Picosecond control of coherent magnetisation dynamics in permalloy thin films by picosecond magnetic field pulse shaping

We have measured the response of a 20×10 μm, 8 nm thin NiFe (80:20) permalloy film due to excitation by short in-plane magnetic field pulses. We will show that using a two-pulse-technique a complete control of the precessional motion of the magnetisation can be achieved on picosecond timescales. Furthermore, we will present numerical calculations which show that a complete suppression of magnetisation ringing after switching can only be realised by a cascade of short field pulses.     

Magnetisation reversal in low-coercivity PrFeB materials

Models proposed to analyse magnetisation reversal in high-coercivity magnets should, in principle, apply to low-coercivity materials. Magnetisation reversal in low-coercivity PrFeB (μ₀H_c≈0.1 T at 4.2 K) is examined in the present paper within the so-called global model. For such low-μ₀H_c values, pinning is expected to occur. The parameters deduced from the analysis are consistent with this process. As expected from simple considerations on magnetisation reversal processes, the activation volume, v_a, is larger in low-coercivity PrFeB than in high-coercivity magnets. The quantitative analysis shows, however, that the reversal mechanisms are not identical.     

Role of oxygen impurity in growth and magnetic properties of Ni83Fe17 permalloy thin films

To study the influence of oxygen impurities in the sputtering atmosphere on microstructure, and the magnetic and magnetotransport properties, thin films of Ni₈₃Fe₁₇ were deposited under dc magnetron sputtering technique into which regulated oxygen gas was introduced. The partial pressure of oxygen was varied from 2×10⁻⁷ to 3×10⁻⁶ mbar. X-ray diffraction patterns indicate the reduction of grain growth with increasing the oxygen partial pressure. The grain microstructure and the composition were confirmed through high resolution transmission electron microscopy attached with Scanning Transmission Electron Microscopy (STEM). Transition from canted to rectangular magnetic hysteresis loop was observed through magnetization measurements for samples prepared under higher oxygen partial pressure which implies the structural changes in the magnetic domain formation. These observations were further confirmed through the measurements of anisotropic magnetoresistance properties.     

Superconductivity in Li3Ca2C6 intercalated graphite

In this paper, we report the discovery of superconductivity in Li₃Ca₂C₆. Several graphite intercalation compounds (GICs) with electron donors, are well known as superconductors [T. Enoki, S. Masatsugu, E. Morinobu, Graphite Intercalation Compounds and Applications, Oxford University Press, Oxford, 2003]. It is probably not astonishing, since it is generally admitted that low dimensionality promotes high superconducting transition temperatures. Superconductivity is lacking in pristine graphite, but after charging the graphene planes by intercalation, its electronic properties change considerably and superconducting behaviour can appear. Li₃Ca₂C₆ is a ternary GIC [S. Pruvost, C. Hérold, A. Hérold, P. Lagrange, Eur. J. Inorg. Chem. 8 (2004) 1661-1667], for which the intercalated sheets are very thick and poly layered (five lithium layers and two calcium ones). It contains a great amount of metal (five metallic atoms for six carbon ones). Its critical temperature of 11.15 K is very close to that of CaC₆ GIC [T.E. Weller, M. Ellerby, S.S. Saxena, R.P. Smith, N.T. Skipper, Nat. Phys. 1 (2005) 39-41; N. Emery, C. Hérold, M. d’Astuto, V. Garcia, Ch. Bellin, J.F. Marêché, P. Lagrange, G. Loupias, Phys. Rev. Lett. 95 (2005) 087003] (11.5 K). Both CaC₆ and Li₃Ca₂C₆ GICs possess currently the highest transition temperatures among all the GICs.     

Novel MR image contrast mechanisms in epilepsy

A range of magnetic resonance (MR) parameters are introduced, which can give rise to image contrast by using suitable pulse sequences, and that can be measured quantitatively. Their relationship to tissue pathology is given as far as possible. Techniques for their measurement, and results from multiple sclerosis, stroke, and epilepsy are given. The parameters are proton density, T₁, T₂, transverse magnetisation decay, which gives estimates of extracellular water and myelin concentrations, magnetisation transfer ratio and T_1sat, and diffusion (including trace and anisotropy measured from the tensor matrix).     

