On the stability of bcc Co in Co/Fe superlattices an NMR and XRD study

While RHEED observations show that 10 to 11 As is the stability limit for an open bcc Co layer when grown on an Fe substrate, our XRD and NMR studies have shown that, in MBE grown Co/Fe superlattices, cobalt can be stabilised in a bcc structure up to a critical Co thickness of 21 Ås. In order to understand this apparent discrepancy, NMR experiments have been carried out in Co_x/Fe_y multilayers with thickness varying in the range 5 Å < x < 42 Å and 24 Å < y < 60 Å, grown on GaAs (1 1 0) as well as on MgO (1 0 0) substrates. The analysis of the chemical short range order by NMR concludes that the larger bcc Co thickness observed in superlattices results from the formation of a rather homogeneous CoFe_20% bcc alloy which contains the supplementary 10–11 As of Co and which coexists with pure Co grains. The concentration of about 20% Fe in the alloyed part of the Co layer happens to be close to the stability limit for a bcc structure in the equilibrium phase diagram of bulk CoFe alloys. However, while a mixture of bcc and fcc phases is observed in bulk alloys, the bcc structure is preserved in all phases under the MBE growth conditions and below the critical thickness. Above the critical thickness amixture of bcc Co, bcc CoFe and hcp Co is observed.     

Stabilisation of fcc cobalt layers by 0.4 nm thick manganese layers in Co/Mn superlattices

Epitaxial Co/Mn multilayers (0.75 to 6 nm Co, 0.4 nm Mn layer thickness) have been grown on mica substrates covered by a (0002) Ru buffer layer. The structural properties of these layers have been studied using X-ray diffraction, nuclear magnetic resonance (NMR), and high resolution transmission electron microscopy (HRTEM). The Co layers, grown as face centred cubic (fcc), were found to be stabilised by the very thin Mn layers. Data obtained using X-ray diffraction and NMR were analysed and found to be in good agreement, while Monte-Carlo simulations were used to interpret the data and calculate the expected diffracted intensity and NMR spectra. The HRTEM data show that the Mn layers give rise to a large strain contrast extending, in the growth direction, over a distance which exceeds the thickness of the Mn layers. The superlattices could be described as having an fcc structure containing randomly located stacking faults with varying densities. The results verify the presence of a dominant, almost perfect phase of fcc stacking, and of a faulted hcp phase, while the number of defects increases with the Co layer thickness. Received 27 October 1999 and Received in final form 29 May 2000     

New phases and chemical short range order in co-deposited CoFe thin films with bcc structure: an NMR study

A ⁵⁹Co NMR study has been carried out on several series of co-evaporated Co₁-_xFe_x thin-film alloys prepared on MgO (001), GaAs (100), and GaAs (110) substrates at deposition temperatures between 175°C and 500°C. The sample thicknesses varied between 100 Å and 1000 Å and the alloy concentrations were in the range 0:1 < x < 0:3. X-ray diffraction and NMR show that the stability limits of the bcc phase in CoFe alloys is shifted from the x = 0:25 observed in the bulk alloys down to about x = 0:11 in thin films. For x = 0:27 and at the deposition temperature of 500°C, a new ordered phase has been stabilised where Co has two Fe atoms only in its first coordination shell. Other samples, grown at lower temperatures, also exhibit an exotic chemical short range order (CSRO) where Co coordinations with zero and two Fe neighbours dominate. A mixture of bcc Co (and not fcc Co as in the bulk alloys) and unknown ordered bcc intermetallics can account for the observed CSRO. Theoretical ground-state phases for the bcc lattice are considered in order to explain the observations.     

31P and 11B NMR in amorphous NiPB alloys near the paramagnetic-ferromagnetic transition

Knight-shift and nuclear spin-lattice relaxation time measurements have been performed between 4.2°K and room temperature on ¹¹B and ³¹P in amorphous NiPB alloys near the para-ferromagnetic transition. The EFG parameters on ¹¹B were found to be ν_Q=200(±20)kHz and η = 0.35 (±0.10). Knight-shift and Korringa spin-lattice relaxation are mainly due to mechanisms involving p electrons. The effect of Ni magnetic clouds results in a broadening of the linewidth. We observed also the occurence of a Giovannini-Heeger-like contribution to the spin-lattice relaxation rate.     

Absence of antiferromagnetism in cobalt diselenide compounds with pyrite structure

We summarize some recent N.M.R. and susceptibility measurements performed on cobalt diselenide samples having the pyrite structure. Our results show unambiguously that the low temperature anomalies observed on the magnetic susceptibility are not related with any magnetic ordering.     

Hyperfine interaction studies of amorphous materials using NMR

Through hyperfine interactions the electronic structure of amorphous materials can be investigated by NMR. Furthermore, the local character of these interactions makes them adequate to study the local environment and the local symmetry around the probe nuclei through the measurement of hyperfine fields and electric field gradients. We review recent NMR studies of amorphous metals from the following two points of view: (1) Electronic structure: Knight shifts and spin relaxation times in paramagnetic materials and hyperfine fields in ferromagnetic materials. (2) Local amorphous structure: topological and chemical short-range order. Emphasis will be given to the comparisons between amorphous materials and corresponding crystalline materials.     

Hyperfine field and ordering in bcc CoFe bulk alloys studied by 59Co NMR and Monte-Carlo simulation

Bcc CoFe alloys and ordered compounds with Fe concentrations of 50 at.%, 40 at.% and 30 at.% have been studied by ⁵⁹Co NMR. Spectra with regularly spaced satellites are observed and interpreted as resulting from a step increase of the resonance frequency with each additional Fe impurity in the first coordination shell. The influence of the second-neighbour shell is also demonstrated. Satellite position analysis leads to the relationship between the hyperfine field and the first- and secondneighbour shell composition, which is found valid for the three compositions both in ordered compounds and disordered alloys. This relationship is used to determine the chemical short range order ruling the compound structure. The analysis reveals that the probability for Co to be found in perfectly ordered stoichiometric regions is higher than expected from a random distribution of the excess Co in off-stoichiometric compounds. Monte-Carlo simulations indicate that such short range order would result not only from first-neighbour interactions but also from rather large second- and/or third-neighbour interactions. However, the observations made during isothermal annealing show that the phase transformation takes place through a nucleation and growth process and not by way of homogeneous ordering. Therefore, the final short range order is also discussed in terms of kinetics of the phase transformation.     

NMR analysis of buried metallic interfaces

Hyperfine Interactions - 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...