Robust perpendicular magnetization of Co nanomagnets against alloy composition

Abstract

${\mathrm{Co}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x}$ bilayer nanomagnets formed on a Cu(111) substrate with the alloy composition of $x=0$, 0.2, 0.5, 0.8, and 1 were investigated using scanning tunneling microscopy (STM). The islands with $x=0$, 0.2, and 0.5 were further studied using spin-polarized STM. With $x=0.2$, the nanoislands exhibit triangular shapes similar to the pure Co islands with an upward shift in the peak energy of the minority $d$ state. On the other hand, at $x=0.5$, the appearance of the nanoislands evolve to a round shape, and the peak energy is distributed irregularly within the islands. A hexagonal shape and inhomogeneous electronic states similar to pure Fe islands are observed on the $x=0.8$ islands. The magnetic field dependence of the spin-sensitive signal revealed that the perpendicular anisotropy of the ${\mathrm{Co}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x}$ alloy islands is persistent up to half of the Fe alloy composition.