Single-shot time-domain studies of spin-torque-driven switching in magnetic tunnel junctions

We report single-shot measurements of resistance versus time for thermally assisted spin-torque-driven switching in magnetic tunnel junctions. We achieve sufficient sensitivity to resolve the resistance signals leading up to switching, including the variations between individual switching events. Analyses of pre-switching thermal fluctuations allow detailed measurements of coherence times and variations in magnetization precession amplitude. We find that with a small in-plane hard-axis magnetic field the magnetization dynamics are more spatially coherent than for the case of zero field.Comment: 13 pages, 4 figure

Bias and angular dependence of spin-transfer torque in magnetic tunnel junctions

We use spin-transfer-driven ferromagnetic resonance (ST-FMR) to measure the spin-transfer torque vector T in MgO-based magnetic tunnel junctions as a function of the offset angle between the magnetic moments of the electrodes and as a function of bias, V. We explain the conflicting conclusions of two previous experiments by accounting for additional terms that contribute to the ST-FMR signal at large |V|. Including the additional terms gives us improved precision in the determination of T(V), allowing us to distinguish among competing predictions. We determine that the in-plane component of has a weak but non-zero dependence on bias, varying by 30-35% over the bias range where the measurements are accurate, and that the perpendicular component can be large enough to be technologically significant. We also make comparisons to other experimental techniques that have been used to try to measure T(V).Comment: 30 pages, 8 figures. Expanded with additional data and discussion. In press at PR

Quantum and classical fidelities for Gaussian states

We examine the physical significance of fidelity as a measure of similarity for Gaussian states, by drawing a comparison with its classical counterpart. We find that the relationship between these classical and quantum fidelities is not straightforward, and in general does not seem to provide insight into the physical significance of quantum fidelity. To avoid this ambiguity we propose that the efficacy of quantum information protocols be characterized by determining their transfer function and then calculating the fidelity achievable for a hypothetical pure reference input state.Comment: 9 pages, 5 figures, to be published in J. Opt. Soc. Am. B special issue on Optical Quantum Information Scienc

Sensitivity of spin-torque diodes for frequency-tunable resonant microwave detection

We calculate the efficiency with which magnetic tunnel junctions can be used as resonant detectors of incident microwave radiation via the spin-torque diode effect. The expression we derive is in good agreement with the sensitivities we measure for MgO-based magnetic tunnel junctions with an extended (unpatterned) magnetic pinned layer. However, the measured sensitivities are reduced below our estimate for a second set of devices in which the pinned layer is a patterned synthetic antiferromagnet (SAF). We suggest that this reduction may be due to an undesirable coupling between the magnetic free layer and one of the magnetic layers within the etched SAF. Our calculations suggest that optimized tunnel junctions should achieve sensitivities for resonant detection exceeding 10,000 mV/mW.Comment: 17 pages, 2 figure

Universal cross-over behavior of a magnetic impurity and consequences for doping in spin-1/2 chains

We consider a magnetic impurity in the antiferromagnetic spin-1/2 chain which is equivalent to the two-channel Kondo problem in terms of the field theoretical description. Using a modification of the transfer-matrix density matrix renormalization group (DMRG) we are able to determine local and global properties in the thermodynamic limit. The cross-over function for the impurity susceptibility is calculated over a large temperature range, which exhibits universal data-collapse. We are also able to determine the local susceptibilities near the impurity, which show an interesting competition of boundary effects. This results in quantitative predictions for experiments on doped spin-1/2 chains, which could observe two-channel Kondo physics directly.Comment: 5 pages in revtex format including 3 embedded figures (using epsf). The latest version in PDF format is available from http://fy.chalmers.se/~eggert/papers/crossover.pdf . Accepted by PR

Simulation of alnico coercivity

Micromagnetic simulations of alnico show substantial deviations from Stoner-Wohlfarth behavior due to the unique size and spatial distribution of the rod-like Fe-Co phase formed during spinodal decomposition in an external magnetic field. The maximum coercivity is limited by single-rod effects, especially deviations from ellipsoidal shape, and by interactions between the rods. Both the exchange interaction between connected rods and magnetostatic interaction between rods are considered, and the results of our calculations show good agreement with recent experiments. Unlike systems dominated by magnetocrystalline anisotropy, coercivity in alnico is highly dependent on size, shape, and geometric distribution of the Fe-Co phase, all factors that can be tuned with appropriate chemistry and thermal-magnetic annealing

Differential Use of Diagnostic Ultrasound in U.S. Emergency Departments by Time of Day

Background: Survey data over the last several decades suggests that emergency department (ED) access to diagnostic ultrasound performed by the radiology department is unreliable, particularly outside of regular business hours.Objective: To evaluate the association between the time of day of patient presentation and the use of diagnostic ultrasound services in United States (U.S.) EDs.Methods: This was a cross-sectional study of ED patient visits using the National Hospital Ambulatory Medical Care Survey for the years 2003 to 2005. Our main outcome measure was the use of diagnostic ultrasound during the ED patient visit as abstracted from the medical record. We performed multivariate analyses to identify any association between ultrasound use and time of presentation for all patients, as well as for two subgroups who are more likely to need ultrasound as part of their routine workup: patients at risk of deep venous thrombosis, and patients at risk for ectopic pregnancy.Results: During the three-year period, we analyzed 110,447 patient encounters, representing 39 million national visits. Of all ED visits, 2.6% received diagnostic ultrasound. Presenting to the ED “off hours” (defined as Monday through Friday 7pm to 7am and weekends) was associated with a lower rate of ultrasound use independent of potential confounders (odds ratio [OR] 0.73, 95% confidence interval [CI]: 0.65 - 0.82). Patients at increased risk of deep venous thrombosis who presented to the ED during “off hours” were also less likely to undergo diagnostic ultrasound (OR 0.34, 95% CI: 0.15 - 0.79). Similarly, patients at increased risk of ectopic pregnancy received fewer diagnostic ultrasounds during “off hours” (OR 0.56, 95% CI 0.35 - 0.91).Conclusion: In U.S. EDs, ultrasound use was lower during “off hours,” even among patient populations where its use would be strongly indicated. [West J Emerg Med. 2011;12(1):90-95.