Magnetic ordering in (110) Eu films in an applied magnetic field

Below its ordering temperature (T _N = 90 K), bulk bcc Eu has a helical magnetic state with propagation vectors along the three equivalent 〈100〉 directions. In contrast, epitaxial (110)Eu films exhibit a unique magnetic ordering: the domain with a magnetic helix propagating along the in-plane 001] direction vanishes on cooling, at the expense of other domains with helices propagating along 100] and 010]. This paper is devoted to the study of the stability of the magnetic domains in an external magnetic field using neutron scattering experiments and macroscopic magnetization measurements. The helix propagating along the 001] direction can be restored by the application of an external field along this direction. On the contrary, when a magnetic field is applied along an intermediate direction, specifically 10], the domain with a helix propagating along 001] is suppressed. Both effects depend on the film thickness. They are explained if one considers that, because of the low magnetic anisotropy of Eu, a helix with a propagation vector parallel to (or close to) the applied magnetic field is energetically more favourable than cycloidal structures with unchanged propagation vectors. Finally, the amplitudes of the propagation vectors and their directions (that are modified in films compared to bulk) do not vary under magnetic field.