Precise estimate of fundamental in-vivo MT parameters in human brain in clinically feasible times

A methodology is presented for extracting precise quantitative MT parameters using a magnetisation-prepared spoiled gradient echo sequence. This method, based on a new mathematical model, provides relaxation parameters for human brain in-vitro and in-vivo. The in-vivo parameters have been obtained from three different regions of normal white matter: occipital white matter, frontal white matter and centrum semiovale; two regions of normal grey matter: cerebral cortex and cerebellum, and from five regions with MS lesions. All this has been achieved using MT images collected within a timeframe that is clinically feasible. We hope that this new technique will shed light on the properties and dynamics of water compartments within the brain.     

Application of the Stoner-Wohlfarth model with interaction for the determination of the saturation magnetisation, anisotropy field, and mean field interaction in bulk amorphous ferromagnets

Magnetic hysteresis curves of bulk amorphous ferromagnet alloys of composition Nd₆₀Fe₃₀Al₁₀, Nd₆₀Fe₂₀Co₁₀Al₁₀ and Pr₅₈Fe₂₄Al₁₈ have been measured in applied magnetic fields up to 9 T at temperatures in the range 10-350 K. The behaviour of the demagnetisation curve in the first quadrant is interpreted using a mean field interaction model as proposed by Callen et al. [Phys. Rev. B 16 (1977) 263], which extends the Stoner-Wohlfarth model [Philos. Trans. Roy. Soc. A 240 (1948) 599] for a random distribution of non-interacting uniaxial grains. Application of the mean field interaction model enables the determination of the saturation magnetisation M_s, anisotropy field H_a, and interaction parameter d, and from these other magnetic parameters, such as the anisotropy constant, K, are deduced. For the three alloys, the temperature dependent behaviour of M_s, H_a, d and K over the range 20-350 K are found to be qualitatively similar, though there are quantitative differences. In all cases M_s increases with decreasing temperature, both H_a and K increase with decreasing temperature, reaching a peak in the range 75-120 K, and then decreasing, and d decreases approximately linearly as the temperature decreases. The physical mechanisms responsible for coercivity in these materials are discussed in the context of random anisotropy and a strong pinning model of domain walls.     

Nucleation of magnetisation reversal, from nanoparticles to bulk materials

We review models for the nucleation of magnetisation reversal, i.e. the formation of a region of reversed magnetisation in an initially magnetically saturated system. For small particles, models for collective reversal, either uniform (Stoner–Wohlfarth model) or non-uniform like curling, provide good agreement between theory and experiment. For microscopic objects and thin films, we consider two models, uniform (Stoner–Wohlfarth) reversal inside a nucleation volume and a droplet model, where the free energy of an inverse bubble is calculated, taking into account volume energy (Zeeman energy) and surface tension (domain wall energy). In macroscopic systems, inhomogeneities in magnetic properties cause a distribution of energy barriers for nucleation, which strongly influences effects of temperature and applied field on magnetisation reversal. For these systems, macroscopic material parameters like exchange interaction, spontaneous magnetisation and magnetic anisotropy can give an indication of the magnetic coercivity, but exact values for nucleation fields are, in general, hard to predict. To cite this article: J. Vogel et al., C. R. Physique 7 (2006).     

Centric-scan SPRITE magnetic resonance imaging with prepared magnetisation

The combination of contrast preparation with centric-scan SPRITE imaging readout is investigated. The main benefit of SPRITE, its ability to image objects with short T2, is retained. We demonstrate T1 and T2 mapping as examples of magnetisation preparation followed by magnetisation storage and spatially resolved encoding. A strategy for selection of the most advantageous imaging parameters for contrast mapping is presented.     

Kinetic investigation of a ruthenium metathesis catalyst

The complex [(IMesH₂)(PPh₂Cy)Cl₂RuCHPh] was synthesised and shown to be an active catalyst in ring-closing metathesis of a diallylmalonate. Its phosphine exchange was investigated in C₆D₆ using magnetisation transfer ³¹P NMR spectroscopy and it was found to operate via a dissociative mechanism with k³⁵³ = 4.1 ± 0.9 s⁻¹, ΔH^‡ = 84 ± 10 kJ mol⁻¹ and ΔS^‡ = 4 ± 28 J mol⁻¹ K⁻¹.