ALBERT

Notes: We have recently shown that utilizing double frequency sweeps (DFSs) instead of pulses can lead to increased efficiencies in population and coherence transfer in half-integer quadrupolar spin systems. Cosine modulation of the carrier amplitude corresponds to the simultaneous irradiation of two frequencies symmetrically around the rf-carrier frequency. Convergent or divergent DFSs can be generated by appropriate time-dependent cosine modulation of the rf field. Population and coherence transfer induced by sweeping the modulation frequency through the quadrupolar satellite transitions is investigated in detail. The time dependence of such passages determines the adiabaticity of the transfer processes. Insight into the involved spin dynamics is of utmost importance in the design and optimization of experiments based on amplitude modulation, such as DFS enhanced multiple-quantum magic angle spanning, where multiple to single-quantum conversion is performed by a DFS. Vega and co-workers have provided a theoretical basis of adiabatic coherence transfer in spin-3/2 systems induced by the combined action of simple time independent cosine amplitude modulation (CAM) of the rf field and sample spinning [Madhu et al., J. Chem. Phys. 112, 2377 (2000)]. In our report we will extend this theory to DFS induced adiabatic transfer phenomena in spin-3/2 and spin-5/2 systems. A fully analytical description will be presented covering the whole adiabaticity range resulting in an accurate description of actual experiments. In this context it will be shown that both population and coherence transfer are governed by the same principles and one unique adiabaticity parameter for each pair of spectral satellites. The transfer phenomena derived for spin-3/2 systems will be studied and quantified experimentally for 23Na in a single crystal of NaNO3. In a static and spinning sample the combination with DFS and CAM irradiation will be studied showing the equivalence of the transfer in all these situations. Further we will demonstrate the greater flexibility of a DFS compared to a CAM pulse to manipulate the adiabaticity and thus to maximize the transfer efficiency. Finally, the 27Al resonance in an α-Al2O3 single crystal will be inspected to demonstrate that the efficiency of DFS-induced population and coherence transfer in spin-5/2 systems depends on the direction of the DFS. © 2001 American Institute of Physics.

Notes: The time-dependent dielectric breakdown of Co/Al2O3/Co(-Fe) magnetic tunnel junctions is investigated. At voltages larger than 1.2 V, almost immediate breakdown of the junction is observed, leading to a decreased (magneto)resistance. The shorts, which are local hot spots, were visualized by making use of a liquid crystal film on top of the junction. The breakdown voltages of a series of nominally identical tunnel junctions measured in a voltage-ramp experiment are shown to increase with increasing ramp speed. The results are analyzed in the framework of several models for the voltage dependent breakdown probability. © 1998 American Institute of Physics.

Notes: Due to their very thin tunnel barrier layer, magnetic tunnel junctions show dielectric breakdown at voltages of the order of 1 V. At the moment of breakdown, a highly conductive short is formed in the barrier and is visible as a hot spot. The breakdown effect is investigated by means of voltage ramp experiments on a series of nominally identical Co/Al2O3/Co tunnel junctions. The results are described in terms of a voltage dependent breakdown probability, and are further analyzed within the framework of a general model for the breakdown probability in dielectric materials, within which it is assumed that at any time the breakdown probability is independent of the (possibly time-dependent) voltage that has been previously applied. The experimental data can be described by several specific forms of the voltage breakdown probability function. A comparison with the models commonly used for describing thin film SiO2 breakdown is given, as well as suggestions for future experiments. © 1999 American Institute of Physics.