MAGNETIC STRUCTURES - Rare Earth Magnetism: Structures and Excitations - page 133

moment falls below the saturation value of 10 µ B at low temperatures,
even though the total integrated magnetic scattering corresponds to the
fully saturated moment. At higher temperatures, the coupling of the net
ferromagnetic moment in a Dy layer to an applied magnetic field breaks
the coherence of the helical structure between the layers well before the
transition to the true ferromagnetic phase occurs. A b -axis superlattice,
on the other hand, fails to form a coherent magnetic structure even when
the Y layer is as thin as 9 planes, since the static susceptibility at q = 0 ,
which is required to propagate the ferromagnetic coupling between the
basal planes of the Dy layers through the Y, is not particularly high. The
Dy layers therefore form helical magnetic structures with wave-vectors
in the plane of the layers, but no coherence of phase or chirality between
the layers.