We present two approaches for studying the uniformity of a tunnel barrier. The first approach is based on measuring single-electron and two-electron tunneling in a hybrid single-electron transistor. Our measurements indicate that the effective area of a conduction channel is about one order of magnitude larger than predicted by theoretical calculations. With the second method, transmission electron microscopy, we demonstrate that variations in the barrier thickness are a plausible explanation for the larger effective area and an enhancement of higher order tunneling processes.

Double-barrier magnetic tunnel junctions with two MgO barriers and three CoFeB layers exhibiting tunneling magnetoresistance ͑TMR͒ values of more than 100% were fabricated. The bias voltage dependence of the TMR ratio is highly asymmetric after annealing at low temperatures, indicating dissimilar CoFeB / MgO interfaces. The TMR effect decays very slowly for positive bias and is only reduced to half of its maximum value at V 1/2 = 1.88 V when the junctions are processed at 200°C. The largest output voltage, 0.62 V, is obtained after annealing at 300°C, a temperature that combines high TMR ratios with a considerable asymmetric bias dependence. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2362977͔Following the demonstration of large tunneling magnetoresistance ͑TMR͒ ratios and low resistance-area products in magnetic tunnel junctions with a crystalline MgO͑001͒ barrier, MgO is now considered as the key barrier material for next-generation device applications. Giant TMR ratios well above 100% at room temperature have been obtained with epitaxial Fe/ MgO / Fe junctions, 1-5 textured magnetic tunnel junctions ͑MTJs͒, 6 and CoFeB / MgO / CoFeB structures. [7][8][9][10][11] In the latter case, the MgO barrier grows with a highly oriented ͑001͒ texture on top of an amorphous CoFeB layer. A postdeposition annealing process then crystallizes the CoFeB electrodes, thereby creating the different Bloch state symmetries with dissimilar decay rates in the MgO barrier that are necessary for giant TMR values. 12,13 In practical applications, MTJ structures are biased to increase the output voltage and decrease its signal-to-noise ratio. Electronic structure effects, magnon excitations, and spin-flip scattering on interface defects, however, reduce the TMR effect at elevated bias voltage and this limits the output signal of MTJ sensors and memory elements. A possible solution is to use double-barrier magnetic tunnel junctions ͑DMTJs͒ where the applied voltage is divided over two single junctions. Experimental studies on DMTJs with amorphous Al 2 O 3 barriers [14][15][16] and fully epitaxial Fe/ MgO double-barrier structures 17 have indeed confirmed a slower decay of the TMR ratio with bias voltage. In this letter, we demonstrate that high quality double-barrier junctions with crystalline MgO barriers and CoFeB electrodes can be fabricated by magnetron sputtering and postdeposition annealing. The bias voltage dependence of the TMR is asymmetric and depends strongly on the annealing temperature. These results can be used to engineer MTJs with high output signals.The DMTJs consisting of a 5 Ta/50 Ru/5 Ta/5 NiFe/10 IrMn/2 CoFe/0.7 Ru/4 CoFeB/2.5 MgO/3 CoFeB/2.5 MgO/4 CoFeB/0.7 Ru/2 CoFe/10 IrMn/5 NiFe/5 Ta ͑thickness in nanometers͒ multilayer stack were grown by magnetron sputtering on thermally oxidized Si substrates in our Shamrock deposition tool. All metallic layers were deposited by dc sputtering and a Co 40 Fe 40 B 20 ͑at. %͒ alloy target was used for the ferromagnetic electrodes. The MgO barriers were fabricated by rf ...

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A compact incident beam monitor for use with protein crystallography has been developed at Argonne's Structural Biology Center at the National Synchrotron Light Source (beamline X8C). Monitoring the incident x-ray beam at a synchrotron source is eli tical if accurate normalization factors are to be recorded. and a beam monitor must be capable of operating through a wide dynamic range. In the present design, incident beam intensity is monitored by measuring radiation scattered from a thin polymer film into a PIN diode. The response of the diode is linear over a range of 10 orders of magnitude. For improved statistical accuracy, the scat--tering film can be replaced with a thin metal foil. Results from measurements with Cr. Mn, Fe and Co foils indicate that fluorescent radiation emitted by a metal foil can increase the beam monitoring signal up to 50 times that produced from scattering alone. With this detector design, the direction of the forward scattered radiation is restricted to an area not larger than the beam stop, minimizing excessive background radiation in diffraction measurements. Suppm1 materials have been optimized so that no unwanted absorption edge effects are present between 6-19.5 ke V. This design makes the detector useful for monitoring incident beam intensities over a wide range of absorption edges often associated with multi wavelength anomalous diffraction (MAD) expeliments. Accurate incident beam monitoring also simplifies optimization of the x-ray beam through the diffractometer collimation after each new electron orbit of the synchrotron is established. This device would be particularly well-suited for operations on third-generation synchrotron source beamlines. An induction furnace has been constructed to provide a temperature range of 290-2500 K and a cryogenic chamber using a closed-cycled refrigerator has been assembled to cool powder samples from room temperature down to 20 K. The furnace and cryostat are designed as parts of an integrated program to enhance the powder diffraction facilities at the SRS, Dares bury Laboratory. In this work we describe the commissioning of these devices for high resolution powder diffractometry. Powder diffraction results from standard tungsten and silver samples are presented to demonstrate the operation of the apparatus and some of the early experimental results are included. The first KOSSEL experiments using synchrotron radiation were pe1formed in 1992. The exposure time was drastically reduced due to the high intensity of polychromatic synchrotron radiation at HASYLAB. When using imaging plates, serveral seconds of exposure times are sufficient. When using X-ray films, the required time increases to the few minutes. KOSSEL patterns of crisp contrast could be obtained from elements, alloys, intermetallics, semiconductor compounds and minerals.The following problems received special attention: PS01.02.19 KOSSEL-TECHNIQUE BY MEANS OF SYN-CHROTRON BEAM EXCITATION-•Proof of minor tetragonal distortions (c/a :e: 1,02) in FeAl cylindrical specimen, as a f...

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