Dipolar coupling effect in magnetic bilayer system

Equilibrium micromagnetic structures in a bilayer system composed of two thin cobalt films separated by a non magnetic spacer are systematically analysed. These 2D magnetization distributions are obtained by numerical computations according to different set of magnetic and geometric parameters. The coupling effect due to the dipolar long range interaction (or stray-field effect) between the two Co layers is studied through the evolution of the magnetic pattern in the stack with or without an applied field and compared to a continuous film of same thickness. Special attention is paid to the hysteresis process in a bilayer. Even though the general aspect of the magnetization distribution looks like a Landau-Lifshitz structure, the absence of any core in the vortex of the magnetic structure is analysed in relation to a possible disappearence of (topological) hysteresis

Transverse magnetic susceptibility of thin films and multilayers exhibiting perpendicular anisotropy

A full analysis of domain structure using a micromagnetic model is developed in order to get a clear insight into the behaviour of transverse initial susceptibility as a function of dc applied field for thin films and bilayers exhibiting both in-plane and perpendicular anisotropy. The numerical simulations are in good agreement with available experimental results in case of single layers with the so-called stripe domain pattern while some predictions are done for bilayers. As the main result, it is shown that in low field, the transverse initial susceptible cannot be interpreted without the knowledge of the static domain pattern while, above saturation, it is only affected by the in-plane anisotropy

Magnetic domains studies in strongly and weakly exchange coupled Co/NiO bilayers

Results of the Auger electron spectroscopy measurements with depth profiling showed that at the Co-NiO interface only oxygen is present, making it very likely that only NiO is formed and no other nickel or cobalt compounds, which grow apparently with smaller probability. It has been also found that the average exchange coupling energy for the Co-NiO interface strongly depends on the preparation conditions. For the Co layers with strong interface exchange coupling, we have observed large uniform domains and 180° walls. On the other hand, the Co layers with the weak interface coupling showed large domains with a strong ripple structure and non-uniform 180° walls

Appareil de mesure des chaleurs spécifiques entre 0,3 °K et 3 °K en régime thermique oscillant

An apparatus for measurements on samples with low heat capacities between 0,3 °K and 3 °K is described, including a He3 cryostat, a vapor pressure thermometer and a CMN magnetic thermometer. An ac temperature calorimetry technique has been used. The various physical and technical problems involved are discussed in detail. Measurements of the heat capacities of two copper samples are presented which illustrate the capabilities of the apparatus.On décrit un appareil destiné à mesurer des capacités calorifiques faibles entre 0,3 °K et 3 °K et comprenant un cryostat à He3, un thermomètre à tension de vapeur et un thermomètre magnétique à CMN. On a mis en oeuvre une technique calorimétrique en régime thermique oscillant. Les différents problèmes physiques et techniques qui se posent sont discutés en détail. Pour illustrer les performances de cet appareil, on présente des mesures de la capacité calorifique de deux échantillons de cuivre

NMR evidence for critical fluctuations of the electronic density in the one-dimensional conductor K2Pt(CN)4Br0.33 H2O

The Peierls transition in the one-dimensional conductor K2Pt(CN) 4Br0.33 H2O has been studied by 195Pt nuclear magnetic resonance. The Pt Knight shift, constant in the temperature range of the Peierls insulator, increases monotonically between 120 and 330 K indicating that the metallic state is not yet established at room temperature. We attribute the extremely short spin-lattice relaxation time T1 and the large line width observed between 150 and 300 K to one-dimensional critical fluctuations of the electronic density of states at the Fermi level. These results clearly place the mean-field Peierls transition temperature well above 300 K.Nous avons étudié la transition de Peierls dans le conducteur à une dimension K2Pt(CN)4 Br0,33 H2O, par résonance magnétique nucléaire du 195Pt. Le déplacement de Knight du Pt, constant dans le domaine de température de l'isolant de Peierls, augmente de façon monotone entre 120 et 330 K, ce qui indique que l'état métallique n'est pas encore atteint à température ambiante. Nous attribuons le temps de relaxation spin réseau T1 extrêmement court, et la forte largeur de raie observée entre 150 et 300 K à des fluctuations critiques de la densité d'état électronique au niveau de Fermi. Ces résultats situent la température de Peierls définie à partir de théories de champ moyen nettement au-dessus de 300 K

Static and dynamic properties of ferromagnetic thin films with stripe domain structure

We investigate the properties of ferromagnetic films deposited by magnetron sputtering, with a stripe domain structure. The domain structure is visualized by Kerr microscopy. We propose a model based on the minimization of a free energy of the stripe system, which allows us to fit the complete hysteresis curve. We can deduce the domain period and the magnetic parameters from the experimental hysteresis curve. We obtain a reasonable agreement on the stripe width with our observations, and with previous works. The dynamic response in the 10 MHz to 3 GHz range is also studied. The resonance frequencies are computed by the Smit and Beljers method. The calculated resonance frequencies are in good agreement with our observations

Magnetization reversal via bloch points nucleation in nanowires and dots: a micromagnetic study

Micromagnetic studies of the reversal of stripe domains in elements of different geometries are reported. Various 2D and 3D codes are used in order to allow comparison between an infinite slab, nanowires and dots. Starting from a saturated state the establishment of stripe domains through the sample is first studied. Contrary to the thin film case, the nucleation of this pattern is not uniform and is very different to the geometry of a wire or an isolated dot. Special attention is paid to the reversal of the core of the vortex (the remnants of inner walls between up and down domains) which requires the insertion of point singularities called Bloch points (BP) either at surfaces or created in a pair depending on the length allowed for the stripe in the corresponding element. The magnetization distribution around the core of the various BP is described in detail as well as the key characteristics of their motion. Finally, some experiments are suggested predicting the behaviour of stripe domains under an applied field. Copyright EDP Sciences/Società Italiana di Fisica/Springer-Verlag 2005