MAGNETIC STRUCTURES - Rare Earth Magnetism: Structures and Excitations - page 131

spin slips. Such ferromagnetic terminations at the surfaces of slabs con-
taining planes of rotating moments are a general feature, reflecting the
predominantly ferromagnetic interaction between closely neighbouring
planes in the magnetic rare earths. This coupling normally gives rise to
a net moment in the slab, and is calculated to stabilize ferromagnetic
ordering at 4 K in samples thinner than about nine atomic planes (Bohr
et al. 1989).

Fig. 2.11. Mean-field calculation of the orientation of the magnetic
moments in a 15-plane slab of Ho at 4 K. The inner planes are close
to a bunched commensurable helix, but there is a strong tendency to
ferromagnetism near the surfaces.

The effect of the epitaxial strain is strikingly illustrated by the be-
haviour of thin films and superlattices of Dy and Er grown on Y, in both
of which ferromagnetism is suppressed, by somewhat different mecha-
nisms, in favour of periodic magnetic ordering. In 16-plane Dy films
embedded in Y in a variety of [Dy 16 |

Y m ] multilayers, with the c -axis
normal to the plane of the slab, Rhyne et al. (1989) found that the helix
persists to the lowest temperatures, and the ferromagnetic state is only
induced if a field of the order of 10 kOe is applied in the easy direction.
An obvious mechanism for this quenching of ferromagnetism is the con-
straint which the Y slabs impose on the Dy layers, so that the γ -strains
which provide the principal driving force for the transition cannot be
fully developed.
The ferromagnetic ordering of the axial moment is also suppressed