Amorphous Co79x-yErxB21+y alloys, with x<10 at.% and y<3 at.%, belong
to the series of ternary transition metal-rare earth-metalloid glasses
of great interest for their high coercivity and magnetization. An ano
malous wide angle X-ray scattering study has been performed on a serie
s of metallic glasses in order to relate their magnetic behaviour to t
heir local and medium range order. Due to the large difference in size
and electronic number, Z, between the three components, this techniqu
e allows the determination of the surroundings of the cobalt and erbiu
m atoms and the discrimination of the Go-Co, Er-Er and Co-Er correlati
ons in the first coordination shell. A competition between the topolog
ical disorder and a chemical ordering in the short and medium range oc
curs when the Er content increases, which may influence their magnetic
and thermodynamic properties.