Large wavelength magnetic modulation in (0001)Tb films

A large wavelength magnetic modulation, which has never been observed up to now, has been evidenced by neutron scattering experiments in (0001) terbiu m single crystal films. With decreasing temperature, this magnetic arrangem ent appears when the helical one vanishes and before the long range ferroma gnetic order sets in. The large wavelength magnetic modulation is squared a nd it varies between 180 and 700 Angstrom, depending on the temperature and the film thickness. We assume that it is due to the stacking of ferromagne tic blocks whose net magnetization points along a basal plane direction and rotates by a given angle between two consecutive blocks. The occurrence of this phase seems to be independent of the strains induced by epitaxy but i t would be related to the lattice clamping between epitaxied layers. A temp erature-magnetic field phase diagram of a terbium film is presented.