  Sputtered thin films of binary YxCo1-x with 0 < x < 0.54 and thickness 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 x0 =
0.50, where the appearance of magnetism is marked by a para-process 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 lies in a narrow range around 0.633 and Co is
coordinated by 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 Bohr
magnetons, respectively for amorphous Y0.25Co0.75. 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
Hall effect and magnetoresistance are modelled in terms of effective uniform
rotation of the magnetization, with spontaneous and band contributions.
Amorphous YxCo1-x is contrasted with amorphous YxFe1-x, which exhibits random
noncollinear magnetic order that is very sensitive to the film density.
