Evidence for nanoscale two-dimensional Co clusters inCoPt3 films with perpendicular magnetic anisotropy

Abstract

The length scale of the local chemical anisotropy responsible for thegrowth-temperature-induced perpendicular magnetic anisotropy of face-centered cubicCoPt3 alloy films was investigated using polarized extended x-ray absorption fine structure(EXAFS). These x-ray measurements were performed on a series of four (111)CoPt3 films epitaxially grown on (0001) sapphire substrates. The EXAFS data show a preferencefor Co–Co pairs parallel to the film plane when the film exhibits magnetic anisotropy, andrandom chemical order otherwise. Furthermore, atomic pair correlation anisotropy wasevidenced only in the EXAFS signal from the next neighbors to the absorbing Co atomsand from multiple scattering paths focused through the next neighbors. This suggests thatthe Co clusters are no more than a few atoms in extent in the plane and onemonolayer in extent out of the plane. Our EXAFS results confirm the correlationbetween perpendicular magnetic anisotropy and two-dimensional Co segregation inCoPt3 alloy films, and establish a length scale on the order of 10 Å for the Co clusters.