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Three different types of interfaces; a perfect interface (a), an interface with roughness (b) and an interface with interdiffusion (c).
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Epitaxial Co/Cr(001) superlattices were prepared by Molecular‐Beam‐Epitaxy (MBE). The crystal structure was determined both with Reflection High Energy Electron Diffraction (RHEED) and x‐ray diffraction experiments. The Co layers grow in an hcp phase with their (11.0) planes oriented parallel to the Cr(001) planes. The magnetic anisotropy was measu...
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... with high surface energy minimize the energy by growing as three- dimensional islands, whereas materials with low surface energy can grow layer by layer. The interfaces between the layers can be smooth or rough, sharp or diffuse. Roughness is in general the result of limited surface diffusion during growth. High growth temperatures promote surface diffusion but can also lead to unwanted interdiffusion between the layers. Three different types of interfaces are shown in Fig. 3. The interfaces in a real multilayer or superlattice are never perfect, i.e. some roughness and interdiffusion are always present. Two different types of interfacial roughness can be distinguished, one which is correlated between different layers and one which is random or uncorrelated. The correlated roughness is often induced by the substrate which is not exactly flat but contains steps which are replicated from layer to layer. In most cases, the roughness can be described as lying between these two extremes, the roughness is partially correlated. The roughness can also increase from layer to layer, this is refered to as cumulative roughness. If the growth is not phase locked the incomplete formation of the last monolayer of each individual layer will also give rise to an ...
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Citations
... This model was introduced by Behler and al [194,195]. Both radial and angular informations around each atom are represented by sums of pairwise functions. ...
... 2. Neural network using radial and angular symmetry functions [194] or CGCNN [197]. ...
... Random Searching The phases from YH 4 to YH 7 are explored using the active learning process employing mainly a SNAP type numerical potential. For complementary structural searches, Neural Network (NN) [194] type numerical potentials are also used. As presented in 3.1.1.3, ...
Alloys characterized by the presence of a large amount of hydrogen in their crystal structure, superhydrides are promising materials for dense energy storage and high temperature superconductivity. The main obstacle to the use of superhydrides for applications remains their very high synthesis pressure, in the range of one million atmosphere. To reduce it, knowledge about these new materials must be deepened. This thesis thus aims at making a contribution to the search of transition metal type superhydrides. For this purpose, the ”Ab-Initio Random Structure Searching” (AIRSS) algorithm was firstly implemented. Studies of hydrides and superhydrides of copper, manganese, yttrium as well as a ternary yttrium-iron system were carried out. The case of yttrium superhydrides is the place of a scientific challenge, because they probably have large primitive cells, which are not accessible by the AIRSS method. To reach a better description of these compounds machine learning tools were developed and used.
... For more detailed information on the structure and magnetic anisotropy, we refer to our previous papers [11,12,14,[19][20][21][22]. Here we present a detailed study of the exchange coupling in Co/Cr(0 0 1) superlattices. ...
... For the analysis of the interlayer exchange coupling a knowledge of the anisotropy properties is indispensable. The room temperature results can be summarized as follows [14,19,20]. An interface anisotropy constant K 1 +!0.65 mJ/m causes a transition of the orientation of the magnetization vector from in-plane to the out-of-plane direction with decreasing Co thickness t ! . ...
We have investigated the exchange coupling of Co/Cr(001) superlattices by polar and longitudinal magneto-optical Kerr effect measurements, by varying both the Co and Cr film thicknesses. At a Co thickness of ≈10Å a nearly perpendicular anisotropy is found with antiferromagnetic order in the range of 5–15Å of Cr thickness. For these superlattices the magnetization curve starting from remanence to saturation is characterized by a surface spin-flip transition at low field, followed by domain wall nucleation and motion, and finally by a coherent spin rotation with increasing field. Antiferromagnetic coupling is also observed for superlattices with thicker Co layers and with in-plane magnetic anisotropy.
A series of Co/Cr/Pd modulated multilayers with Cr layer thickness XCr ranging from 0 to 1.13 nm have been prepared by magnetron sputtering system on 40 nm thick Pd buffer. With combimed small- and large-angle X-ray analyses, the layered and crystal structure of the samples were ivestigated. The magnetic hysteresis loops and domain structure were obtained by means of alternating-gradient magnetometer (AGM) and Magnetic Force Microscopy (MFM) methods. The perpendicular magnetic anisotropy and the coercivity of the multilayers decrease with the increase of Cr thickness, which was ascribed to the reduction of the polarization of Pd atoms and the interlayer roughness, respectively. The Kerr rotation and ellipticity of the multilayers were also studied. The decrease of Kerr rotation with the increase of the Cr thickness was assumed to be due to the decrease of the polarization of Pd atoms.
We have studied the effect of Cr capping on the magnetic properties of Co nanoparticles (NPs). The NPs have an average diameter of 2.2 nm. The blocking temperature T-B of the bare Co particles is 13.2 K. By capping with a thin Cr layer up to a thickness of t(Cr) = 0.52 nm, we first observe a decrease of T-B up to t(Cr) = 0.14 nm, followed by an increase of T-B for larger thicknesses 0.14 nm <= t(Cr) <= 0.52 nm. X-ray magnetic circular dichroism measurements at the resonant Co and Cr L-3 edges confirm a magnetic polarization of Cr which is opposite to the magnetization of Co. The antiparallel alignment of Co and Cr spins at the Co/Cr interface can explain the decrease at low capping layer thickness. However, for larger Cr capping layer thicknesses, the Cr film bridges the Co NPs, mediating interparticle exchange coupling and enhancing dipolar coupling that leads to an increase of the blocking temperature. (C) 2013 AIP Publishing LLC.
