J.P. Jay

NMR analysis of buried metallic interfaces

Owing to the sensitivity of the hyperfine field to the topological and chemical environment of the probe nuclei, NMR spectra can be considered as detailed histograms of the short range order ruling the structure of the material under investigation. Complementary to diffraction techniques this gives a local insight into the structure in the direct space. We present here recent developments of the method which have been used for structural investigations of cobalt layers imbedded in Co/X multilayers and particularly of buried interfaces. Indeed the use of spectrum modelling allows a detailed, quantitative, description of the way intermixing takes place at the interfaces. Co/Cu multilayers, a case of weakly miscible elements, have been specially investigated. Such a case is the most likely to yield sharp interfaces. Actually, the interface structure is shown to depend drastically on the preparation conditions and techniques, resulting in very different interfaces from abrupt to largely mixed. Solid solution forming elements (Co/Ru or Co/Cr) have also been studied for which interdiffusion at the interfaces is more expected. Indeed thick diffusion profiles, across several atomic planes, are observed which are much less dependent on the preparation technique than in the former case. The case of compound forming elements (Co/Fe) shows the largest interface admixture with a structure that seems also reminiscent of the bulk alloy phase diagram.

Growth of Co/Ru strained superlattices

Abstract We demonstrate the growth of epitaxial Co/Ru superlattices — despite the large lattice mismatch (≈ 8%) — consisting of hep (0001) Co and Ru sublayers. The evaporation of the first Co monolayers onto the Ru buffer follows a layer by layer mechanism after which the Co nucleates into islands. Throughout the sample growth, the average in-plane lattice parameter relaxes towards the bulk values within 15 A suggesting two growth regimes which induce dramatic effect on the magnetic properties.

Structural properties and oscillatory magnetoresistance of Co(hcp)/Cu sandwiches

Abstract We present structural and magnetoresistance studies for a series of Co(hcp)/Cu sandwiches. RHEED patterns obtained during growth indicate a hcp structure for at least the first Co layer. NMR measurements show a single resonance peak at a frequency of 228 MHz, clearly confirming the hcp structure of both Co layers. The occurrence of the hcp phase for the Co atoms is mainly due to the growth of a thin 8 A Cu seed layer on the Ru buffer layer. We performed magnetoresistance studies at room temperature and found oscillatory behaviour of the magnetoresistance with a period of about 13 A, which is smaller than the values usually observed for fcc (111)Co/Cu systems. The magnetoresistance value at the first maximum reaches 4% at room temperature, which indicates the good quality of these samples.

Nmr Studies of Bulk and Interface Structure in Co Based Multilayers

Owing to the sensitivity of the hyperfine field to the topological and chemical environment of the probe nuclei, NMR spectra can be considered as histograms of the short range order ruling the structure of the material under investigation. Complementary to diffraction techniques this gives a local insight on the structure in the direct space. We review recent structural investigations of cobalt layers imbedded in Co/X multilayers and particularly of buried interfaces. Special attention has been given to the way intermixing takes place at the interfaces as its influence on the multilayer properties may be of considerable importance. Co/Cu multilayers, a case of weakly miscible elements, have been specially investigated owing to their GMR properties. But also cases of solid solution forming elements (Co/Ru or Co/Cr) or compound forming elements (Co/Fe) have been thoroughly studied. The latter case, which shows a stabilization of a bcc Co phase, will be discussed against the bulk alloy phase diagram.

Structure and oscillatory magnetoresistance of Co(hcp)Cu sandwiches

Abstract We present structural and magnetoresistance studies performed on a series of Co(hcp) Cu sandwiches. We report reflection high-energy electron diffraction analysis and nuclear magnetic resonance measurements showing clearly the hcp structure of the Co layers and we find an oscillatory behaviour of the magnetoresistance with a period of about 13 A.

Structure and oscillatory magnetoresistance of sandwiches

Abstract We present structural and magnetoresistance studies performed on a series of Co(hcp) Cu sandwiches. We report reflection high-energy electron diffraction analysis and nuclear magnetic resonance measurements showing clearly the hcp structure of the Co layers and we find an oscillatory behaviour of the magnetoresistance with a period of about 13 A.

59Co NMR study in Co–Fe alloys/Co magnetite composites

Abstract Iron–cobalt compounds containing 20–30 at.% Fe with unusual crystallographic structure have been observed in metal-oxide composite synthesized in an aqueous media at 120–140°C. The oxide is a cobalt-containing magnetite. The metallic component is found in two crystallographic structures depending on the preparation conditions: a b.c.c. (α-Fe) structure and an α-Mn, also called Re 24 Ti 5 , structure (space group I- 43 m ). The α-Mn-like CoFe alloy is encountered for the first time and transforms into the b.c.c. structure during annealing at temperatures as low as 160°C. In the b.c.c. structure itself, the cobalt and iron atoms present a chemical short-range order totally different from the regular B2 phase of the ordered bulk CoFe compound. This particular order is similar to the one encountered in high vacuum co-deposited thin films. For annealing temperatures larger than 400°C, the metallic component loses some Fe and transforms into nearly pure f.c.c. cobalt. This study shows that chemical as well as physical low temperature preparation techniques favor original CoFe ordered phases not achievable by conventional metallurgy.

Structure and giant magnetoresistance in Co/Cu sandwiches with thin Ag layers at the interfaces

Abstract We present structural and magnetoresistance studies performed on Co/Cu sandwiches with thin Ag layers at the interfaces. We study the effect on the Co structure induced by the insertion of Ag. We find a phase shift of half a period in the magnetoresistance oscillations for the samples including Ag at the interfaces.

Coupling mechanism in Co/Ru sandwiches with thin spacers

Abstract A study of the indirect exchange coupling of Co/Ru sandwiches for small Ru spacer thicknesses has been performed. For critical thicknesses of the spacer, a large curvature of the M - H loops can be observed and its origin lies in a competition between areas with different couplings. These areas are smaller than the magnetic coherence length of the cobalt layers, and the existence of a large extrinsic biquadratic coupling is likely to occur. A strong interdiffusion at the interfaces seems to be determinant for the coupling mechanism.

Stabilization of Thick Fcc Cobalt Layers by One Monolayer of Manganese in Co/Mn Superlattices

Epitaxial Co/Mn superlattices (0.6 to 4.8 nm thick Co) have been grown on (0002) hcp Ru buffer layer on mica substrates. The face centered cubic (fcc) phase of cobalt is stabilized by the very thin manganese layer. The structural properties of these layers have been studied through x ray diffraction and nuclear magnetic resonance.