Showing papers by "Albert Fert published in 1992"

Conductivity and magnetoresistance of magnetic multilayered structures.

Abstract: We consider multilayered structures consisting of magnetic and nonmagnetic metals. We derive analytic expressions for the conductivities by treating the scattering at the interfaces between layers in the same way as that throughout the layers (bulk). The application of an external magnetic field reorients the magnetization in the magnetic layers, which in turn alter the mean free path of the conduction electrons. This is the origin of the giant magnetoresistance seen in iron-chromium (Fe/Cr) superlattices. We present analytic results on the magnetoresistance in limiting cases where either the mean free path is much greater or less than the layer thickness, as well as numerical results for the realistic situation found in Fe/Cr superlattices when they are comparable.

Structural characterization of Fe/Cu multilayers by x-ray absorption spectroscopy.

Abstract: The local structure of Fe/Cu multilayers prepared by sputtering is studied by x-ray absorption spectroscopy (XAS). We find that thin Cu layers grow with the bcc structure imposed by the Fe layers. For larger thicknesses, the Cu films are more disordered and beyond 13 \AA{} the films relax to fcc structure. The thickness dependence of the magnetoresistance of the multilayers is discussed in the light of the XAS results.

Interplay between oscillatory exchange coupling and coercivities in giant magnetoresistive [Ni80Fe20/Cu/Co/Cu] multilayers

Abstract: We have observed giant magnetoresistance in [Ni80Fe20/Cu/Co/Cu] multilayers with well‐defined oscillations in magnitude as a function of the Cu layer thickness both at 300 and 4.2 K. The phase and period are found to be very similar to those previously measured in Co/Cu and, more recently, Ni81Fe19/Cu multilayers. However, the existence of a strong contrast in coercive fields between Ni80Fe20 and Co leads to a significant magnetoresistance for Cu layer thicknesses where coupling is zero or even moderately ferromagnetic. At room temperature, resistance changes as large as 7% are observed within a few tens of oersted of zero field.

Structure cristallographique de multicouches métalliques et magnétiques étudiées par spectroscopie d'absorption X

Abstract: The results of X-ray Aborption Spectroscopy (XAS) measurements on magnetic Co/Cu and Fe/Cu multilayers show the capability of this technique to point out the dependence of crystallographic phase on the layers thickness and the presence of structural anisotropies and strain in these multilayers

Study of vortex dynamics in YBa2Cu3O7−x by hysteresis cycle measurements

Abstract: An interesting point of investigation in high T c superconductors concerns the problems of pinning of the vortices. It is known that their motion can be monitored on the one hand by the ratio between their pinning energy and the thermal activation energy, and on the other hand by the driving forces generated by an applied magnetic field. For a temperature near T c , a flux flow behaviour is expected, characterized by a frequency dependence of the motion of flux lines. Hysteresis cycles in a.c. magnetic field were measured for a YBa2Cu3O7−x single crystal with the applied field perpendicular to the c axis. The hysteresis losses were studied as a function of field amplitude (0–250 Oe), frequency (0.5–100 Hz) and temperature (4.2–90 K). It is pricipally shown that near T c the hysteresis loops are frequency dependent, and that the associated losses present, for aiven amplitude, a maximum for a temperature T ∗ corresponding to full field penetration in the sample. The behaviour of the hysteresis losses as a function of frequency is analysed on the basis of a flux flow model.

Mechanisms of the giant magnetoresistance in magnetic multilayers

Abstract: t m d ~ T h z a n t magnetoresistance (GMR) or spin valve effect was first observed in the FdCr system [ 1-21. In the Cr thickness ranges where the interlayer exchange coupling is antiferromagnetic (Am, the resistivity drops when an applied field overcomes this coupling and aligns the magnetic moments of all the layers. Similar MR effects have now been found in many multilayered magnetic structures, primarily in multilayers with AF interlayer exchange 13-61 and also in systems, such as CdCu/NiFe or NiFdCu/NiFefFeMn. where the AF arrangement arises from some difference in the coercivity of neighbor magnetic layers [7]. Recently MR effects have also been found with the current perpendicular to the layer and they are even larger in this configuration [8]. Finally, other transport properties are also affected by the flip from antiferromagnetic (Am to ferromagnetic (F) arrangement. Recently a "giant magnetothermcclectric poweSj.e. a large field induced change of the thermoelectric power, has been found in CdCu, FdCu, Fe/Cr and CdCu/NiFe srmctures 191 . The GMR is ascribed to spin dependent electron scattering in the magnetic layers (spin dependent s c a r i n g , i.e.different scattering rates for the majority and minority electrons, is a wellknown property of the itinerant ferromagnets [ 101 ). Review of Exwrimental Results on FdCr, CoICn and FdCo. In FdCr, the MR is very high at low temperature-up to 108% in Fe(M)l)/Cr(OOl) superlattices grown on MgO(001)-but decreases substancially as the temperature increases. The microstructure of the FdCr interfaces appears to the particularly importaot for the magnitude of the effect. Small changes of the p r e p a d o n parameters or addition of impurities at the interfaces can change significantly the MR ratio. FdCr is an example of multilayer in which the spin dependent scattering responsible for the h4R takes place at the interfaces. In Co/Cu[4], the MR is a!so very large but , compared to FdCr, the saturation fields are lower and there IS a relatlvely small reduction of the MR with temperature. For ColSA/Cu9A, we have found that MR reaches 78% at 4.2K and 48% at room temperature. The variarion of the MR on the thickness of Cu for 15A thick CO layers, is shown in fig. 1. The MR exists only if there is some degree of AF arrangement at low field , so that the succession of thickness ranges with AF and F exchanges gives rise to oscillations of the MR. The decrease of the amplitude of the MR peaks at increasing Cu thickness is mainly due to the decoupling of the spin dependent scatterings in different magnetic layers, as predicted by theory . Another contribution comes from the crossover from strong coupling (perfect AF arrangement at low field) to uncoupled limit (random arrangement at low field. In FdCu, the behavior of the MR is very similar to that in Co/Cu, but the MR is smaller y d more dependent on temperature[6]. Jbfscand Tbeoretienl Models The mechanism of GMR can be described simply. When the magnetic moments of all the layers arc parallel, the scattering is weak in one of the spin channel and strong 4