Magnetism, transport, and atomic structure of amorphous binary YxCo1–x alloys

Sputtered thin films of binary ${\mathrm{Y}}_x{\mathrm{Co}}_1--x$ with $0<x\ensuremath{\le}0.54$ and thickness $\ensuremath{\approx}15$ nm are investigated to help understand the ferromagnetism of cobalt in amorphous rare-earth cobalt alloys. The magnetic moment per cobalt falls to zero at ${x}_c\ensuremath{\approx}0.50$, where the appearance of magnetism is marked by a paraprocess with a dimensionless susceptibility of up to 0.015. All films are magnetically soft, with densities that fall between those of crystalline Y-Co intermetallic compounds and the density of a relaxed 10 000-atom binary random close-packed model of hard spheres with an Y:Co volume ratio of 3:1, where the packing fractions for all films lie in a narrow range, 0.633 \ifmmode\pm\else\textpm\fi{} 0.004, and Co is coordinated by an average of 3.2 Co and 3.2 Y atoms at $x=0.5$. All films with $x<0.4$ exhibit in-plane shape anisotropy that is about six times as great as an intrinsic perpendicular component. Average cobalt spin and orbital moments obtained by x-ray magnetic circular dichroism were 1.31 and $0.32\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}$, respectively, for amorphous ${\mathrm{Y}}_0.25{\mathrm{Co}}_0.75$. Strong local anisotropy is associated with the large cobalt orbital moment, but there is little influence of anisotropy on the ferromagnetic order because of exchange averaging. The films are soft ferromagnets. Hall effect and magnetoresistance are modeled in terms of effective uniform rotation of the magnetization, with spontaneous and band contributions. Amorphous ${\mathrm{Y}}_x{\mathrm{Co}}_1--x$ is contrasted with amorphous ${\mathrm{Y}}_x{\mathrm{Fe}}_1--x$, which exhibits random noncollinear magnetic order that is very sensitive to the film density.