The Silences of the Archives, the Reknown of the Story.
The Martin Guerre affair has been told many times since Jean de Coras and Guillaume Lesueur published their stories in 1561. It is in many ways a perfect intrigue with uncanny resemblance, persuasive deception and a surprizing end when the two Martin stood face to face, memory to memory, before captivated judges and a guilty feeling Bertrande de Rols. The historian wanted to go beyond the known story in order to discover the world of the heroes. This research led to disappointments and surprizes as documents were discovered concerning the environment of Artigat’s inhabitants and bearing directly on the main characters thanks to notarial contracts. Along the way, study of the works of Coras and Lesueur took a new direction. Coming back to the affair a quarter century later did not result in finding new documents (some are perhaps still buried in Spanish archives), but by going back over her tracks, the historian could only be struck by the silences of the archives that refuse to reveal their secrets and, at the same time, by the possible openings they suggest, by the intuition that almost invisible threads link here and there characters and events.
Artificial magnetic superlattices composed of successive ferromagnetic and paramagnetic layers have received much attention in recent years due to their scientific and technological relevance. While the strength of the exchange coupling and the oscillation period depend on the details of the Fermi surfaces involved, the overall features appear universal. More complex couplings are observed for magnetic superlattices with chromium spacer layer. This is due to the intrinsic spin density wave of Cr. Extensive synchrotron and neutron scattering experiments have now unraveled the Neel- state of thin Cr layers and proximity effects between Fe and Cr, elucidating the mutual interdependence of Cr spin structure and Fe exchange coupling. In Co/Cr superlattices the structural mismatch between hcp Co and bcc Cr adds another complexity, which affects strongly the magnetic anisotropy. Both, the current status and understanding of the exchange coupling in Fe/Cr(001) and Co/Cr(001) superlattices is briefly reviewed here.
An anomalous magnetization process was observed in multilayers of Cr(8 nm)/[Co(1 nm)/Cr(t nm)]19/Co(1 nm). A magnetic moment per Co atom, σsCo, larger than 200 emu/g is obtained for t-values of 1.5 and 3 nm from magnetization curves measured for magnetic fields up to 5 T at temperatures down to 5 K with a SQUID magnetometer. Also, a transition between a state of a larger magnetic moment and a smaller moment were observed at temperatures below 60 K.
We report about synchrotron and neutron-scattering studies investigating incommensurate spin-density waves (I-SDW’s) in epitaxially grown thin Cr(001) films, including surface and interface effects. These studies show that thin ferromagnetic cap layers of Fe, Ni, and Co with a thickness of only 2–3 nm have a strong effect on the propagation and orientation of the I-SDW’s in Cr. For thick Cr films there exist essentially only transverse I-SDW’s propagating parallel to the film plane with the spins oriented normal to the plane and at right angles to the in-plane magnetization of the ferromagnetic cap layers. With decreasing Cr thickness a different transverse I-SDW grows at the expense of the in-plane ones, now propagating normal to the plane and with spins parallel or antiparallel to the film magnetization. At a Cr thickness of about 250 Å, the transverse out-of-plane I-SDW completely dominates the phase diagram of Cr. All other domains are suppressed and a spin-flip transition does not occur above 10 K in strong contrast to bulk. For in-plane propagation of the I-SDW we find a coexisting commensurate spin-density wave (C-SDW) which vanishes during the reorientation to out-of-plane propagation with Cr thickness. Finally, for Cr thicknesses well below the period of the I-SDW, the Cr can only order as a C-SDW. The behavior of the SDW’s in thin Cr films with ferromagnetic cap layers can be understood in terms of competing interactions at the rough interfaces inducing frustration and by finite-size and strain effects. We have also investigated the effect of Cu and Pd cap layers on the SDW. The Cu cover is similar to a Cr/vacuum interface, whereas the effect of the Pd cover is intermediate between the ferromagnetic layers and Cu.
We have studied the giant magnetoresistance (GMR) of antiferromagnetically coupled Fe1−xCox/Cr superlattices in the temperature range between 77 K and 296 K. We find that the field induced net change of the magnetoresistance (ΔR) increases with increasing temperature. This unusual temperature dependence of the GMR effect can be modeled by a positive linear temperature dependent term together with a negative quadratic temperature dependent term. The temperature induced GMR enhancement is attributed to inelastic phonon scattering, which becomes spin-dependent in a system with strong spin asymmetry of the electronic band structure. © 1997 American Institute of Physics.
We have investigated the magnetization reversal and the magnetic anisotropy of epitaxial and Fe(0 0 1) films and compared them with epitaxial trilayers with spin valve characteristics. Furthermore, by e-beam lithography we have cut the single layers and the trilayers into the shape of wires to study the effect of the shape anisotropy and the dipolar interaction between the wires on their switching characteristics. We found that patterning very much affects the magnetization reversal of the single films by introducing an additional anisotropy term. Dipolar interaction between the different wires may exist but is less obvious. However, the remagnetization process of the patterned trilayers resembles more those of the continuous single films than a combination of the Co and Fe wires. We argue that this is due to a weak but remaining interlayer exchange coupling between Co and Fe, mediated by the intervening Cr layer.
