Transmission electron microscopy study of unhydrided,dehydrided and annealed LaNi5

The influence of hydrogen absorption on the microstructure of LaNi5 powders has been investigated by transmission electron microscopy. At the surface of the unhydrided and dehydrided LaNi5 grains a reaction layer is observed. By means of selected area electron diffraction this layer is found to consist of metallic nickel and a second constituent, most probably La203. This observation suggests that this surface layer is not induced by the hydrogen absorption reaction. Within dehydrided LaNi5 grains planar-like defects, which are formed during previous hydrogen absorption, were observed. The habit plane, strain field and average spacing of these planar-like defects have been determined. From the correspondence between the above characteristics and the reduced coherence length perpendicular to the c-axis, as derived from anisotropic peak broadening in X-ray diffraction analyses, it was concluded that a correlation exists between the presence of planar-like defects and peak broadening. This correlation is further sustained by the fact that they both disappear upon annealing at 600°C

In-situ X-ray diffraction : a useful tool to investigate hydride-formation reactions

A high-pressure X-ray diffraction (XRD) cell has been designed which allowed us to study simultaneously hydrogen absorption/desorption isotherms and XRD powder diffraction patterns on (de)hydrided intermetallic compounds. The hydride formation reaction was investigated in the case of LaNi5 under both dynamic and steady-state conditions. The a-to-ß phase transition was monitored throughout the plateau region. The discrete a-to-ß lattice expansion, which amounted to more than 21 vol.%, was found to be much larger than the crystallographic expansion of both solid solutions in the pressure range measured (= 10 bar). The discrete phase transformation is thought to be responsible for the formation of planar-like defects, as observed by transmission electron microscopy, and, subsequently, for the cracking of powder particles. The formation of these crystallographically oriented defects has been correlated with the reduced coherence length and increased strain, which could be calculated from the anisotropic peak broadening found in the XRD diagrams. Annealing of dehydrided LaNi5 induced the planar-like defects to disappear and, consequently, to sharpen the relevant XRD reflections

On the ferromagnetic interlayer coupling in exchange-biased spin-valve multilayers

The ferromagnetic interlayer coupling in sputter-deposited permalloy/copper/permalloy exchange-biased spin valve multilayers has been measured as a function of the copper thickness. The variation with thickness may, for tcu>1.7 nm, be analyzed in terms of the Neel model for magnetostatic coupling due to correlated interface roughness, using parameters which are consistent with the observed microstructur

In-situ X-ray diffraction : a useful tool to investigate hydride-formation reactions

A high-pressure X-ray diffraction (XRD) cell has been designed which allowed us to study simultaneously hydrogen absorption/desorption isotherms and XRD powder diffraction patterns on (de)hydrided intermetallic compounds. The hydride formation reaction was investigated in the case of LaNi5 under both dynamic and steady-state conditions. The a-to-ß phase transition was monitored throughout the plateau region. The discrete a-to-ß lattice expansion, which amounted to more than 21 vol.%, was found to be much larger than the crystallographic expansion of both solid solutions in the pressure range measured (= 10 bar). The discrete phase transformation is thought to be responsible for the formation of planar-like defects, as observed by transmission electron microscopy, and, subsequently, for the cracking of powder particles. The formation of these crystallographically oriented defects has been correlated with the reduced coherence length and increased strain, which could be calculated from the anisotropic peak broadening found in the XRD diagrams. Annealing of dehydrided LaNi5 induced the planar-like defects to disappear and, consequently, to sharpen the relevant XRD reflections

Grain Size and Strain in Sputter Deposited Ni0.8Fe0.2 and Cu Films

The average grain size and strain in the direction parallel to the surface of thin Ni0.8Fe0.2 and Cu films, sandwiched between Ta layers, have been determined as a function of layer thickness by grazing incidence X-ray diffraction. The in-plane grain size and grain size distribution were also assessed by plan-view transmission electron microscopy. Standard ¿-2¿ X-ray powder diffraction was used to determine the uniform strain in the direction perpendicular to the surface. Both for Ni0.8Fe0.2 and Cu, an elongation of the lattice parameter perpendicular to the surface and a compression of the lattice parameter in the plane of the film is observed, which decreases with increasing film thickness. Additionally, for Ni0.8Fe0.2 a non-uniform elongation of the perpendicular interactomic distance at the Ta interfaces is deduced by fitting a kinematical model to the ¿-2¿ diffraction spectrum. This study illustrates the strength and the complementary character of standard powder X-ray diffraction, grazing incidence X-ray diffraction and transmission electron microscopy for the structural analysis of thin metal films

Grain Size and Strain in Sputter Deposited Ni0.8Fe0.2 and Cu Films

The average grain size and strain in the direction parallel to the surface of thin Ni0.8Fe0.2 and Cu films, sandwiched between Ta layers, have been determined as a function of layer thickness by grazing incidence X-ray diffraction. The in-plane grain size and grain size distribution were also assessed by plan-view transmission electron microscopy. Standard ¿-2¿ X-ray powder diffraction was used to determine the uniform strain in the direction perpendicular to the surface. Both for Ni0.8Fe0.2 and Cu, an elongation of the lattice parameter perpendicular to the surface and a compression of the lattice parameter in the plane of the film is observed, which decreases with increasing film thickness. Additionally, for Ni0.8Fe0.2 a non-uniform elongation of the perpendicular interactomic distance at the Ta interfaces is deduced by fitting a kinematical model to the ¿-2¿ diffraction spectrum. This study illustrates the strength and the complementary character of standard powder X-ray diffraction, grazing incidence X-ray diffraction and transmission electron microscopy for the structural analysis of thin metal films

Influence of grain size on the transport properties of Ni80Fe20 and Cu thin films

The conductivity of sputtered Ta/Cu/Ta and Ta/Ni80Fe20/Ta thin films was measured for a film thickness ranging from 20 to 1500 Angstrom. The measured data were analyzed using a semi-classical model for the electron transport, that includes grain boundary scattering. It was found that in these films grain boundaries are an important source of electron scattering, in Ni80Fe20 leading to an effective spin dependence of the scattering which is considerably smaller than the intrinsic spin dependence of the scattering