Catherine Dufour (1964-2011)

Gperhard Heinzmann, Rédacteur en chef La philosophie est la mère des sciences. Il faut avoir une bonne mémoire pour se le rappeler. Quel bonheur lorsque ce rappel vient d’une collègue qui ne l’a pas appris, mais qui éprouve la nécessité de questionner sa pratique scientifique et actualise ainsi l’ancienne métaphore ! C’était le cas de Catherine Dufour. On se connaissait vaguement depuis l’école maternelle des Trois Maisons que fréquentaient nos enfants. Je fus donc très heureux de découvrir, ..

Catherine Dufour (1964-2011)

Gperhard Heinzmann, Rédacteur en chef La philosophie est la mère des sciences. Il faut avoir une bonne mémoire pour se le rappeler. Quel bonheur lorsque ce rappel vient d’une collègue qui ne l’a pas appris, mais qui éprouve la nécessité de questionner sa pratique scientifique et actualise ainsi l’ancienne métaphore ! C’était le cas de Catherine Dufour. On se connaissait vaguement depuis l’école maternelle des Trois Maisons que fréquentaient nos enfants. Je fus donc très heureux de découvrir, ..

Intrinsic versus shape anisotropy in micro-structured magnetostrictive thin films for magnetic surface acoustic wave sensors

International audienceThis work aims at studying the interaction between surface acoustic waves (SAW) and micro-structured magnetostrictive layers under a magnetic field with a perspective to develop magnetic field sensors. The impact of the competition between the strong intrinsic magnetic anisotropy of the magnetic material and the shape anisotropy of the interdigitated transducer (IDT) fingers introduced by the micro-structuration is investigated. Therefore, the macroscopic and microscopic magnetic properties of the IDT and their influence on the magneto-acoustic response are studied. A SAW resonator with the IDTs made of the magnetostrictive thin film was elaborated and the magnetic surface acoustic wave (MSAW) response under a magnetic field was performed and discussed. Depending on the energy balance, the anisotropy gets modified and a correlation with the MSAW sensitivity to an externally applied magnetic field is made

Couplage magnétique et effets magnétostrictifs dans des films épitaxies et super-réseaux à base de terres rares

Not availableCe travail concerne l'étude des propriétés magnétiques de films de dysprosium épitaxies entre des couches d'yttrium et d'erbium (Y/Dy/Y et Er/Dy/Er) et de super réseaux de terres rares Dy/Er, élaborés dans une enceinte d'épitaxie par jet moléculaire. Les films de dysprosium épitaxies sur yttrium subissent une diminution de paramètre c (déformation alpha2 négative), alors que les films de dysprosium épitaxies sur Er sont déformés positivement. Les expériences de diffraction neutronique et les mesures magnétiques montrent que la température de curie des films épitaxies dépend fortement du signe et de l'amplitude de la déformation induite par épitaxie. Les évolutions des champs critiques Hc, des angles de rotation entre spins et des paramètres précisent l'amplitude des effets magnétostrictifs. Dans les super réseaux Dy/Er, le dysprosium développe un ordre hélicoïdal cohérent à travers l'erbium paramagnétique qui s'interprète par la propagation d'une hélice virtuelle à travers l'erbium. À basse température, le dysprosium devient progressivement ferromagnétique dans les couches avec un ordre antiferromagnétique à grande distance. Malgré son état de contrainte, le dysprosium ne voit pas sa température de curie augmenter, ce qui est interprété par une modification de la barrière énergétique entre les configurations ferromagnétique et hélicoïdale. L’erbium ne présente pas de transition ferromagnétique à basse température, conformément à son état de déformation négative. Les termes d'énergie d'échange, élastique et magnétoélastique, sont discuté

Elastic and inelastic neutron scattering for the study of magnetic thin films and heterostructures

Neutron-based techniques have proved for decades to be essential tools in the investigation of magnetic materials. The strong experience gathered in neutron centers together with high performance instruments have also largely contributed to the researches on magnetic nanosystems. We will present here a few examples of the use of elastic and inelastic neutron scattering for the study of Dysprosium-based thin films and heterostructures. Elastic neutron scattering in rare earth-based nanosystems provides crucial information both on the crystal structure and the modulated magnetic order. This has especially permitted to connect the drastic change in Curie temperature observed in Dy epitaxial films to modifications of both magnetoelastic and exchange contributions. In superlattices, elastic neutron scattering has been successfully used to analyze the magnetic coupling phenomena, in particular the long range coherent propagation of the Dy helical order. The example of Dy/Er superlattices highlights this fascinating coupling mechanism through either a paramagnetic or magnetic spacer. Inelastic Neutron Scattering is the technique of choice to investigate magnetic excitations over the entire Brillouin Zone. The very low inelastic scattering cross section can however make any investigation of nanosystems very difficult and very few studies have indeed been reported so far on nanosystems. We will show that relevant information could be obtained from high quality single crystalline Dy films and superlattices. These promising results are paving the way to new spin wave explorations in a wider range of magnetic nanosystems

