M. Hennion

Magnetoelastic and Exchange Contributions to the Helical-Ferromagnetic Transition in Dysprosium Epitaxial Films

The Curie temperature of dysprosium films epitaxially grown on yttrium and on erbium is shown to increase continuously with the epitaxial strain induced along the c-axis. This variation is correlated to modifications of the α strains, of the γ distortion and of the exchange energy barrier. The evolution of these different terms is discussed from the experimental thermal variations of the lattice parameters and turn angles determined by neutron diffraction.

Stabilization of the helical phase in Tb and in alloys films grown epitaxially on Y

Neutron diffraction experiments and macroscopic magnetization measurements show evidence of the stabilization of the helical magnetic phase over large temperature ranges for Tb and alloy films grown epitaxially on yttrium. In particular, the temperature of the transition between the helical and the ferromagnetic states is shifted from 220 K for bulk terbium to 160 K for a pure terbium film grown epitaxially on yttrium, despite the low stability of the helical phase in the bulk element. The decrease of the Curie temperature is due to the negative c-axis strain induced by the epitaxial growth on yttrium. The epitaxial strains also induce modifications of the Fermi surface, which leads to an increase of the turn angle even at . At low temperature, a long-wavelength-modulated phase, whose origin still remains to be explained, has been observed.

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) terbium single crystal films. With decreasing temperature, this magnetic arrangement appears when the helical one vanishes and before the long range ferromagnetic order sets in. The large wavelength magnetic modulation is squared and it varies between 180 and 700 A depending on the temperature and the film thickness. We assume that it is due to the stacking of ferromagnetic 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 it would be related to the lattice clamping between epitaxied layers. A temperature-magnetic field phase diagram of a terbium film is presented.

Shifts of Curie temperature of epitaxial dysprosium films

Abstract From magnetic measurements and neutron scattering, dysprosium epitaxial films present an enhanced Curie temperature, when they are grown on erbium, and a reduced (even suppressed) one, when they are grown on yttrium. A magnetostrictive model reproduces the continuous increase of the Curie temperature with the c -axis strain induced by epitaxy.

Polarized neutron reflection study of an Er/Fe multilayer

Abstract Low angle polarized neutron scattering experiments have been performed on an Er91 A/Fe39 A multilayer. The evolution of the intensity of the first peak with the temperature and the external magnetic field shows that the average magnetizations of Fe and Er layers are in the applied field direction. The study of the intensities of the second and the third peaks indicates that there is an interface effect: some erbium atoms remain antiferromagnetically coupled to iron at the interface.

Antiferromagnetic and biquadratic coupling in NiFe/Ag and Ni/Ag multilayers

Low-angle polarized neutron scattering has been used to study the coupling between Ni layers and between permalloy layers through silver. It is shown that the coupling between Ni layers is purely bilinear. The coupling between permalloy layers presents bilinear and biquadratic contributions.

Magnetic structure of Tb films and Tb/Y superlattices

Abstract Thin terbium films and Tb/Y superlattices have been grown by molecular beam epitaxy on sapphire substrates. They have been studied by magnetization measurements and neutron scattering experiments. Tb directly undergoes a transition from the paramagnetic phase to the ferromagnetic one. Neutron scattering experiments performed on superlattices below 220 K show that there is no long range magnetic order except a small tendency to antiferromagnetic coupling between ferromagnetic Tb layers.

Neutron scattering evidence for antiferromagnetic coupling in Ag/Ni superlattices

Abstract Low angle neutron scattering experiments performed on Ag/Ni superlattices show the appearance of a low angle peak attributed to the doubling of the chemical period, as a consequence of the antiferromagnetic coupling of the magnetic moments in adjacent nickel layers. This is confirmed by polarized neutron experiments. The variation of the intensity of this peak with temperature is in accordance with a classical model of antiferomagnetism.

Clamping effect on the magnetic behavior of europium epitaxial thin films

Epitaxial thin films of europium experience significant strains when deposited on a sapphire substrate with a niobium buffer. We demonstrate that the relative population of the magnetic domains is affected, in particular, in the presence of a slight tilt of the film growth axis relative to the substrate. The application of a magnetic field further modifies the population of the domains giving rise to unusual irreversible effects. These results could help to elucidate the magnetoelastic properties of bulk europium, which are still unknown.

Polarized neutron study of antiferromagnetic ordering in Dy/Er superlattices

Dy/Er superlattices, grown on sapphire, exhibit a succession of magnetic phases. Below the ordering temperature of dysprosium, the helix of dysprosium propagates coherently through paramagnetic erbium. At low temperature, dysprosium becomes ferromagnetically ordered in each layer but the layers are antiferromagnetically stacked. A polarized neutron scattering study of the low-temperature phase is presented. It shows that the antiferromagnetic peaks are never purely spin-flip.