Effect(s) of Cobalt Substitution in $L{1}_0$-(Fe,Co)Pt Thin Films

We have studied the effect of cobalt substitution in $L{1}_0$-Fe${}_{1-x}$Co${}_x$Pt films by means of x-ray magnetic circular dichroism (XMCD) and first-principles calculations. The magnetic moments of Fe ($\sim$2.5 ${\mu }_{\mathrm{B}}$) and Co ($\sim$1.5 ${\mu }_{\mathrm{B}}$) deduced using XMCD were almost unchanged upon Co doping, and the net magnetization decreases with increasing Co content. Calculation also showed that the 3$d$ electrons that have been added by Co substitution occupy only spin-down bands.