High-frequency measurements of spin-valve films and devices (invited)

The emergence of very stable traveling intrinsic localized modes (ILMs) locked to a uniform driver is demonstrated in a discrete electrical transmission line. The speed of these traveling ILMs is tunable by the driver amplitude and frequency. It is found to be quite sensitive to the ratio of intersite to on-site nonlinearity. The number of traveling ILMs can also be selected via the driving conditions and appears to be the result of a spatiotemporal pattern selection process.

The low-frequency resistance noise in sputtered-deposited magnetic tunnel junctions with MgO barriers has been measured as a function of annealing time at different annealing temperatures. The noise has a 1 / f spectrum and it is quantified by a Hooge-like parameter ␣ given in units of m 2. Unannealed devices have the highest noise levels and their ␣ parameters exhibit a pronounced dependence on the voltage bias across the junction. A significant increase in tunneling magnetoresistance ͑TMR͒ is observed for short annealing times ͑on the order of minutes͒ at high temperatures and it is correlated with a large reduction in noise and in its bias dependence. The maximum TMR and minimum noise levels are reached at a later time that depends on temperature, being shorter at higher annealing temperatures. Devices annealed at 380 and at 430°C exhibit the same minimum noise levels, ␣ Ϸ 2 ϫ 10 −10 m 2. The origin of the resistance noise, its annealing time evolution, and its bias dependence are discussed and they are attributed to vacancy defects in the MgO barriers.

Reported are the synthesis, the structural characterization, and the electronic band structures of two new Zintl phases: BaGa2P2 and BaGa2As2. Both compounds are isoelectronic and isotypic and crystallize in a monoclinic system with a new structure type (Pearson symbol mP20). The structures have been established by single-crystal X-ray diffraction, space group P2(1)/c (Z = 4), with lattice parameters as follows: a = 7.3363(13)/7.495(5) Å; b = 9.6648(17)/9.901(6) Å; c = 7.4261(13)/7.643(5) Å; beta = 115.373(2) degrees/115.381(8) degrees for BaGa2P2/BaGa2As2, respectively. The atomic arrangements in both cases are devoid of disorder and are best rationalized as polyanionic layers, (infinity)(2)[Ga2Pn2]2- (Pn = P, As), with Ba2+ cations separating them. The layers, in turn, can be viewed as the result of condensation of Ga2Pn6 units, which are isosteric with the ethane molecule in its staggered conformation. Structural parallels with other known Zintl phases are presented. The electronic structures, computed using the tight-binding linear muffin-tin orbital methods (TB-LMTO), are discussed as well.

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Magnetic 1/f noise is compared in magnetic tunnel junctions with electron-beam evaporated and sputtered MgO tunnel barriers in the annealing temperature range 350-425 C. The variation of the magnetic noise parameter (a mag) of the reference layer with annealing temperature mainly reflects the variation of the pinning effect of the exchange-bias layer. A reduction in a mag with bias is associated with the bias dependence of the tunneling magnetoresistance. The related magnetic losses are parameterized by a phase lag e, which is nearly independent of bias especially below 100 mV. The similar changes in magnetic noise with annealing temperature and barrier thickness for two types of MgO magnetic tunnel junctions indicate that the barrier layer quality does not affect the magnetic losses in the reference layer. V C 2012 American Institute of Physics.

We report the crystallization study of CoFeB/MgO/CoFeB magnetic tunnel junctions using in-situ, time-resolved synchrotron-based x-ray diffraction and transmission electron microscopy. It was found that the crystallization of amorphous CoFeB electrodes occurs on a time scale of seconds during the postgrowth high temperature annealing. The crystallization can be well fit by the Johnson-Mehl-Avrami model and the effective activation energy of the process was determined to be 150 kJ/mol. The solid-state epitaxy mode of CoFeB was found to involve separate crystallization at different locations followed by subsequent merging of small grains, instead of layer-by-layer growth of CoFeB film along the MgO template.

Experimental evidence for the generation of intrinsic localized modes (ILMs) in a nonlinear electrical transmission line is presented both via modulational instability (MI) of the uniform mode and via driving the lattice locally. The spatial profiles of these modes localized on the order of the lattice spacing can be directly measured in this macroscopic lattice, in contrast to most other systems where ILMs have so far been detected.