Edmund R. Nowak

Reversibility and irreversibility in the packing of vibrated granular material

Abstract We report on the settling of loosely packed, cohesionless granular material under mechanical vibrations. Monodisperse spherical beads were confined to a long vertical cylinder that was driven by an electromagnetic vibration exciter. Under vibrations the bead packing evolves from an initial, low-density configuration towards higher density. Ramping the vibration intensity repeatedly up and back down again reveals the existence of both an irreversible and a reversible branch in the response. The reversible branch represents a steady state in which the packing density depends monotinically on the vibration intensity. We have investigated the bead size, depth, and ramp rate dependence of the compaction process. Our results indicate how the occupied volume fraction can be optimized by slowly reducing the vibration intensity along the reversible branch.

Electrical noise in hysteretic ferromagnet-insulator-ferromagnet tunnel junctions

Low frequency noise has been measured in magnetic tunnel junctions that have Al2O3 tunnel barriers and magnetoresistance values up to 35% at 295 K. Fluctuations in voltage were found to cross over from Johnson noise to shot noise at low bias voltages, in quantitative agreement with theories of noise in quantum ballistic systems. 1/f resistance noise, where f is frequency, predominates at larger biases and is proportional to the mean current squared. This noise is attributed to trapping processes and it depends sensitively on the relative position of the oxide edge and the ferromagnet–Al interface.

Slow relaxation in granular compaction

Abstract Experimental studies show that the density of a vibrated granular material evolves from a low density initial state into a higher density final steady state. The relaxation towards the final density follows an inverse logarithmic law. As the system approaches its final state, a growing number of beads have to be rearranged to enable a local density increase. A free volume argument shows that this number grows as N = ϱ (1−ϱ) . The time scale associated with such events increases exponentially ∼ eN, and as a result a logarithmically slow approach to the final state is found ϱ ∞ − ϱ(t) ∼ 1 ln t . Furthermore, a one-dimensional toy model that captures this relaxation dynamics as well as the observed density fluctuations is discussed.

Noise properties of ferromagnetic tunnel junctions

We report measurements of voltage fluctuations in magnetic tunnel junctions which exhibit both high and low magnetoresistance (MR). The voltage noise power normalized to the square of the junction bias voltage was 10−14/Hz at a frequency of 1 Hz in a high MR junction. Low MR junctions had significantly higher noise power at 1 Hz and the origin of the noise was not magnetic. In these junctions, random telegraph noise was observed over a wide range of temperatures and junction biases. The results are consistent with a two-channel model of conduction, one of which is spin independent and gives rise to large noise. A noise measuring technique provides evidence for bias-dependent current-path rearrangements. The data support the existence of an inhomogeneous (filamentary-like) current-flow pattern across the tunnel junction associated with the spin-independent channel.

BaGa2Pn2 (Pn = P, As): new semiconducting phosphides and arsenides with layered structures.

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.

Glassy behavior of the parking lot model

We present a theoretical discussion of the reversible parking problem, which appears to be one of the simplest systems exhibiting glassy behavior. The existence of slow relaxation, nontrivial fluctuations, and an annealing effect can all be understood by recognizing that two different time scales are present in the problem. One of these scales corresponds to the fast filling of existing voids, the other is associated with collective processes that overcome partial ergodicity breaking. The results of the theory are in a good agreement with simulation data; they provide a simple qualitative picture for understanding recent granular compaction experiments and other glassy systems.

Magnetic-Field Dependence of the Noise in a Magnetoresistive Sensor Having MEMS Flux Concentrators

We report the dc and ac magnetic field dependence of the low-frequency noise in a microelectromechanical system (MEMS) flux concentrator device containing a giant magnetoresistance spin valve (SV). The noise is dominated by resistance fluctuations having a magnetic origin. Under nominally zero magnetic-field biasing conditions, the noise power is large and varies rapidly with small changes in magnetic field. Metastability between distinct resistive states is observed and can be suppressed with the application of a moderate longitudinal field. Stationary flux concentrators do not contribute excess noise, rather the dominant source of noise is the SVs themselves. This result indicates that the device is likely to increase the sensitivity of many magnetic sensors at low frequencies by orders of magnitude

Magnetic tunneling junction based magnetic field sensors: Role of shape anisotropy versus free layer thickness

Al2O3- and MgO-based magnetic tunnel junction (MTJ) sensors were designed and fabricated using microfabrication techniques. This study revealed that in the case of Al2O3-based sensors, the shape anisotropy in the free NiFe electrode resulted in a linear and hysteresis-free tunneling magnetoresistance (TMR) curve. These sensors exhibited TMR values between 27% and 30% and sensitivity up to 0.4%/Oe over a magnetic field range of − 40 to 40 Oe. In the case of CoFeB/MgO/CoFeB MTJ sensors, shape anisotropy alone was not sufficient to achieve a linear and hysteresis-free MR response. A superparamagnetic free layer was used to achieve the desired sensor response. MgO-based sensors had about 90% TMR and 1.1%/Oe sensitivity over the same field range as Al2O3-based MTJs.

Electron tunneling and noise studies in ferromagnetic junctions

Abstract Transport properties of exchange-biased magnetic tunnel junction structures are reported as a function of tunnel barrier thickness. The temperature dependence of the tunneling conductance and magnetoresistance (MR) is consistent with recent theories relating it to the magnon excitations at the electrode–barrier interface or the temperature-dependent surface magnetization. We have also measured the bias voltage dependence of the MR. For CoFe magnetic electrodes, the reduction in MR is approximately 50% at biases of ∼250 mV. At low temperatures, we observe a cusp-like dip in the tunneling conductance at zero-bias. The conductance increases as the square-root of the bias voltage, indicating that electron–electron interactions as in disordered media may be important. Coating a magnetic electrode or the oxide barrier with an alternate, thin magnetic layer of higher spin polarization is shown to generally increase the MR by several percent but at the expense of higher 1/f noise.

Onset of hysteresis measured by scanning tunneling microscopy

The feasibility of adapting scanning tunneling microscope (STM) technology to investigate the transition from reversible to irreversible behavior in mechanical systems is demonstrated. Using an STM with a graphite sample, both regimes of reversible and irreversible response to electrostrictive cycling are observed.