Direct evidence of the anisotropy of magnetization in rare-earth metals and rare-earth/Fe-2 alloys

Équipe 101 : Nanomagnétisme et électronique de spinInternational audienceWe report on the genuine origin of the anisotropy of the magnetization M in rare-earth (RE) metals and RE-based alloys. Taking Ho-based layered nanostructures as testing ground, we prove that the anisotropy of M is substantial despite that the sixfold magnetic anisotropy constant K-6(6) vanishes, which contradicts the established wisdom [E. R. Callen and H. B. Callen, J. Phys. Chem. Solids 16, 310 (1960)]. Furthermore, we show that the symmetric anisotropic contributions to M and K-6(6) vary with temperature distinctively from one another, which indicates that both anisotropic effects are unrelated and stem from dissimilar microscopic sources. Our findings are discussed according to the theory [R. J. Elliott and M. F. Thorpe, J. Appl. Phys. 39, 802 (1968)] that predicts the emergence of symmetric anisotropic indirect-exchange terms under the presence of orbital moments. We show evidence that the anisotropy of M is caused by the indirect-exchange coupling among localized 4f magnetic moments mediated by spin-orbit coupled conduction electrons, which ultimately generates a spatially nonuniform spin polarization that replicates the lattice symmetry

Magnetic control of the zero-magnetization ferromagnet Sm1-xGdxAl2

Equipe 101 : Nanomagnétisme et électronique de spinInternational audienceX-ray magnetic circular dichroism experiments have been performed up to +/- 17 T to investigate the magnetic configuration and magnetization reversal of an original zero-magnetization ferromagnet Sm1-xGdxAl2 (x = 0.028), both as a single epitaxial layer and as a pinning layer in an exchange-coupled system. The Sm0.972Gd0.028Al2 single layer appears to exhibit an extraordinary large coercivity that exceeds 20 T below its magnetic compensation (T-comp). Despite such huge magnetic stability in the single layer, interface exchange coupling in the bilayer drives the formation of domains in Sm0.972Gd0.028Al2 and their reversal upon field, both below and at magnetic compensation. Increasing the cooling field yields the increase in exchange-favored domains in Sm0.972Gd0.028Al2, surprisingly also at T-comp, whereas, Zeeman energy does not favor this specific orientation neither at compensation nor during the cooling process. We propose a possible scenario for those domains' formation and highlight the way the external magnetic field may tune the magnetic configuration in such a zero-magnetization ferromagnet. The cooling field also consequently influences the SmAl2 magnetization reversal which is biased by pinned magnetic components in Sm0.972Gd0.028Al2; the bias field is satisfactorily explained in considering different pinned contributions with opposite magnetic orientations

Phase diagram in exchange-coupled CoTb/[Co/Pt] multilayer-based magnetic tunnel junctions

International audienceMagnetic and magneto-transport properties of [Co/Pt]/MgO/[Co/Pt]CoTb magnetic tunnel junctions have been investigated. Depending on the thickness of the Pt layer, temperature, and field history, the [Co/Pt]CoTb hard layer shows complex behaviors that can be reproduced by micromagnetic calculations. The magnetic tunnel junctions appear to exhibit conventional behavior but also exchange bias and spring magnet and domain duplication phenomena that have been observed combining magnetometry and interface-sensitive spin-dependent tunnel transport

Towards shaping picosecond strain pulses via magnetostrictive transducers

Using time-resolved x-ray diffraction, we demonstrate the manipulation of the picosecond strain response of a metallic heterostructure consisting of a dysprosium (Dy) transducer and a niobium (Nb) detection layer by an external magnetic field. We utilize the first-order ferromagnetic–antiferromagnetic phase transition of the Dy layer, which provides an additional large contractive stress upon laser excitation compared to its zero-field response. This enhances the laser-induced contraction of the transducer and changes the shape of the picosecond strain pulses driven in Dy and detected within the buried Nb layer. Based on our experiment with rare-earth metals we discuss required properties for functional transducers, which may allow for novel field-control of the emitted picosecond strain pulses