Amorphous ErCo₂ exhibits at low temperature a sperimagnetic structure: the cobalt moments are parallel, while the erbium moments are opposed but strongly connected to their local easy-magnetisation axes. The present study with polarised neutrons and polarisation analysis shows that there exists a correlation between the erbium magnetic components perpendicular to the cobalt magnetisation. Their arrangement is ferro- or antiferromagnetic, depending on the angle between the cobalt moments and the easy axes. There is evidence for correlation between the local easy-magnetisation axes which are not distributed at random. As a result, the existence of magnetic subdomains may be deduced. The correlation between erbium moments vanishes when the temperature increases and the anisotropy disappears.