P. Baules

Growth and stress relaxation of Co/NiO bilayers on MgO(001)

Co/NiO bilayers have been grown on a MgO(001) surface in an ultrahigh vacuum sputtering chamber under various deposition conditions. Investigations by x-ray diffraction, atomic force microscopy, and high resolution transmission electron microscopy give evidence for a close relationship between structure and deposition conditions for the two different layers. The role of the deposition rate on the oxide layer surface roughness is particularly emphasized. At high deposition rates, surface roughness increases with temperature, whereas the surface remains flat for low deposition rates. Whatever the growth conditions, the NiO layer grows epitaxially on the MgO substrate with the orientation relationship NiO(001)[100]//MgO(001)[100]. The misfit strain (about 1%), elastically accommodated in the thinner layers (3 nm), is fully relaxed in 50-nm-thick layers. A temperature dependence of the cobalt layer structure is observed: at room temperature it grows in its high temperature face-centered cubic structure, where...

Permalloy thin films on MgO(001): Epitaxial growth and physical properties

Permalloy thin films were deposited onto MgO(001) substrates by standard sputtering technique at growth temperatures ranging from 200 to 800 °C. Both reflection high-energy electron diffraction (RHEED) experiments and atomic force microscopy observations reveal that the substrate temperature for two-dimensional epitaxial growth should not exceed 350 °C. A series of permalloy films with thicknesses ranging from 3.5 to 100 nm was prepared at 300 °C. All films were grown as (001) single crystal with the same crystallographic orientations as the MgO(001) substrates. As evidenced by RHEED and x-ray diffraction, films with thicknesses larger than about 20 nm present a structural relaxation. The in-plane magnetization hysteresis loops obtained by longitudinal Kerr loop measurements exhibit a 〈100〉 fourfold cubic anisotropy and an enhancement of the magnetic softness with decreasing film thickness, both likely originating from the large interfacial strain configuration.

Modulation of interlayer exchange coupling by ion irradiation in magnetic tunnel junctions

Ion implantation of epitaxial Fe/MgO/Fe magnetic tunnel junctions exhibiting antiferromagnetic exchange coupling by spin polarized tunneling has been performed. X-ray reflectivity and transmission electron microscopy experiments were performed to extensively study the structural changes of the junctions. Electron holography and vibrating sample magnetometry experiments show that the amplitude and the nature of the coupling are modified by the irradiation. A gradual increase in the bilinear and the biquadratic coupling has been measured when the ion dose increases. For large doses, the net coupling becomes ferromagnetic. This corresponds to an intermixing of the MgO barrier with the adjacent Fe layers which is demonstrated by x-ray reflectivity.

The high Tc superconductor Bi2Sr2CaCu2O8 studied by electron microscopy. Influence of the partial substitution of Yb for Ca on the modulated structure

The incommensurate modulation of high-Tc superconductors of the type Bi2Sr2Ca1−yYbyCu2O8+x is characterised by electron diffraction. The results are in favour of an atomic displacive one-dimensional modulation with both transverse and longitudinal components. In a four-dimensional description of the modulated structure, the reflection conditions that are evidenced obey the relation: h+l+m=2n. The partial substitution of Yb cations for Ca proves to affect the electrical properties of these materials. Analysis of compounds having different Yb contents (0≤y≤0.66) reveals that the modulation wave vector decreases with increasing y until y=0.5.

Towards a unified description of the AMOB2O5 (A = K, Rb, Cs, Tl; M = Nb, Ta) family of compounds

Single-crystal X-ray diffraction data (Mo Kalpha radiation) are used to re-refine the structure of RbNbOB2O5, rubidium niobium oxo pyroborate. The structure is refined as an incommensurate modulated structure with superspace group symmetry Pmn2(1)(0,0.4,0)s and lattice parameters a = 7.406 (2), b = 3.939 (2) and c = 9.475 (2) A. Refinement on 3242 unique reflections converged to R = 0.031, while a previous conventional superstructure refinement led to R = 0.090. This lowering of the R factor goes hand-in-hand with a substantial reduction in the number of refined parameters. The refinement strongly suggests that the structure is effectively incommensurately modulated, despite an apparently rational magnitude of the primary modulation wave-vector and overlap of satellite reflections.

