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One of the critical challenges during fabrication of magnetic tunnel junctions is to avoid the magnetic bottom electrode oxidation, considered as a spin depolarization source that lowers the tunneling magnetoresistance effect. In this paper, we present a study of the magnetic properties of Co/MgO bilayers as a function of several sputtering growth parameters by using a superconducting quantum interference device magnetometer. We demonstrate the high sensitivity of the magnetic properties to the oxidation of the Co layer and identify the relevant parameters to optimize the MgO growth process. [ABSTRACT FROM AUTHOR]

We have studied the current-induced displacement of a domain wall (DW) in the permalloy (Py) layer of a Co/Cu/Py spin-valve structure. At zero and very small applied fields (<10 Oe), the displacement is in opposite direction for opposite dc currents, and the current density required to move DW is of the order of a few 106 A/cm2. At higher applied magnetic fields, the DW motion, even though triggered by the current, has its direction controlled by the field. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We have investigated, by means of ac susceptibility experiments, the magnetic relaxation of layers of Co nanoclusters (D ≃ 2.6 nm) embedded in Al[SUB2]O[SUB3]. Superparamagnetic blocking takes place at higher temperatures as the number of layers increases. These results are interpreted using a simple model in which dipole--dipole interactions between nearest neighbor particles increase the relaxation time. The influence of interactions is affected by the application of bias magnetic field H. As the magnetic moments of the particles become polarized by H, the blocking temperature approaches the behavior expected for noninteracting particles. [ABSTRACT FROM AUTHOR]

We present experimental results on the displacement of a domain wall by injection of a dc current through the wall. The samples are 1-μm-wide long stripes of a CoO/Co/Cu/NiFe classical spin-valve structure. The stripes have been patterned by electron-beam lithography. A neck has been defined at 1/3 of the total length of the stripe and is pinning center for the domain walls, as shown by the steps of the giant magnetoresistance curves at intermediate levels (1/3 or 2/3) between the resistances corresponding to the parallel and antiparallel configurations. We show by electric transport measurements that, once a wall is trapped, it can be moved by injecting a dc current higher than a threshold current of the order of magnitude of 10[SUP7] A/cm[SUP2]. We discuss the different possible origins of this effect, i.e., local magnetic field created by the current and/or spin transfer from spin-polarized current. [ABSTRACT FROM AUTHOR]

We report on the structure of cobalt cluster films obtained by aggregation of metal atoms sputter-deposited on amorphous alumina. The cluster layers were encapsulated with a second amorphous alumina layer, also made by sputtering in the same chamber. Electron energy loss spectroscopy (EELS) indicates that encapsulation with sputtered alumina keeps the clusters free from cobalt oxide. Transmission electron microscopy (TEM) exhibits relatively narrow distributions of the cluster sizes and of the inter-cluster distances, which results in a noticeable local order. Size distributions appear quasi-Gaussian, but TEM misses an important number of small particles. Extended X-ray absorption fine structure (EXAFS) data shows that the actual average sizes are smaller, and Monte Carlo simulations suggest that the actual distributions could be bimodal, with a secondary peak in the small-size range. [ABSTRACT FROM AUTHOR]

We have combined in situ reflection high energy electron diffraction, high-resolution transmission electron microscopy, and magnetotransport experiments to investigate the role of a thin inserted Mg layer on the crystalline texture of MgO barriers in magnetic tunnel junctions grown in a standard sputtering system. It was found that an ultrathin Mg layer of 2–6 Å can efficiently promote a MgO (001) texture prior to any annealing. For thicker Mg layers, the MgO (001) texture was found to degrade due to the hexagonal structure of Mg. For tunneling magnetoresistance (TMR), the efficient role of the MgO crystallization induced by the Mg layer appears after a 400 °C annealing. The optimum was found for a 4 Å inserted Mg layer with a TMR of 120% at room temperature (210% at 3 K) which could be considerably improved for fully (001) oriented magnetic tunnel junctions. [ABSTRACT FROM AUTHOR]

The growth of a nanocluster assembly on a processed AlOx and its electronic transport behavior are investigated. An Ar+ plasma was used in order to create defects of high trapping energy on an AlOx layer. This process promotes a shift of the cluster distribution towards smaller sizes and allows to sharpen it. In a second part, the authors show that the application of this techniqueto clusters inserted in a tunnel junction can lead to a dramatic increase in the temperature range over which Coulomb blockade can be observed. [ABSTRACT FROM AUTHOR]

We show that a single current pulse as short as 0.4 ns can trigger domain wall (DW) displacement in spin-valve stripes of 0.3 μm width inserted into a coplanar waveguide. The experiments were carried out with varying current pulse amplitude, duration, polarity, and applied static magnetic field. In zero field, DW displacement occurs in the same direction as the conduction electron current. In finite applied field, the direction of DW displacement is that favored by the field orientation. In both cases, the DW displacement occurs only above a critical current density jc of the order of 106 A/cm2. The distance traveled by the DW along the stripe increases with the current pulse amplitude and applied field strength, but it does not depend on the pulse duration between 0.4 and 2 ns. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We have studied the current-induced displacement of a domain wall (DW) in the permalloy (Py) layer of a Co/Cu/Py spin valve structure at zero and very small applied field. The displacement is in opposite direction for opposite dc currents, and the current density required to move DW is only of the order of 10[SUP6]A/cm[SUP2]. For H = 3 Oe, a back and forth DW motion between two stable positions is observed. We also discuss the effect of an applied field on the DW motion. [ABSTRACT FROM AUTHOR]

The influence that dipole-dipole interactions exert on the dynamics of the magnetization of nanometer-sized Co clusters has been studied by means of ac and dc susceptibility experiments. These clusters grow in a quasiordered layered structure, where all relevant parameters can be tailored and measured independently. Our data show without ambiguity that the magnetic relaxation becomes slower as the degree of interaction increases. The effective activation energy increases linearly with the number of nearest neighbor clusters, evolving from the value for a 2D layer to the fully 3D behavior, which is nearly reached for five layers. The experimental results agree quantitatively with the predictions of a simple model.

We study the 3D growth of clusters during the deposition of Au atoms on amorphous Al2O3. By comparing transmission electron microscopy images of the growth with Monte Carlo simulations, we show that nucleation takes place on substrate defects, but that further stages of growth imply that clusters leave the defects after they have reached a given critical size, and diffuse. An interesting consequence of this property is that, in contrast to intuition, and in a certain range of size, larger clusters are more mobile than smaller ones in this system.