ films. Large perpendicular magnetic anisotropy is also observed in the as-deposited films (of both orientations) grown near $400{}^{\ensuremath{\circ}}$C.