Sputter growth and magnetic properties of exchange-biased La1/4Ca3/4MnO3–La2/3Sr1/3MnO3 epitaxial bilayers

We have grown La1/4Ca3/4MnO3–La2/3Sr1/3MnO3 (LCMO–LSMO) thin films by reactive facing target radio frequency sputtering on SrTiO3(001) substrates. After 1 bar oxygen annealing at 800 °C, the films have the desired oxygen stoichiometry and perovskite structure. Transport properties of single LCMO films reveal the charge order (CO) transition between 100 and 180 K depending on the LCMO layer thickness while LSMO films have a Curie temperature greater than 330 K. Kerr effect measurements show evidence of an increase of the LSMO coercive field HC from ∼50 Oe at room temperature up to 110 Oe at 40 K and a significant decrease in the squareness of the hysteresis loop. This effect is accompanied by a slight shift of the hysteresis loop HE due to exchange bias between the antiferromagnetic charge ordered LCMO layer and the LSMO ferromagnet which reaches 20 Oe at 40 K. Both the onset of HE and the drop in squareness are observed to be close to the charge order transition of a similar LCMO single layer and are assu...

Controlled modifications of electron injection on Au/Si and Au/SiO2/Si contacts using ballistic electron emission microscopy

In this article, ballistic electron emission microscopy (BEEM) induced modifications on Au/Si and Au/SiO2/Si contacts are presented. BEEM current can be locally enhanced or reduced in a controlled manner. These observations are attributed to tip induced modifications on the gold surface. According to Au thickness, x-ray reflectivity experiments show different surface evolutions correlated to the size variations of the modifications introduced as a function of time.

Ferroelastic behaviour of the (Ln)Ba2Cu3O6+x orthorhombic phase

Abstract The orthorhombic phase of the high Tc superconductor (Ln)Ba2Cu3O6+A is observed in transmission electron microscopy. This phase is ferroelastic and exhibits two different orientation domains. A full description of the twin boundaries is given. The behaviour of the ferroelastic domains at the intersection of orthogonal twin walls is more particularly investigated.

Interlayer Exchange Coupling in Single Crystal Magnetic Tunnel Junctions Studied by Electron Holography

Ion irradiation has been performed on epitaxial Fe/MgO/Fe magnetic tunnel junctions exhibiting antiferromagnetic exchange coupling. Electron holography and VSM experiments show that the amplitude and the nature of the coupling are modified by the irradiation. We demonstrate that the roughness induced by the irradiation at the Fe/MgO interfaces destroys the AF coupling. The biquadratic coupling experimentally observed in the irradiated samples is the result of the competition between local antiferromagnetic and ferromagnetic coupling. For the largest irradiation doses, the net coupling becomes ferromagnetic.

Strain Accommodation and Relaxation Mechanisms in Epitaxially Grown Manganites

Epitaxial growth of La 1-x (SrorCa) x MnO 3 manganites (LSMO or LCMO) has been successfully achieved on SrTiO 3 (001) and MgO(001) substrates. We report the influence of the deposition conditions, in particular growth rate, annealing and nature of the substrate, on the state of strain in the manganite layers. Fully strained layers are reported at very slow growth rate on SrTiO 3 substrates while misfit dislocations are clearly observed in layers grown on MgO and alsoin layers grown on SrTiO 3 , after annealing at high temperature. Besides, evidence is given for a rhombohedral (respectively orthorhombic) distortion relaxation in LSMO (respectively LCMO) layers. The microstructure of the manganite layers is analyzed taking into account their non-isomorphic growth on the cubic substrates.